Carbohydrates

Question 1

chemical properties of monosaccharides

Answer 1

• very water soluble
• poorly soluble in organic solvents
• colourless
  formula: (CH2O)n

Question 2

Monosaccharides

Answer 2

combine a carbonyl group which is either an aldehyde or ketone AND at least 2 carbons bearing hydroxyl groups (alcohol)
- can contain 3-7 carbon atoms
- hexoses are the most common

Question 3

Emil Fischer

Answer 3

studied analysis, synthesis and stereochemistry of simple sugars and developed Fischer projections

Question 4

Fischer projection and perspective formulas

Answer 4

Fischer projection: vertical bonds project behind the plane horizontal bonds to project out of the plane
Perspective formula: solid wedge shaped bonds project in front, dashed bonds point away

Question 5

asymmetric carbon atoms

Answer 5

• all monosaccharides except dihydroxyacetone contain one or more chiral carbon atoms
• gives rise to occurrence of optically active isomeric forms

Question 6

enantiomers

Answer 6

• mirror images, differ in configuration at every chiral carbon
• “left-handed” or “right-handed”
• have identical chemical properties
• differ in optical activity: plane of polarization of polarized light is bent in opposite directions

Question 7

diasteromers

Answer 7

• monosaccharides with more than one chiral carbon that differ in handedness at only some carbons
• do not have identical properties because of different spatial relationships

Question 8

D-sugars

Answer 8

chiral carbon furthest away from the carbonyl group has the same configuration as D-glyceraldehyde (OH on right)
- most common in nature

Question 9

L-sugars

Answer 9

chiral carbon furthest away from the carbonyl group has the same configuration as L-glyceraldehyde (OH on left)

Question 10

Epimers

Answer 10

pair of sugars that are identical except for the configuration at one carbon atom
- special case of diastereomers

Question 11

how many stereoisomers will a sugar have?

Answer 11

2^n
n= number of chiral centres
- every time a carbon is added the number of isomers doubles

Question 12

Hemiacetals ands Hemiketals

Answer 12

hemiacetal: aldehyde + alcohol
hemiketal: ketone + alcohol
- the original carbonyl carbon becomes chiral upon formation

Question 13

cyclization of sugars - basic info

Answer 13

• sugars have BOTH alcohol and aldehyde or ketone functional groups
• hemiacetal or hemiketal formation is intramolecular
• hemiacetal and hemiketal formations turn into ring structures

Question 14

cyclization of glucose

Answer 14

• the OH group at C5 reacts with the carbonyl carbon of the aldehyde group to form a stable ring
• this renders C1 asymmetric, giving rise to alpha and beta forms

Question 15

anomers

Answer 15

isomeric form of a molecule that differs only in configuration around hemiacetal or hemiketal carbon
- alpha and beta forms

Question 16

cyclization chemistry for a ketose

Answer 16

• same as for an aldose, except the OH group of C5(furanose) or C6(pyranose) reacts with the ketone group (usually C2)
• the electrophilic carbonyl carbon reacts with the nucleophilic O of an OH group

Question 17

mutarotation

Answer 17

• when dissolved in water a sugar slowly converts into an equilibrium mixture of the alpha, beta and linear forms
• a solution of alpha glucose and one of beta glucose will form identical equilibrium mixtures over time (1/3 alpha, 2/3 beta and trace amounts of linear)

Question 18

what forms can sugars take on when they cyclize

Answer 18

pyranose: 6-membered
furanose: 5-membered
- size of the ring formed depends on relative thermodynamic stabilities of possible ring structures

Question 19

Haworth projections

Answer 19

for D sugars the anomeric OH is above in the beta form and below in the alpha form
- OH groups that are left in the fisher formula are above in Haworth

Question 20

converting Fischer structures to Haworth projections

Answer 20

• carbonyl carbon is always the electrophile in the reaction
• any OH group can act as the nucleophile
• OH group that reacts depends on if the ring is pyranose or furanose
• fructose can cyclize to a pyranose or furanose
• if the OH of C6 is the nucleophile then fructose will be a 6 membered ring

Question 21

reducing sugars

Answer 21

carbonyl carbon can be oxidized to a carboxyl group by oxidizing agents (ex: Cu2+)
- Cu2+ gets reduced to Cu+ which forms red precipitate
- sugars that react like this are known as reducing sugars

Question 22

non-reducing sugars

Answer 22

sugars that can’t be oxidized by oxidants such as Cu2+
- lack a free aldehyde or ketone group

Question 23

conditions for sugars to be oxidized by Cu2+

Answer 23

can only occur in the linear form (with a carbonyl carbon; the aldehyde or ketone form) which exists in equilibrium with cyclic forms

Question 24

glycosides

Answer 24

the anomeric carbon of a sugar is electrophilic - its at this position most reactions involving ring sugars take place
- condensation of the anomeric carbon with the nucleophilic OH or NH is the most important sugar rxn
- resulting molecule is a glycoside
- bond formed is glycosidic (C-O) or glycosilic (C-N)

Question 25

What happens if the anomeric carbon is involved in a glycosidic bond?

Answer 25

- it becomes a non-reducing sugar - if the anomeric carbon is in a glycosidic bond the sugar can no longer open up from ring to linear form - since only open chain forms can undergo oxidation by Cu2+ the sugar becomes non-reducing - when dissolved remains in ring structure

Question 26

disaccharides

Answer 26

- formed when 2 monosaccharides are linked through a glycosidic bond - the aromatic carbon (electrophile) of one sugar reacts with the hydroxyl group (nucleophile) of another - disaccharides are glycosides

Question 27

naming disaccharides

Answer 27

state configuration the sugar with the anomeric carbon reacting followed by an arrow and which C the hydroxyl group of the other sugar reacting is on Lactose: gal(B1->4)glc

Question 28

reducing and non-reducing disaccharides

Answer 28

- the end of a chain with a free anomeric carbon is called the reducing end - anomeric carbon of galactose is unable to undergo maturation since it is in a glycosidic bond - the anomeric carbon of glucose is free and able to undergo maturation - lactose is a reducing sugar

Question 29

basic info on disaccharides

Answer 29

- formation of a disaccharide must involve the anomeric C of one sugar as its the only electrophilic carbon - the OH on any carbon of the sugar could be the nucleophile - galactose and glucose could form lots of disaccharides that would be structural isomers of lactose

Question 30

sucrose

Answer 30

- disaccharide of glucose and fructose - anomeric carbons of both glucose and fructose are involved in the glycosidic bond, therefore is non-reducing (no reducing end) -

Question 31

2 of many glucose disaccharides

Answer 31

maltose: glc(a1->4)glc trehalose: glc(a1<->a1)glc - maltose has a reducing end, trehalose does not

Question 32

polysaccharides

Answer 32

- often highly branched - branched since sugars have many OH groups that can act as a nucleophile in forming glycosidic bonds - differ from each other in types of sugars that are linked, length of the chains, type of bonds and the degree of branching

Question 33

chiral carbon centers

Answer 33

carbon atoms that are attached to 4 different substituents - not involved in double bonds

Question 34

anomeric carbon

Answer 34

- the carbon of a cyclic sugar bearing a hemiacetal or hemiketal - C1 in aldoses C2 in ketoses

Question 35

lactose

Answer 35

- disaccharide of galactose and glucose gal(B1->4)glc - able to act as a reducing sugar because it has a free reducing end