Carbohydrate

Question 1

Molecular formula of carbohydrates

Answer 1

(CH2O)n

Question 2

What is an aldose

Answer 2

A carbohydrate with an aldehyde group

Question 3

What is a ketose?

Answer 3

A carbohydrate with a ketone group

Question 4

Trioses

Answer 4

Carbohydrates with the formula C3H6O3

Question 5

Tetroses

Answer 5

Carbohydrates with the formula C4H8O4

Question 6

Pentoses

Answer 6

Carbohydrates with the formula C5H10O5

Question 7

Hexoses

Answer 7

Carbohydrates with the formula C6H12O6

Question 8

Triose example

Answer 8

D-glyceraldehyde

Question 9

Pentose example

Aldose
Ketose

Answer 9

Aldose:
D-Ribose
D-Arabinose
D-Xylose

Ketose:
D-Ribulose

Question 10

Hexoses examples

Aldose
Ketose

Answer 10

Aldose:
D-glucose
D-Mannose
D-galactose

Ketose:
D-fructose

Question 11

How to draw Fischer projections do work out L and D enantiomers

Answer 11

1. Draw sugar in a vertical line of C’s with Carbonyl on top
2. Draw side chain bonds imagining they are coming towards you
3. Identify chiral centre
4. Draw again but at the chiral centre(s) swap the position of the OH and H.

L = OH on left
D = OH on Right

Question 12

Do organisms prefer D or L carbohydrates ?

Answer 12

D carbohydrates

Question 13

How many isoforms do aldoses (with the following amount of carbons) C3, C4, C5 and C6 have?

Answer 13

C3 = 2
C4 = 4
C5 = 8
C6 = 16

Question 14

How many isoforms do ketoses with (the following amount of carbons) C3, C4, C5 and C6 have?

Answer 14

C3 = 1
C4 = 2
C5 = 4
C6 = 8

Question 15

If an aldose or ketone has multiple chiral centres which one do you use to determine if it’s an L or D enantiomer?

Answer 15

The one furthest from the carbonyl

Question 16

Define enantiomer

Answer 16

Mirror images of each other

Question 17

Define diastereomers

Answer 17

Stereoisomers that are not mirror images

Same covalent bonding but spatially different

Question 18

Define epimers

Answer 18

Two molecules that only differ at one stereo centre

Question 19

Example of an epimer

Answer 19

D-arabinose and D-ribose

Question 20

How many chiral centres do aldotrioses have?

Answer 20

2 chiral centres

Question 21

Pyranose

Answer 21

6 membered ring

Question 22

Furanose

Answer 22

5 membered ring

Question 23

Why don’t we get four membered rings of sugars?

Answer 23

Unstable

Question 24

By what process do sugar rings form?

Answer 24

Intramolecular cycliziation

Question 25

What happens in the cyclisation of aldoses?

Answer 25

The secondary hydroxyl group on the aldose reacts with the carbon in it’s carbonyl, closing the ring and forming a hemiacetal linkage.

Question 26

Why is the secondary hydroxyl preferable in the cyclisation of aldoses?

Answer 26

It is more reactive

Question 27

Is cyclisation reversible or irreversible?

Answer 27

Reversible

Question 28

What is a hemiacetal linkage?

Answer 28

A carbon connected to an oxygen in an -OH (alcohol) and an oxygen in an ether (-O-R)

Question 29

What type of linkage is formed in the cyclisation of aldoses?

Answer 29

Hemiacetal

Question 30

What type of linkage is formed in the cyclisation of ketoses?

Answer 30

Hemiketal

Question 31

What is an anomeric carbon?

Answer 31

Carbon in the hemiacetal/hemiketal bond which is derived form the carbonyl (C=O)

Question 32

What are two possible conformations that pyranose can be?

Answer 32

Chair

Boar

Question 33

Chair conformation is

Answer 33

Energetically favourable

Less steric hindrance between equatorial groups

Question 34

What conformation are six membered rings usually in?

Answer 34

Chair

Question 35

Why is there a higher % beta version of pyranose rings formed than alpha?

Answer 35

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.

Question 36

What type of conformation do 5 membered rings have?

Answer 36

Envelope

Question 37

What does the cyclisation of fructose produce?

Answer 37

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)