Sugars Flashcards
What are two types of carbohydrates?
Simple - monosaccharide
Complex - disaccharide and polysaccharides
What is a monosaccharide?
A simple form of carbohydrate
Made up of a single carbon chain (3-6 carbons long)
They contain either a ketone (ketose) or aldehyde (aldose) functional group
They contain one or more hydroxyl groups
How do you name carbohydrates?
Aldo- or keto- depending on functional group
- tri-, -tetra-, -pent-, -hex- depending on number of carbons
- ose to indicate that its a carbohydrate
How do you form enantiomers of a carbohydrate?
For a pair of enantiomers to exist, all of the stereogenic centers of the two enantiomers must have opposite stereochemistry (R or S)
How do you calculate the number of stereoisomers possible from the number of stereocenters?
No. of stereoisomers = 2^n
where n is the number of stereocenters
What does D/L indicate?
Notation used for sugars indicating the direction of rotation of plane polarised light
D - clockwise
L - anticlockwise
How do you determine whether a sugar is D or L?
Go to the stereogenic center furthest from the carbonyl group
If the stereochemistry is R -> D, S -> L
How do you form a cyclic hemiacetal?
Nucleophilic addition occurs usually between the alcohol group on the 5th or 6th carbon and the carbonyl group due to their relative stability
What effect does formation of cyclic hemiacetals from open chain sugars have on stereochemistry?
All carbons have the same stereochemistry except C-1 - the carbon across which nucleophilic addition occurs (as it is sp2 hybridised)
What are anomers?
A type of diastereomer in which the stereochemistry of a pair of compounds only differs at a single carbon -> anomers form when nucleophilic addition of an open chain occurs to form cyclic hemiacetal
How are anomers named?
The relative positions of -OH on C1 and -CH2OH on C5 influence naming
alpha-anomer = trans beta-anomer = cis
What is mutorotation?
When a monosaccharide sugar is left in a solution, it will form an equilibrium between its open-chain form and its two anomers. This means the sugars in the solution will have a specific rotation that is different to the rotation of the individual anomers
This scrambling of anomeric stereocenter in a solution is called mutorotation
What is a glycoside?
The acetal form a sugar
How are glycosides formed?
The hemiacetal is reacted with an alcohol with an acid catalyst forming the acetal. (nucleo sub.)
Do glycosides exhibit mutorotation?
No, glycosides are stable in water in their cyclic acetal form and therefore will not form an equilibrium
