Carbohydrates - David klein Flashcards
1
Q
Would you expect an aldohexose and a ketohexose to be
constitutionally isomeric? Explain why or why not.
A
Yes, an aldohexose and a ketohexose are constitutional isomers.
Explanation:
Definition of Constitutional Isomers
Constitutional isomers are compounds that have the same molecular formula but different connectivity of atoms.
Aldohexose vs. Ketohexose Structure
Aldohexoses (e.g., glucose) contain an aldehyde (-CHO) functional group at carbon 1.
Ketohexoses (e.g., fructose) contain a ketone (-CO-) functional group, typically at carbon 2.
Comparison of Molecular Formula and Connectivity
Both aldohexoses and ketohexoses have the molecular formula C₆H₁₂O₆.
However, they differ in the position of the carbonyl group (aldehyde vs. ketone), which changes the connectivity of atoms in the molecule.
Since they have the same molecular formula but different connectivity, aldohexoses and ketohexoses are constitutional isomers.
2
Q
What is Glyceraldehyde?
A
Glyceraldehyde is one of the smallest compounds considered to be a carbohydrate. It contains a single chiral center and can exist as a pair of enantiomers.
3
Q
What are Enantiomers?
A
Enantiomers are mirror-image isomers that are non-superimposable. They rotate plane-polarized light in opposite directions.
3
Q
What is the difference between (+)-Glyceraldehyde and (−)-Glyceraldehyde?
A
(+)-Glyceraldehyde is dextrorotatory (rotates plane-polarized light clockwise) and is abundant in nature.
(−)-Glyceraldehyde is levorotatory (rotates plane-polarized light counterclockwise) and is generally not found in nature.
4
Q
What is the D and L notation in sugars?
A
D and L notation is based on the configuration of glyceraldehyde:
D-glyceraldehyde has the OH group on the right in a Fischer projection.
L-glyceraldehyde has the OH group on the left in a Fischer projection.
5
Q
How does glucose relate to D-glyceraldehyde?
A
Most naturally occurring carbohydrates degrade to form D-glyceraldehyde.
Since glucose degrades into D-glyceraldehyde, it is classified as a D-sugar.
6
Q
What determines whether a sugar is D or L?
A
The configuration of the chiral carbon farthest from the carbonyl group determines whether a sugar is D or L.
If the OH group on this carbon is on the right, it is a D-sugar.
If the OH group is on the left, it is an L-sugar.
7
Q
What makes glyceraldehyde a carbohydrate?
A
Glyceraldehyde is considered a carbohydrate because it follows the general formula CₙH₂ₙOₙ (where n = 3 for glyceraldehyde). It contains a carbonyl (C=O) functional group and hydroxyl (-OH) groups, which are characteristic features of carbohydrates.
8
Q
Why can glyceraldehyde exist as two enantiomers?
A
Glyceraldehyde has a single chiral center at the central carbon (C2), which means it can exist in two non-superimposable mirror-image forms:
D-(+)-Glyceraldehyde (rotates plane-polarized light clockwise)
L-(−)-Glyceraldehyde (rotates plane-polarized light counterclockwise)
These two forms are enantiomers because they are mirror images that cannot be superimposed on one another.
9
Q
How can you distinguish between D- and L-glyceraldehyde?
In the Fischer projection of glyceraldehyde:
A
If the OH group on the chiral carbon (C2) is on the right, it is D-glyceraldehyde.
If the OH group is on the left, it is L-glyceraldehyde.
10
Q
What is the significance of D-glyceraldehyde in carbohydrate chemistry?
A
D-glyceraldehyde serves as the reference compound for classifying all other sugars as either D-sugars or L-sugars. Sugars that degrade into D-glyceraldehyde are classified as D-sugars.
Those that degrade into L-glyceraldehyde are classified as L-sugars.
Most naturally occurring sugars (e.g., glucose) belong to the D-series.
11
Q
Why is naturally occurring glucose classified as a D-sugar?
A
Glucose is classified as a D-sugar because the hydroxyl (-OH) group on the chiral carbon farthest from the carbonyl group (C5 in glucose) is on the right in the Fischer projection.
Additionally, when glucose is degraded, it produces D-glyceraldehyde, confirming its D-configuration.
12
Q
How does the structure of glyceraldehyde relate to the stereochemistry of other sugars?
A
The stereochemistry of larger monosaccharides (e.g., glucose, fructose) is determined by comparing their chiral centers to D- or L-glyceraldehyde. The D/L notation of any sugar depends on the position of the hydroxyl group on the chiral carbon farthest from the carbonyl group (usually the penultimate carbon).
If it matches D-glyceraldehyde (OH on the right), the sugar is a D-sugar.
If it matches L-glyceraldehyde (OH on the left), the sugar is an L-sugar.
13
Q
Explain the relationship between optical rotation and the D/L notation in carbohydrates
A
Optical rotation (+ or −) refers to how a molecule rotates plane-polarized light but does not determine whether a sugar is D or L.
D/L notation is based on the spatial arrangement of atoms relative to glyceraldehyde, not on how the sugar rotates light.
For example, D-glucose is dextrorotatory (+), but D-fructose is levorotatory (−), showing that D/L classification is independent of optical activity.
14
Q
What happens to the chiral centers of glucose when it degrades into D-glyceraldehyde?
During degradation, cleavage of carbon-carbon bonds occurs, reducing the number of chiral centers.
A
The original chiral center at C5 of glucose remains and becomes the only chiral center in D-glyceraldehyde.
Since the OH group on C5 was originally on the right, the final product is D-glyceraldehyde, confirming glucose as a D-sugar.
15
Q
Why is the D-isomer of glyceraldehyde more commonly found in nature?
