Carbohydrate Flashcards
Molecular formula of carbohydrates
(CH2O)n
What is an aldose
A carbohydrate with an aldehyde group
What is a ketose?
A carbohydrate with a ketone group
Trioses
Carbohydrates with the formula C3H6O3
Tetroses
Carbohydrates with the formula C4H8O4
Pentoses
Carbohydrates with the formula C5H10O5
Hexoses
Carbohydrates with the formula C6H12O6
Triose example
D-glyceraldehyde
Pentose example
Aldose
Ketose
Aldose:
D-Ribose
D-Arabinose
D-Xylose
Ketose:
D-Ribulose
Hexoses examples
Aldose
Ketose
Aldose:
D-glucose
D-Mannose
D-galactose
Ketose:
D-fructose
How to draw Fischer projections do work out L and D enantiomers
- Draw sugar in a vertical line of C’s with Carbonyl on top
- Draw side chain bonds imagining they are coming towards you
- Identify chiral centre
- Draw again but at the chiral centre(s) swap the position of the OH and H.
L = OH on left D = OH on Right
Do organisms prefer D or L carbohydrates ?
D carbohydrates
How many isoforms do aldoses (with the following amount of carbons) C3, C4, C5 and C6 have?
C3 = 2 C4 = 4 C5 = 8 C6 = 16
How many isoforms do ketoses with (the following amount of carbons) C3, C4, C5 and C6 have?
C3 = 1 C4 = 2 C5 = 4 C6 = 8
If an aldose or ketone has multiple chiral centres which one do you use to determine if it’s an L or D enantiomer?
The one furthest from the carbonyl
Define enantiomer
Mirror images of each other
Define diastereomers
Stereoisomers that are not mirror images
Same covalent bonding but spatially different
Define epimers
Two molecules that only differ at one stereo centre
Example of an epimer
D-arabinose and D-ribose
How many chiral centres do aldotrioses have?
2 chiral centres
Pyranose
6 membered ring
Furanose
5 membered ring
Why don’t we get four membered rings of sugars?
Unstable
By what process do sugar rings form?
Intramolecular cycliziation
What happens in the cyclisation of aldoses?
The secondary hydroxyl group on the aldose reacts with the carbon in it’s carbonyl, closing the ring and forming a hemiacetal linkage.
Why is the secondary hydroxyl preferable in the cyclisation of aldoses?
It is more reactive
Is cyclisation reversible or irreversible?
Reversible
What is a hemiacetal linkage?
A carbon connected to an oxygen in an -OH (alcohol) and an oxygen in an ether (-O-R)
What type of linkage is formed in the cyclisation of aldoses?
Hemiacetal
What type of linkage is formed in the cyclisation of ketoses?
Hemiketal
What is an anomeric carbon?
Carbon in the hemiacetal/hemiketal bond which is derived form the carbonyl (C=O)
What are two possible conformations that pyranose can be?
Chair
Boar
Chair conformation is
Energetically favourable
Less steric hindrance between equatorial groups
What conformation are six membered rings usually in?
Chair
Why is there a higher % beta version of pyranose rings formed than alpha?
Hydroxyl group sterically clash when in the axial position; this is chemically unfavourable. Hence the a higher % beta version is formed because in beta the hydroxyl group at the anomeric carbon is in an equatorial position where there is no clashing or steric hindrance.
What type of conformation do 5 membered rings have?
Envelope
What does the cyclisation of fructose produce?
A 6 membered ring caused by reaction of primary hydroxyl with carbonyl of the ketose.
(Usually the secondary hydroxyl reacts with the carbonyl of the ketose to form a five membered ring however in the case of fructose the formation of a six membered ring is more stable)
