JEE Chemistry Biomolecules Notes Set 01

Carbohydrates: Definition and Classification

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. Many fit the formula C_x(H₂O)_y, but the formula is not a test: rhamnose, C₆H₁₂O₅, is a carbohydrate and acetic acid, C₂H₄O₂, is not.

Sugars are the sweet, water-soluble carbohydrates (mono- and oligosaccharides). A reducing sugar has a free hemiacetal or hemiketal group and reduces Tollens' reagent and Fehling's solution. A non-reducing sugar has no such group.

Glucose

Glucose, C₆H₁₂O₆, is an aldohexose. Its natural form is D-(+)-glucose. In the Fischer projection (CHO at the top) the OH groups on C-2 to C-5 point right, left, right, right. The D or L label depends only on the OH on the last stereocentre, C-5: it is on the right for D. D and L do not tell the direction of optical rotation; (+) and (-) do.

1. Evidence for the open chain: gives an oxime with hydroxylamine and a cyanohydrin with HCN (carbonyl group), forms a pentaacetate (five OH groups), is oxidised by bromine water to gluconic acid (aldehyde) and by nitric acid to saccharic acid (aldehyde and terminal CH₂OH), and gives n-hexane with HI (six carbons in a straight chain).
2. Evidence that the open chain is not the whole story: glucose does not give Schiff's test, does not react with NaHSO₃ and the pentaacetate does not react with hydroxylamine. Also, it shows mutarotation and exists in two crystalline forms. All of this is explained by a cyclic hemiacetal structure.
3. Ring formation: the OH on C-5 adds to the C-1 aldehyde to give a six-membered pyranose ring. This creates a new stereocentre at C-1, the anomeric carbon, so two anomers form. Alpha-D-glucose has the C-1 OH on the opposite side to the CH₂OH group in the Haworth drawing (down), beta has it on the same side (up).
4. Specific rotations: alpha form +111 degrees (melting point 419 K), beta form +19.2 degrees (melting point 423 K). In water the two interconvert through the open chain until the equilibrium value of +52.5 degrees is reached. This change in rotation with time is mutarotation.

Fructose

Fructose is a ketohexose, laevorotatory (-92.4 degrees) and the sweetest sugar. In the Fischer projection C-2 is the keto group, and the OH groups on C-3, C-4 and C-5 point left, right, right. In the free state it exists as a six-membered pyranose ring, and in sucrose as a five-membered furanose ring. It reduces Tollens' reagent although it is a ketose, because in basic medium it rearranges through an enediol to glucose and mannose.

Reactions Worth Remembering

Disaccharides

Polysaccharides

1. Starch: stored by plants as a mixture of 15 to 20 percent amylose (unbranched, alpha-1,4 links, water soluble, gives blue colour with iodine) and 80 to 85 percent amylopectin (branched with alpha-1,6 links at branch points, insoluble).
2. Cellulose: unbranched chains of beta-D-glucose joined by beta-1,4 links. It is the main structural material of plant cell walls. Humans cannot digest it because they lack the enzyme for beta links.
3. Glycogen: the energy store of animals, in liver and muscles. Its structure is like amylopectin but more highly branched.
4. None of these reduces Tollens' reagent in a practical test, and none is sweet.

Importance

Carbohydrates supply energy (glucose is the fuel of cells), form the structure of plants (cellulose) and make part of nucleic acids (ribose and deoxyribose). Honey is mostly invert sugar. Cellulose is used for paper, cotton and rayon.