A
D-glyceraldehyde is more common because most naturally occurring carbohydrates (including glucose, ribose, and other sugars used in metabolism) belong to the D-family. This preference is due to the evolution of enzymes that selectively process D-sugars in biological pathways like glycolysis and the pentose phosphate pathway.
16
Q
How does the presence of a single chiral center in glyceraldehyde influence the classification of larger carbohydrates?
A
Glyceraldehyde is the simplest chiral sugar, serving as the basis for determining the D/L configuration of all other carbohydrates. When a larger carbohydrate (e.g., glucose, ribose) has multiple chiral centers, only the chiral carbon farthest from the carbonyl group determines whether it is classified as D or L, based on its similarity to D-glyceraldehyde.
17
Q
What happens when naturally occurring glucose is degraded?
A
The loss of three carbon atoms from glucose produces D-glyceraldehyde.
This pattern is seen in most naturally occurring carbohydrates.
17
Q
How do synthetic sugars degrade compared to naturally occurring sugars?
A
Synthetic sugars degrade into a mixture of D- and L-glyceraldehyde, unlike natural sugars that predominantly yield D-glyceraldehyde.
18
Q
What is the Fischer–Rosanoff convention?
A
The convention assigns the letter D to any sugar that degrades into (+)-glyceraldehyde.
The chiral center farthest from the carbonyl group determines the D/L designation.
19
Q
What defines a D-sugar?
A
A D-sugar has the OH group on the right at the chiral center farthest from the carbonyl group in the Fischer projection.
20
Q
What defines an L-sugar?
A
An L-sugar has the OH group on the left at the chiral center farthest from the carbonyl group in the Fischer projection.
It is the enantiomer of the corresponding D-sugar.
21
Q
Does being a D-sugar mean the molecule is dextrorotatory?
A
No. While D-glyceraldehyde is dextrorotatory by definition, other D-sugars are not necessarily dextrorotatory.
Example: D-erythrose and D-threose are actually levorotatory.
22
Q
What determines whether a sugar is dextrorotatory or levorotatory?
A
Optical rotation (dextrorotatory + or levorotatory −) is experimentally determined.
It is not related to the D/L classification.
23
What is the significance of the lowest chiral center in sugar classification?
The lowest chiral center (farthest from the carbonyl group) in D-sugars has the same R configuration as D-glyceraldehyde.
Similarly, L-sugars have the opposite configuration of D-sugars.
24
Why does glucose degrade into D-glyceraldehyde?
Naturally occurring D-glucose has a specific stereochemistry that results in the selective formation of D-glyceraldehyde when degraded.
25
How do synthetic sugars degrade differently from natural sugars?
Synthetic sugars degrade into a mixture of D- and L-glyceraldehyde, whereas natural sugars predominantly degrade into D-glyceraldehyde.
26
What does the Fischer–Rosanoff convention state?
The D/L classification of sugars is determined by the position of the OH group on the lowest chiral center in the Fischer projection.
If OH is on the right → D-sugar.
If OH is on the left → L-sugar.
27
Does a D-sugar always rotate light clockwise (dextrorotatory)?
No. The D/L system is independent of optical activity.
Some D-sugars (e.g., D-erythrose, D-threose) are levorotatory.
28
What determines the optical rotation of a sugar?
Optical rotation is determined experimentally by measuring how the molecule rotates plane-polarized light.
It cannot be predicted solely from the D/L configuration.
29
How do we determine whether a sugar is D or L?
Look at the chiral carbon farthest from the carbonyl group:
OH on the right → D-sugar.
OH on the left → L-sugar.
30
How do D- and L-sugars relate to each other?
They are enantiomers—mirror images of each other.
31
What does the D/L notation refer to in modern carbohydrate chemistry?
The D/L notation refers to the absolute configuration of the sugar relative to D- and L-glyceraldehyde.
It does not indicate whether the sugar is dextrorotatory or levorotatory.
32
Classification of Carbohydrates as Aldose or Ketose
To classify each carbohydrate, we identify:
Aldose: Contains an aldehyde (-CHO) group at C1.
Ketose: Contains a ketone (-CO-) group at C2.
Then, we count the total number of carbon atoms to name them correctly.
Analysis of Each Structure:
(a) Aldohexose (6 carbon aldose) – Aldehyde at C1, six carbons.
(b) Aldopentose (5 carbon aldose) – Aldehyde at C1, five carbons.
(c) Ketopentose (5 carbon ketose) – Ketone at C2, six carbons.
(d) Aldtetrose (4 carbon aldose) – Aldehyde at C1, five carbons.
(e) ketohexose (6 carbon ketose) – ketone at C2, six carbons.
33
Would you expect an aldohexose and a ketohexose to be constitutionally isomeric? Explain why or why not.
Answer:
Yes, aldohexose and ketohexose are constitutional isomers because:
They have the same molecular formula (C₆H₁₂O₆).
They differ in the connectivity of atoms:
Aldohexose has an aldehyde (-CHO) at C1.
Ketohexose has a ketone (-CO-) at C2.
This difference in functional group placement makes them constitutional isomers (same molecular formula, different structure).
34
Determination of D or L Sugar & Chiral Centers
To determine D- or L-configuration, look at:
The chiral carbon farthest from the carbonyl group:
If OH is on the right → D-sugar.
If OH is on the left → L-sugar.
Assign R/S configuration for each chiral center.
Analysis of Each Structure:
(a) D-Sugar (OH on the right at the bottom chiral center).
(b) L-Sugar (OH on the left at the bottom chiral center).
(c) D-Sugar (OH on the right at the bottom chiral center).
(d) D-Sugar (OH on the right at the bottom chiral center).
(e) D-Sugar (OH on the right at the bottom chiral center).
Trend Noticed:
D-sugars have the OH group on the right at the lowest chiral center.
