sign in sign up
<<
Home , Aldose, Ketose, Pentose, Tetrose, Threose

Carbohydrates

Typical formula: Cx(H2O)y, eg : C6H12O6. Structure —

Simplest are ; one unit. Monosaccharides can combine to form di-, tri-, . These more complex carbohydrates can be hydrolyzed to give their constituent monosaccharides.

Monosaccharides are usually, at least partly, polyhydroxy or .

Saccharides that are at least partly polyhydroxy aldehydes or ketones are reducing .

D-glucose is a and is the most abundant organic compound in free and combined form.

1 H O O H C C H C OH HO C H HO C H H C O H H O H C OH C HO C H H C OH H C OH HO C H

CH2OH CH2OH CH2OH D-glucose D- L-glucose

The chiral center that determines D- or L- is the one furtherest from the carbonyl. Most naturally occurring sugars are in the D- family.

The family name for sugars has the suffix -ose. Monosaccharides can be characterized as to how many they have: , , , . They can also be characterized as to whether they are aldehydes, , or ketones, . By combination and (below) are aldotetroses.

2 H O O H H O O H C C C C H C OH HO C H HO C H H C O H H C OH HO C H H C O H HO C H

CH2O H CH2OH CH2OH CH2OH D-erythrose L-erythrose D-threose L-threose

Aldohexoses have 4 chiral centers. Meso forms are impossible, so there are 24 = 16 steroisomeric aldohexoses: 8 D,L-pairs, of which the are one.

3 Aldehydes and some ketones react with alcohols to form –

O OH H+ R1 C H(R2) + H OR3 R1 C H(R2)

alcohol OR3 or

R1, R2, and R3 are different R groups; the subscripts do not indicate how many groups.

Aldoses and may form internal hemiacetals (the carbonyl reacts with an -OH in the same molecule), resulting in a ring. If the ring is 5-membered, it is called a ; if it is 6-membered, it is called a . [Note that when the ring is formed, the former carbonyl becomes tetrahedral and a chiral center.]

Glucose forms two when dissolved in water; they differ from each other in configuration around the former carbonyl carbon.

4 H CH2OH 0.2% aldose OH HO H H HO C H OH H O

H H CH OH CH2OH 2 O HO O HO H H OH H HO H HO C H C OH OH H OH H H a-D-glucose, + 113o b-D-glucose, + 19o

99.8% pyranose mixture: + 52.5o Based on the specific rotation of the mixture (assuming no contribution from the aldose form) this means the D-glucose is 64% in b from and 36% in a form.

5 , a ketohexose, forms furanose hemiacetals.

HOCH2 OH CH2OH C H OH O H

OH H ketose O O HOCH2 CH2OH HOCH2 OH C C H OH H OH H OH H CH2OH

OH H OH H a-D-fructose b-D-fructose furanose form, a hemiacetal furanose form, a hemiacetal

6 —

Acetal formation:

OH OR4 H+ R1 C H(R2) + R4OH R1 C H(R2) + H2O

OR3 OR3 hemiacetal acetal

Acetals are stable in neutral, basic solution.

An alkyl a-D- (general term: ) H H CH 2OH CH 2OH O O HO H HO H H H HO C H HO C H OH OH H OH + ROH H OR + HOH

A glycoside (acetal) does not mutarotate in neutral or basic solution (unless R, itself, is a hemiacetal that can open to an aldose or ketose).

A sugar which reduces Tollen’s reagent or Benedict’s solution is called a . It is the aldehyde or a-hydroxyketone group that is oxidized. Therefore, glycosides are not reducing sugars (unless R, itself, is a hemiacetal that can open to an aldose or ketose.)

7 If the alcohol that reacts with the monosaccharide is itself a monosaccharide the glycoside that results is a . Attachment of additional monosaccharide units results in tri-, tetra-, ..., polysaccharides.

Sucrose is a disaccharide: a-glucose is linked to b- fructose.

H CH2OH O HO H HO H H C OH H a link of glucose O O HOCH2 b link of fructose C H OH H CH2OH

OH H

8 : a-glucose .

H CH2OH O HO H H HO C H OH H CH2OH H O O H H HO C H a-glucose OH H CH2OH H O O n H H HO C H n = ~1,000 OH H OH

H CH2OH O O H H HO C H OH H CH2OH H O O H H a-glucose HO C H H OH CH2OH H O O O H H HO C H OH H CH2 H O O H H HO C H ~ 1,000,000 glucose OH units per molecule H O

9 Celluose: b-glucose . H CH2OH O H O H CH2OH O H HO H O H CH2OH C O OH HO H O H C H H OH H HO C O H H OH n H H b-glucose

10