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Flashcards in SNS - Organic Chemistry - Carbohydrates Deck (13):
1


Carbohydrates

Compoinds containing C, H and O with general formula Cn(H2O)n

2


Monosaccharides

Examples

  1. Fructose
  2. Glucose
  3. Mannose
  4. Galactose

3


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

4


Monosaccharides

Epimers

Aldose diastereomers that differ only about the configuration of one carbon atom

5


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

6

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

7

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

8

Monosaccharides

Reactions

  1. Ester Formation
  2. Oxidation
  3. Glycosidic

9

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

10

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

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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

12

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

13

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

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