L-sugars have the OH group on the left at the lowest chiral center.
The fastest way to determine D/L: Look at the bottom-most chiral center in the Fischer projection.
35
Draw Fischer projections for D-Allose and L-Allose.
Identify the stereoisomeric relationship.
Identify which is a D-sugar and which is an L-sugar.
Draw Fischer projections for D-Allose and L-Allose.
D-Allose and L-Allose are aldohexoses with six carbon atoms, and they are mirror images (enantiomers).
D-Allose: All four chiral centers have the R configuration.
L-Allose: All four chiral centers have the S configuration.
D-Allose and L-Allose are enantiomers, meaning they are non-superimposable mirror images.
D-Allose: The OH group on the lowest chiral carbon (C5) is on the right → D-sugar.
L-Allose: The OH group on the lowest chiral carbon (C5) is on the left → L-sugar.
36
Draw both stereoisomeric ketotetrose isomers.
Ketotetroses are four-carbon monosaccharides with a ketone group at C2.
The only ketotetrose is erythrulose, which has two stereoisomers:
Erythrulose and L-Erythrulose are enantiomers.
D-Erythrulose has the OH on the right at C3, while L-Erythrulose has the OH on the left at C3.
37
Draw all four aldotetrose isomers and arrange them into enantiomeric pairs.
Aldotetroses are four-carbon sugars with an aldehyde (-CHo) group at C1.
There are four stereoisomers, grouped into two enantiomeric pairs:
D-Erythrose and L-Erythrose are enantiomers.
D-Threose and L-Threose are enantiomers.
D-Aldotetroses have the OH group on the right at the lowest chiral center.
L-Aldotetroses have the OH group on the left at the lowest chiral center.
Identify which stereoisomers are D sugars and which are L sugars.
D-Sugars: D-Erythrose, D-Threose (OH on the right at C3).
L-Sugars: L-Erythrose, L-Threose (OH on the left at C3
38
How many chiral centers do aldotetroses have?
Aldotetroses have two chiral centers.
There are four possible stereoisomers (two pairs of enantiomers).
39
What are the D-aldotetroses?
D-Erythrose and D-Threose are the naturally occurring D-aldotetroses.
40
What are the L-aldotetroses?
L-Erythrose and L-Threose are the enantiomers of the D-aldotetroses.
41
How many possible aldopentoses exist?
Aldopentoses have three chiral centers, leading to eight stereoisomers (four pairs of enantiomers). 2^3
42
What are the common D-aldopentoses?
The four naturally occurring D-aldopentoses are:
D-Ribose (a key sugar in RNA).
D-Arabinose (found in plants).
D-Xylose (found in wood).
D-Lyxose.
43
How many possible aldohexoses exist?
Aldohexoses have four chiral centers, leading to 16 possible stereoisomers (eight pairs of enantiomers).
44
What are the naturally occurring D-aldohexoses?
The eight D-aldohexoses are:
D-Allose
D-Altrose
D-Glucose
D-Mannose
D-Gulose
D-Idose
D-Galactose
D-Talose
45
How do we classify aldoses based on carbon number?
Aldotetroses → 4 carbons.
Aldopentoses → 5 carbons.
Aldohexoses → 6 carbons.
46
How do we determine if an aldose is a D or L sugar?
Look at the chiral center farthest from the carbonyl group:
OH on the right → D-sugar.
OH on the left → L-sugar.
47
What rule determines the number of stereoisomers in aldoses?
The formula 2ⁿ, where n = the number of chiral centers.
48
How many enantiomeric pairs exist for aldohexoses?
There are eight pairs of enantiomers (16 total stereoisomers).
49
What is the relationship between D-galactose and D-glucose?
They are epimers at C4.
50
What is an epimer?
Epimers are sugars that differ in configuration at only one chiral center.
51
Why are only D-sugars commonly found in nature?
Enzymes in living organisms selectively recognize and process D-sugars.
52
How can you quickly determine whether a sugar is an aldose or ketose?
Aldoses have an aldehyde (-CHO) at C1.
Ketoses have a ketone (-CO) at C2.
53
Why does D-ribose play a key role in biological systems?
D-Ribose is a fundamental component of RNA, ATP, and nucleotides.
54
How many chiral centers do aldoses have based on carbon number?
Aldotetroses → 2 chiral centers.
Aldopentoses → 3 chiral centers.
Aldohexoses → 4 chiral centers.
55
What determines whether an aldose is a tetrose, pentose, or hexose?
The total number of carbon atoms.
56
What is the Fischer–Rosanoff convention?
A D-sugar has the OH group on the right at the lowest chiral center.
An L-sugar has the OH group on the left at the lowest chiral center.
57
What is the starting point for the aldose family tree?
D-Glyceraldehyde is the simplest aldose and the starting point for all aldose sugars.
58
How are aldoses classified by carbon number?
Aldotrioses → 3 carbons (e.g., D-Glyceraldehyde).
Aldotetroses → 4 carbons (e.g., D-Erythrose, D-Threose).
Aldopentoses → 5 carbons (e.g., D-Ribose, D-Arabinose, D-Xylose, D-Lyxose).
Aldohexoses → 6 carbons (e.g., D-Glucose, D-Mannose, D-Galactose, D-Allose, D-Altrose, D-Gulose, D-Idose, D-Talose).
59
How are new chiral centers added to aldoses?
Each new chiral center is introduced just below the carbonyl group, doubling the number of possible stereoisomers.
60
What determines the number of stereoisomers in aldoses?
The formula 2ⁿ, where n = number of chiral centers.
61
How many D-aldohexoses exist?
Eight D-aldohexoses exist, forming four pairs of epimers.
62
What are the epimeric relationships among aldohexoses?
D-Glucose & D-Mannose → Epimers at C2.
