SNS - Organic Chemistry - Carbohydrates Flashcards
Carbohydrates
Compoinds containing C, H and O with general formula Cn(H2O)n
Monosaccharides
Examples
- Fructose
- Glucose
- Mannose
- Galactose
Monosaccharides
L and D sugars
Monosaccharides are assigned L or D configuration depending on their relationship to glyceraldehyde
If the lowest -OH is on the left, is an L sugar, if on the right, is a D sugar
Monosaccharides
Epimers
Aldose diastereomers that differ only about the configuration of one carbon atom
Monosaccharides
Ring Properties
- As contain both -OH and C=O groups, can undergo intramolecular reactions to form cyclic hemiacetals or hemiaketals
- These are stable in solution and may exist as 6-membered pyranose rings (eg glucose) or 5-membered furanose rings.
- Like cyclohexane, pyranose rings adopt chair conformation and the substituents adopt axial or equatorial positions to minimise steric hindrance
- When converting from straight-chain Fischer projection to Haworth projection, any group on the right of the Fischer will be pointing down and vice versa
Monosaccharides
Ring Properties
Anomers
When a straight chain monosaccharide is converted to its cyclic form, C=O carbon becomes chiral
Cyclic steroisomers differing about the new chiral carbon called anomers
Alpha anomers have the C1 -OH trans or axial to the CH2OH substituent, and for Beta this is cis
Monosaccharides
Ring Properties
Mutarotation
When exposed to water, hemiactal rings spontaneously open and reform
Due to bond rotation between C1 and C2 either the alpha or beta anomer may be formed - called mutarotation - and results in a mixture containing both anomers in their equilibrium concentrations
The alpha configuration is less favoured as the C1 -OH is axial making the molecule more sterically strained
The reaction is more rapid when catalysed by an acid or base
Monosaccharides
Reactions
- Ester Formation
- Oxidation
- Glycosidic
Monosaccharides
Reactions
Ester Formation
Monosacchrides contain -OH groups and can undergo many of the same reactions as simple alcohols
Therefore may be converted to esters or ethers
In the presence of acid anhydride and base, all -OH groups will be esterified
Monosaccharides
Reactions
Oxidation
As they switch between anomeric configurations, hemiacetal rings spend a lot of time in the open chain aldehyde form
Like all aldehydes, can be oxidised to carboxylic acids called aldonic acids, thus the aldoses are reducing agents
Any monosaccharide with a hemiacetal ring is considered a reducing sugar and can be oxidised, for example by Benedicts or Tollens reagent
Ketose sugars are also reducing sugars as can isomerise to aldoses via keto-enol shifts
Monosaccharides
Reactions
Glycosidic
Hemiacetal monosaccharides will react with alcohols under acidic conditions
The anomeric -OH is transformed into an alkoxy group to yield a mixture of the alpha and beta acetals
The resulting bond is called a glycosidic linkage and the acetal is known as a glycoside.
An example is the reaction of glucose with ethanol
Disaccharides
A monosaccharide may react with alcohols to yield acetals. When the alcohol is another monosaccharide, generates a disaccharide
The most common glycosidic linkage occurs between C1 of the first sugar and C4 of the second and is designated a 1,4’ link
1,6’ and 1,2’ bonds are also observed
The glycosidic bonds may be either alpha or beta depending on the orientation of the -OH on the anomeric carbon
These glycosidic bonds are ofter cleaved in the presence of aquaeous acid
Polysaccharides
Formed via linkage of monosaccharide units via glycosidic bonds
Most important biological examples are cellulose, glycogen and starch
Cellulose is comprised of D-glucoes linked via 1,4 bonds
Starch and glycogen are energy stores in plants and animals respectively and are formed from glucose unit linked via 1,4 and 1,6 bonds, the latter creating branches
