W10 Carbohydrates

Question 1

Define carbohydrates

Answer 1

• Polyhydroxyl aldehydes and ketones
• General formula approx. C_n(H₂O)_m
• Energy storage material that allows plants to convert sunlight to energy

Question 2

Nomenclature of carbohydrates

Answer 2

1. Aldehyde or ketone (aldo or keto)
2. Number of carbons (tri-, tetra-, penta-)
3. If saccharide, will have -ose as a suffix
4. D- or L- depending on stereochemistry of highest numbered stereogenic carbon

Question 3

Number of enantiomers that a single carbohydrate has

Answer 3

A single carbohydrate will have 2ⁿ isomers where n is the no. chiral centres

Question 4

Fischer projections

and how to draw

Answer 4

• Allows us to determine if carbohydrate is D- or L-

1. Most oxidised carbon needs to be at the top
   
   1. Aldehyde
2. Configurational centre is highest numbered chiral centre (with counting starting from oxidised end)
3. Draw a cross of groups around the central carbon
   
   1. Horizontal line represents bonds coming out of page (will generally be H and OH)
   2. Vertical line represents bonds going into page

Note: might need to do a rotation around C-C bond to get the required orientation

1. D- have hydroxyl to the right of highest numbered chiral centre (2ND LAST CARBON)

L- has to the left

Question 5

Cyclisation of aldoses

Answer 5

• Can undergo nucleophilic addition reaction with hydroxyl group of 5th carbon (highest numbered chiral centre) to form a 6 membered cyclic hemiacetal
• Creates a new stereocentre: the anomeric centre
  
  • α = axial anomeric centre (down on Haworth projection)
  • β = equatorial anomeric centre (up on Haworth projection)
• Note that for cyclic glucose (called D-glucopyranose) all hydroxyls are equatorial -> can use to figure out positions for other sugars

Question 6

Using rings to name cyclic carbohydrates

Answer 6

• hexagonal ring: [carbo]pyran[ose]
• pentagonal ring: [carbo]furan[ose]

Both have O in the cyclic disk and double bonds

Question 7

Mutarotation

Answer 7

Occurs when two anomers are possible (because anomeric centre has a hydroxyl attached)

Each anomer has its own optical rotation

This leads to a change in optical rotation with time -> mutarotation

Question 8

Haworth projections

Answer 8

Molecule is squished so constituents of ring are either pointing up or down

(conformational isomerisation is not displayed

• α anomers have anomeric hydroxyl down (axially)
• β anonmers have anomeric hydoxyl pointing up (equatorially)

For glucose

• All OHs are in equatorial position
• Except for anomeric carbon which could be axial or equatorial
• Can use to decide positions of other isomers of glucose (see what side has switched in fisher projection)

Question 9

Reduction of carbohydrates

Answer 9

Converts carbonyl group to alcohol

Requires NaBH₄/LiAlH₄ or H₂ and catalyst

As ring forms are in equilibrium with open-chain forms, they too are readily reduced

Question 10

Oxidation of carbohydriates - practical purpose

Answer 10

Oxidation of carbohydate is used as a test for the presence of an aldehyde or ketone group

primary alcohol -> aldehyde

secondary alcohol -> ketone

Question 11

Reducing sugars

Answer 11

• Sugars that can oxidise
• MUST have a hydroxyl on anomeric carbon (1st carbon) -> means they can mutarotate
• Aldehyde oxidises to a carboxylic acid (in presence of oxidising agent)
• Tautomers (enol forms) of ketone allow it to oxidise to carboxylic acid by first becoming an aldehyde

Question 12

What are glycosides

How are they formed

How can we test for glycosides

Answer 12

Glycosides are acetals of carbohydrates

Cyclic form of carbohydrate (hemiacetal) is converted to acetal using alchol and acid catalyst

Acetals don’t mutarotate -> aren’t reducing sugars -> Don’t react with Tollen’s reagent or Fehling’s solution

Question 13

Formation of hemiacetal from glycoside

Answer 13