D-Glucose & D-Galactose → Epimers at C4.
D-Allose & D-Altrose → Epimers at C3.
D-Gulose & D-Idose → Epimers at C3.
63
What is the most biologically significant aldohexose?
D-Glucose, as it is the primary energy source for most organisms.
64
What role does D-Ribose play in biology?
D-Ribose is essential for RNA, ATP, and nucleotides.
65
How do L-sugars relate to D-sugars?
L-Sugars are the enantiomers of D-sugars, meaning they have the opposite configuration at every chiral center.
66
Why are D-sugars more common in nature?
Enzymes and biological pathways are highly specific for D-sugars, making them the dominant form in living organisms.
67
How does the number of stereoisomers change as the carbon count increases in aldoses?
Each additional chiral center doubles the number of stereoisomers (2ⁿ rule).
68
Why does D-glucose have multiple epimers?
Because each chiral center can independently change configuration, creating different epimers.
69
What is the relationship between D-erythrose and D-threose?
They are diastereomers (stereoisomers that are not mirror images).
70
How do aldopentoses lead to aldohexoses?
Each aldopentose gains a new chiral center, producing two different aldohexoses.
71
What determines whether a sugar is a D-sugar or L-sugar?
The OH group on the lowest chiral carbon:
Right → D-sugar.
Left → L-sugar
72
Why is D-glyceraldehyde the foundation for the D/L system?
The D/L classification is based on its absolute configuration.
73
How are enantiomers and epimers different?
Enantiomers: Mirror-image isomers (opposite at all chiral centers).
Epimers: Differ at only one chiral center.
74
How are aldoses related to ketoses?
They are constitutional isomers (same formula, different connectivity).
75
Why do some sugars have multiple epimers?
The more chiral centers, the more epimers are possible.
76
Why do aldoses form cyclic structures?
Intramolecular reactions between the carbonyl and hydroxyl groups create hemiacetals.
77
simplest ketose
Dihydroxyacetone is the simplest ketose (C₃H₆O₃).
It has no chiral centers.
78
How many D-ketohexoses exist?
There are four D-ketohexoses:
D-Psicose
D-Fructose
D-Sorbose
D-Tagatose
79
How many D-ketopentoses exist?
There are two D-ketopentoses:
D-Ribulose
D-Xylulose
80
How do ketoses differ from aldoses in chiral centers?
Ketoses have one less chiral center than aldoses with the same number of carbons.
81
What is the most common ketohexose in nature?
D-Fructose is the most abundant ketohexose.
82
How are new chiral centers introduced in ketoses?
A new chiral center is added below the carbonyl group, just like in aldoses.
83
What is the relationship between D-ribulose and D-xylulose?
They are epimers at C3.
83
What are the enantiomers of the D-ketoses?
Each D-ketose has a corresponding L-ketose, which is its mirror image.
84
What is the structural difference between aldoses and ketoses?
Aldoses have an aldehyde (-CHO) at C1.
Ketoses have a ketone (-CO) at C2.
85
Why do ketoses form fewer stereoisomers than aldoses?
Since ketoses have one less chiral center, they form fewer stereoisomers than aldoses.
86
Why do ketohexoses have fewer isomers than aldohexoses?
Ketohexoses have one less chiral center, reducing the total number of possible stereoisomers.
87
How many stereoisomers exist for ketohexoses?
There are four D-ketohexoses, meaning there are eight total stereoisomers (four D-sugars and their L-enantiomers).
88
What is the simplest ketose and why?
Dihydroxyacetone is the simplest ketose because it has no chiral centers.
89
What determines whether a ketose is a D or L sugar?
The OH group on the lowest chiral center:
Right → D-sugar.
Left → L-sugar.
90
Why is D-fructose important biologically?
D-Fructose is found in fruit and honey and is a key component of sucrose (table sugar).
91
What is the epimeric relationship between D-psicose and D-fructose?
They differ at C3, making them epimers.
92
How are D-ribulose and D-xylulose related?
They are epimers at C3.
93
How are ketoses classified by carbon number?
Ketotrioses → 3 carbons (e.g., Dihydroxyacetone).
Ketotetroses → 4 carbons (e.g., D-Erythrulose).
Ketopentoses → 5 carbons (e.g., D-Ribulose, D-Xylulose).
Ketohexoses → 6 carbons (e.g., D-Fructose, D-Psicose, D-Sorbose, D-Tagatose).
94
What is the relationship between ketoses and aldoses?
They are constitutional isomers (same formula, different connectivity).
95
Why do ketoses readily form cyclic structures?
The ketone group reacts with hydroxyl groups, forming hemiacetals in solution.
96
Draw and Name the Enantiomer of D-Fructose
The enantiomer of D-fructose is L-fructose.
L-fructose is a mirror image of D-fructose and has the opposite configuration at all chiral centers.
Key Difference: L-fructose has the OH groups flipped compared to D-fructose.
97
Which term best describes the relationship between D-fructose and D-glucose?
(a) Enantiomers
(b) Diastereomers
(c) Constitutional isomers
Correct Answer: (c) Constitutional Isomers
Explanation:
D-fructose and D-glucose have the same molecular formula (C₆H₁₂O₆).
D-glucose is an aldose (contains an aldehyde at C1).
D-fructose is a ketose (contains a ketone at C2).
They have different connectivity of atoms, meaning they are constitutional isomers.
Why Not the Other Options?
Not enantiomers: Enantiomers are mirror images of each other, but glucose and fructose have different functional groups.
Not diastereomers: Diastereomers differ at one or more chiral centers but have the same functional groups.
98
What is a hemiacetal?
A hemiacetal forms when an aldehyde reacts with an alcohol in the presence of an acid catalyst.
It contains both an -OH (hydroxyl group) and an -OR (ether group) on the same carbon.
98
Why do monosaccharides form cyclic structures?
In monosaccharides, the aldehyde (or ketone) group reacts with a hydroxyl group within the same molecule.
This intramolecular reaction forms a stable cyclic hemiacetal, which is favored in equilibrium.
99
Why does the equilibrium favor cyclic hemiacetals over open-chain forms?
Six-membered rings (pyranoses) are relatively strain-free, making them more stable than their open-chain counterparts.
99
What is the key structural requirement for cyclic hemiacetal formation?
The molecule must contain both an aldehyde (or ketone) and a hydroxyl group in the correct positions to allow ring closure.
99
What is the difference between a hemiacetal and a full acetal?
A hemiacetal contains:
One -OH group
One -OR group
A full acetal forms when a hemiacetal reacts with another alcohol, replacing the -OH group with another -OR group.
100
What mistake should be avoided when drawing cyclic hemiacetals?
Be careful to include the correct number of carbon atoms in the ring.
A common error is drawing too many or too few carbon atoms in the ring structure.
101
What types of sugars commonly form cyclic hemiacetals?
Aldoses (e.g., glucose, ribose) form pyranose (six-membered) rings.
Ketoses (e.g., fructose) form furanose (five-membered) rings.
102
Why is cyclization important for carbohydrates?
Cyclic forms are more stable and prevalent in aqueous solutions.
They play a crucial role in the biochemistry of sugars and their derivatives.
103
How does an aldehyde react with an alcohol to form a hemiacetal?
The carbonyl carbon of the aldehyde is attacked by the oxygen of the alcohol, forming a hemiacetal intermediate.
104
Why do monosaccharides favor cyclic forms?
The equilibrium shifts toward intramolecular hemiacetal formation because it forms a stable six- or five-membered ring.
104
What structural feature allows monosaccharides to form cyclic hemiacetals?
The presence of both a carbonyl group (C=O) and a hydroxyl (-OH) group that can interact.
105
What is the difference between pyranose and furanose rings?
Pyranose: A six-membered ring.
Furanose: A five-membered ring.
106
What determines whether a sugar will form a five- or six-membered ring?
The distance between the carbonyl carbon and the hydroxyl group that participates in the reaction.
107
What kind of reaction converts a cyclic hemiacetal into a full acetal?
A reaction with another alcohol, leading to acetal formation.
108
Why is it important to correctly track carbon atoms in cyclic sugars?
Mistakes in counting carbons can lead to incorrect structures and misinterpretation of sugar chemistry.
109
What is a cyclic hemiacetal?
A cyclic hemiacetal forms when an intramolecular reaction occurs between an aldehyde group (-CHO) and a hydroxyl group (-OH) in the same molecule.
109
What is the first step in drawing a cyclic hemiacetal?
Identify the carbonyl carbon (C=O) and begin numbering the carbon chain.
Continue numbering until reaching the hydroxyl group involved in the reaction.
110
How do you determine the ring size?
Count the number of carbon atoms that will be incorporated into the ring.
The oxygen from the hydroxyl group is also part of the ring.
111
What type of rings do cyclic hemiacetals form?
Five-membered rings (furanoses) are commonly formed.
Six-membered rings (pyranoses) are more stable and frequently found in monosaccharides.
112
What happens during ring formation?
The oxygen atom of the hydroxyl group attacks the carbonyl carbon, forming a hemiacetal bond.
The oxygen must be part of the ring.
113
How do you avoid mistakes when drawing cyclic hemiacetals?
Make sure to:
Include all carbons correctly.
Incorporate the oxygen into the ring.
Ensure the correct ring size.
114
What functional groups are present in a hemiacetal?
A hemiacetal contains:
One -OH (hydroxyl) group.
One -OR (ether) group attached to the same carbon.
115
Why do five- and six-membered rings form more readily than smaller or larger rings?
Five-membered and six-membered rings have minimal strain and are more stable.
Smaller rings (e.g., four-membered) have high angle strain, making them less favored.
116
What is the role of an acid catalyst in hemiacetal formation?
An acid catalyst protonates the carbonyl group, making it more electrophilic, facilitating nucleophilic attack by the hydroxyl oxygen.
117
How does this concept apply to monosaccharides?
Monosaccharides like glucose and fructose form stable cyclic hemiacetals, which dominate in aqueous solutions.
118
What functional groups are necessary for a molecule to form a cyclic hemiacetal?
A molecule must contain both an aldehyde (or ketone) and a hydroxyl (-OH) group.
119
What determines whether a five-membered or six-membered ring forms?
The distance between the carbonyl group and hydroxyl group determines the ring size.
120
Why are cyclic hemiacetals more stable than open-chain forms in monosaccharides?
Cyclic forms reduce strain and stabilize the molecule, favoring equilibrium.
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What is the difference between a hemiacetal and a full acetal?
Hemiacetals have one -OH and one -OR group.
Acetals have two -OR groups and form when a hemiacetal reacts with another alcohol.
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How can you ensure the correct numbering when drawing cyclic hemiacetals?
Always start numbering at the carbonyl carbon and proceed along the chain to the hydroxyl group involved in cyclization.
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What happens when a bifunctional compound is treated with aqueous acid?
The carbonyl group (C=O) reacts with a hydroxyl (-OH) group within the same molecule, forming a cyclic hemiacetal.
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What is the role of the hydroxyl group in hemiacetal formation?
The hydroxyl acts as a nucleophile, attacking the carbonyl carbon to form a new C-O bond, leading to ring closure.
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How do you determine the ring size of a cyclic hemiacetal?
Number the carbon chain starting from the carbonyl carbon.
Count the number of carbons between the carbonyl and the reacting hydroxyl group.
Common ring sizes:
Five-membered rings (furanoses)
Six-membered rings (pyranoses) (more stable)
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Why do five- and six-membered rings form more readily than other ring sizes?
Smaller rings (e.g., 3- or 4-membered) have high angle strain.
Larger rings (e.g., 7+ carbons) have too much flexibility, reducing stability.
Five- and six-membered rings minimize strain, making them more favorable.
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What is the C1 position in a cyclic hemiacetal?
The C1 position is the carbonyl carbon before cyclization.
After forming the cyclic hemiacetal, C1 becomes a chiral center.
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What happens under acidic conditions to promote hemiacetal formation?
Acid protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic.
This enhances the nucleophilic attack by the hydroxyl group.
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How can a molecule have two possible ring sizes?
If the molecule has two hydroxyl groups, either one can nucleophilically attack the carbonyl, leading to different ring sizes.
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How do you predict which ring will be favored?
Six-membered rings (pyranoses) are the most stable.
Five-membered rings (furanoses) can also form, but are slightly less favored.
Three- and four-membered rings rarely form due to ring strain.
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How does numbering the carbon atoms help in drawing cyclic hemiacetals?
It ensures the correct ring size is drawn.
Helps in placing substituents accurately.
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What is the difference between a hemiacetal and a full acetal?
Hemiacetal: One -OH and one -OR group.
Acetal: Two -OR groups, formed when a hemiacetal reacts with another alcohol.
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What is the first step in predicting the cyclic hemiacetal of a compound?
Identify the carbonyl carbon (C=O) and the hydroxyl group (-OH) involved in ring closure.
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Why are six-membered rings more stable than four-membered rings?
Less angle strain and better bond overlap, making them energetically favored.
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What happens if multiple hydroxyl groups are available for hemiacetal formation?
The hydroxyl group closer to the carbonyl carbon usually forms the ring.
If both options are available, the six-membered ring is preferred.
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How does acidic catalysis affect cyclic hemiacetal formation?
Acid protonates the carbonyl, increasing electrophilicity, making it easier for the hydroxyl group to attack.
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What is a pyranose ring?
A pyranose ring is a six-membered cyclic hemiacetal that includes an oxygen atom in the ring.
Named after pyran, a six-membered oxygen-containing ring.
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How does D-glucose form a pyranose ring?
The aldehyde (-CHO) at C1 reacts with the hydroxyl (-OH) at C5, forming a six-membered cyclic hemiacetal.
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Why does D-glucose exist mostly in its cyclic form?
The equilibrium strongly favors the cyclic hemiacetal form due to the stability of the six-membered ring.
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What are the two forms of D-glucopyranose in solution?
α-D-glucopyranose: The OH at C1 (anomeric carbon) is down.
β-D-glucopyranose: The OH at C1 (anomeric carbon) is up.
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What is the open-chain form of glucose?
The open-chain form has a free aldehyde (-CHO) group at C1.
Only a trace amount of glucose exists in this form at equilibrium.
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What type of reaction converts the open-chain form of glucose to the cyclic form?
An intramolecular nucleophilic addition of the hydroxyl (-OH) group at C5 to the aldehyde at C1.
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What functional group forms at C1 when glucose cyclizes?
A hemiacetal functional group (-OH and -OR attached to the same carbon).
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What happens to the chiral centers during cyclization?
C1 becomes a new chiral center (the anomeric carbon).
The configuration at C1 determines if it is α or β anomer.
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What is the relationship between the α and β forms of D-glucose?
They are anomers (differ only in configuration at the anomeric carbon, C1).
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Why is the β-anomer of D-glucose more stable than the α-anomer?
In β-D-glucose, the OH group at C1 is equatorial, reducing steric hindrance, making it more stable.
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How can you identify the anomeric carbon in a cyclic sugar?
The carbon bonded to two oxygen atoms in the cyclic hemiacetal form (C1 in glucose).
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What functional groups are involved in the cyclization of glucose?
Aldehyde (-CHO) at C1 and hydroxyl (-OH) at C5.
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What is the main driving force for glucose cyclization?
The formation of a stable six-membered ring, reducing free energy.
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What is mutarotation?
The process where α- and β-anomers interconvert in solution via the open-chain form.
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What is the major difference between pyranoses and furanoses?
Pyranoses: Six-membered rings (e.g., glucose).
Furanoses: Five-membered rings (e.g., fructose).
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What are anomers?
Anomers are a special type of stereoisomer that differ in configuration at the anomeric carbon (C1) of a cyclic sugar.
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What is the anomeric carbon?
The anomeric carbon is the former carbonyl carbon (C1 in aldoses) that becomes a new chiral center after cyclization.
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What are the two anomers of D-glucose?
α-D-Glucopyranose: The OH at C1 is trans (down) to CH₂OH at C5.
β-D-Glucopyranose: The OH at C1 is cis (up) to CH₂OH at C5.
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Which anomer of D-glucose is more stable?
β-D-Glucopyranose (63%) is more stable than α-D-Glucopyranose (37%) because the OH at C1 is in the equatorial position, reducing steric hindrance.
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What happens to the open-chain form of glucose at equilibrium?
The open-chain form is present in less than 0.01% at equilibrium but is essential for interconversion between anomers (mutarotation).
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What is mutarotation?
Mutarotation is the spontaneous interconversion between α- and β-anomers via the open-chain form in solution.
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What type of reaction forms α- and β-D-glucopyranose?
A nucleophilic attack of the hydroxyl group on C5 to the carbonyl at C1 creates a hemiacetal, resulting in ring closure.
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How does equilibrium establish between the two anomers?
The open-chain form allows reversible conversion between α- and β-anomers in solution.
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Why does β-D-glucose predominate in solution?
The equatorial OH at C1 in β-D-glucose minimizes steric strain, making it more stable than the axial OH in α-D-glucose.
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What is the structural difference between α- and β-D-glucose in the Haworth projection?
α-D-glucose: The OH at C1 is down.
β-D-glucose: The OH at C1 is up.
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How do α- and β-anomers differ in structure?
They differ in the position of the OH group at the anomeric carbon (C1):
α-anomer → OH is down (trans) to CH₂OH.
β-anomer → OH is up (cis) to CH₂OH.
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Why is β-D-glucose more stable than α-D-glucose?
The OH at C1 is equatorial in the β-form, reducing steric hindrance.
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Why does the open-chain form exist in such a small amount?
The cyclic form is more thermodynamically stable than the open-chain aldehyde.
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What allows interconversion between anomers?
The presence of the open-chain form in solution enables mutarotation.
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What is specific rotation?
Specific rotation ([α]D) is the degree to which a compound rotates plane-polarized light.
It is a physical property unique to each stereoisomer.
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What is the specific rotation of α-D-glucopyranose?
[α]D = +112.2° when measured in pure form.
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What is the specific rotation of β-D-glucopyranose?
[α]D = +18.7° when measured in pure form.
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What happens to specific rotation when either anomer is dissolved in water?
Mutarotation occurs, and the specific rotation changes over time until equilibrium is reached.
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What is mutarotation?
Mutarotation is the spontaneous interconversion between α- and β-anomers in aqueous solution via the open-chain form.
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What causes mutarotation?
What causes mutarotation?
In solution, glucose exists in a dynamic equilibrium between:
α-D-glucopyranose (37%)
β-D-glucopyranose (63%)
Open-chain form (<0.01%)
The open-chain form allows the hydroxyl at C1 to freely rotate, interconverting the anomers.
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ow does mutarotation affect optical activity?
How does mutarotation affect optical activity?
The initial specific rotation of a pure anomer changes over time as the system reaches equilibrium.
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Why does β-D-glucose predominate in solution?
Why does β-D-glucose predominate in solution?
β-D-glucose (63%) is more stable because the OH at C1 is equatorial, reducing steric hindrance.
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What is the relationship between α- and β-D-glucopyranose?
What is the relationship between α- and β-D-glucopyranose?
They are diastereomers (stereoisomers that are not mirror images and differ at one chiral center, C1).
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What is the equilibrium composition of D-glucose anomers in water?
63% β-D-glucopyranose, 37% α-D-glucopyranose, <0.01% open-chain form.
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How does specific rotation confirm mutarotation?
If a pure anomer is dissolved, its initial specific rotation shifts over time toward +52.6°, proving equilibrium formation.
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Why does the open-chain form of glucose exist in such low amounts?
The cyclic hemiacetal is far more stable than the reactive open-chain aldehyde.
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How do you experimentally determine if mutarotation is occurring?
Measure the specific rotation at different time intervals and observe if it changes toward an equilibrium value
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What type of reaction allows glucose anomers to interconvert
Acid- or base-catalyzed hemiacetal opening and reformation, allowing rotation at C1.
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What is a Haworth projection?
A Haworth projection is a cyclic representation of sugars, showing their three-dimensional structure.
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What does the name "α-D-galactopyranose" indicate?
α: The OH at C1 (anomeric carbon) is trans (down) to CH₂OH.
D: The CH₂OH at C5 points up.
Galacto: The structure is based on D-galactose.
Pyranose: The sugar forms a six-membered ring (pyranose form).
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What is the convention for drawing pyranose rings?
Oxygen in the back-right position.
Numbering starts from the anomeric carbon (C1) clockwise.
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How is the CH₂OH group positioned in D-sugars?
In D-sugars, the CH₂OH group at C5 always points up in a Haworth projection.
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What does "alpha" (α) mean in α-D-galactopyranose?
The OH at C1 is trans (down) to the CH₂OH group at C5.
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What does "beta" (β) mean in β-D-galactopyranose?
The OH at C1 is cis (up) to the CH₂OH group at C5.
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How do you determine OH group positioning on C2, C3, and C4?
Any OH group on the right side of the Fischer projection is down in the Haworth projection.
Any OH group on the left side of the Fischer projection is up in the Haworth projection.
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What is the anomeric carbon in the cyclic form?
The C1 carbon in aldoses, which forms a hemiacetal bond.
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Why does D-galactose form a pyranose ring?
Six-membered rings (pyranoses) are more stable than five-membered rings (furanoses).
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Why is the CH₂OH group at C5 important in D-sugars?
It determines the D or L configuration:
D-sugars → CH₂OH points up.
L-sugars → CH₂OH points down.
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How do you determine the configuration of the OH groups in a Haworth projection?
Use the Fischer projection:
Right → Down in Haworth.
Left → Up in Haworth.
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What is the key difference between α- and β-anomers in a pyranose ring?
α-anomer: The OH at C1 is trans (down) to CH₂OH.
β-anomer: The OH at C1 is cis (up) to CH₂OH.
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How does the D/L configuration affect Haworth projections?
D-sugars → CH₂OH at C5 points up.
L-sugars → CH₂OH at C5 points down.
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How do you draw a Haworth projection for a pyranose?
Step 1: Draw a six-membered ring with oxygen in the top-right corner.
Step 2: Add the CH₂OH group at C5 (up for D-sugars, down for L-sugars).
Step 3: Determine if the OH at C1 (anomeric carbon) is up or down:
α-anomer → OH down.
β-anomer → OH up.
Step 4: Place remaining OH groups based on the Fischer projection:
Right in Fischer → Down in Haworth.
Left in Fischer → Up in Haworth.
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What is the difference between α- and β-pyranose forms?
α-form: The OH at C1 is trans (down) to CH₂OH.
β-form: The OH at C1 is cis (up) to CH₂OH
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How do you determine whether a sugar is a D- or L-sugar in the Haworth projection?
D-sugars: The CH₂OH group at C5 points up.
L-sugars: The CH₂OH group at C5 points down.
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What happens when D-mannose undergoes mutarotation?
Mutarotation interconverts α- and β-D-mannopyranose via the open-chain form.
The equilibrium mixture consists of both anomers and a tiny fraction of open-chain mannose
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What are the equilibrium percentages of α- and β-D-glucose in solution?
63% β-D-glucopyranose (more stable due to equatorial OH at C1).
37% α-D-glucopyranose.
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What happens when D-talose is dissolved in water?
D-talose exists in two pyranose forms (α and β) at equilibrium, similar to glucose.
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What is the general rule for drawing multiple pyranose forms?
If a sugar forms a pyranose ring, two anomers (α and β) exist in equilibrium.
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What does mutarotation confirm about sugar solutions?
Anomeric interconversion happens spontaneously in water.
The solution's specific rotation stabilizes at an intermediate equilibrium value
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Why does β-D-glucose predominate over α-D-glucose?
The equatorial OH at C1 in β-D-glucose reduces steric hindrance, making it more stable.
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What are the two possible pyranose forms of D-talose?
α-D-talopyranose
β-D-talopyranose
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What is a chair conformation?
A chair conformation is the most stable three-dimensional shape of a six-membered ring.
It minimizes steric strain and torsional strain.
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What determines the stability of a chair conformation?
The largest substituent should occupy an equatorial position to minimize steric hindrance.
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How do you draw a chair conformation for pyranose rings?
Step 1: Draw a six-membered ring with oxygen at the upper-right position.
Step 2: Label each carbon position (C1 to C5 for hexoses).
Step 3: Add substituents as UP or DOWN, based on the Haworth projection.
Step 4: Identify axial (vertical) and equatorial (slanted) positions.
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What is the difference between axial and equatorial positions?
Axial: Straight up or down; increases steric strain.
Equatorial: Angled slightly out; more stable.
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Why is the equatorial position preferred for large groups?
It reduces steric hindrance, making the conformation more stable.
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What happens when a chair flip occurs?
Axial and equatorial positions switch, but up/down orientation remains unchanged.
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What is the key factor in determining the more stable chair conformation?
The conformation where the largest groups (e.g., CH₂OH) are equatorial is the most stable.
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How does α-D-galactopyranose differ from β-D-galactopyranose in chair conformation?
In α-D-galactopyranose, the OH at C1 is axial (less stable).
In β-D-galactopyranose, the OH at C1 is equatorial (more stable).
200
What is the general trend for D-aldohexoses in chair conformations?
The most stable conformation places the CH₂OH group at C5 in the equatorial position.
201
Why is the chair conformation more stable than other forms like the boat conformation?
The boat conformation has steric clashes (1,3-diaxial interactions), making it less favorable.
202
How do you determine the most stable chair conformation for a pyranose ring?
The largest groups should occupy equatorial positions to minimize steric strain.
203
What happens when a chair flips?
Axial positions become equatorial, and equatorial positions become axial.
204
Why is β-D-glucose more stable than α-D-glucose?
In β-D-glucose, the OH at C1 is equatorial, reducing steric hindrance.
205
Why do sugars favor the chair conformation?
It is the lowest energy conformation, reducing strain and making the structure more stable.
206
What is the major factor influencing mutarotation equilibrium in solution?
The relative stability of α- and β-anomers, with β-anomers often predominating due to equatorial OH positioning.
207
What is the most stable chair conformation for β-D-galactopyranose?
The conformation where the CH₂OH group at C5 is equatorial.
The OH groups are arranged to minimize steric hindrance.
208
How does α-D-glucopyranose compare to β-D-glucopyranose in chair stability?
β-D-glucopyranose is more stable because its OH at C1 is equatorial.
α-D-glucopyranose is less stable because its OH at C1 is axial, increasing steric hindrance.
208
How does D-fructose form a furanose ring?
The carbonyl group reacts with the hydroxyl at C5, forming a five-membered ring.
209
What are the equilibrium concentrations of D-fructose forms in water?
70% β-pyranose (most stable).
2% α-pyranose.
23% β-furanose.
5% α-furanose.
0.7% open-chain form.
210
Why does β-pyranose predominate in D-fructose equilibrium?
Six-membered pyranose rings are generally more stable than five-membered furanose rings
211
Which furanose form of D-fructose is most important in biochemical pathways?
β-D-fructofuranose is commonly found in biological processes, despite β-D-fructopyranose being more stable.
212
What is the general rule for determining chair stability?
The most stable conformation places the largest groups in equatorial positions.
213
What happens when β-D-glucopyranose undergoes a chair flip?
All axial groups become equatorial, and vice versa.
This converts the more stable conformation into a less stable one.
214
How does the presence of furanose and pyranose forms affect sugar reactivity?
Furanose rings are more reactive due to increased ring strain.
Pyranose rings are more stable and less reactive.
215
Why does D-fructose participate in metabolic reactions as β-furanose rather than β-pyranose?
The enzyme recognition sites in metabolic pathways are specific for the furanose form.
215
How does D-fructose exist in aqueous solutions?
As a mixture of pyranose, furanose, and open-chain forms, with β-pyranose being the most stable.
216
Why is β-D-glucopyranose more stable than α-D-glucopyranose?
The OH at C1 is equatorial in β-form, reducing steric hindrance.
217
What happens when a pyranose undergoes mutarotation?
The anomeric carbon interconverts between α and β forms via the open-chain form.
218
What is the significance of furanose forms in metabolism?
They are more reactive and recognized by enzymes in biochemical pathways.
219
How do axial and equatorial positions affect stability in chair conformations?
Equatorial positions minimize steric hindrance, making the conformation more stable.
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