Carbohydrates

Carbohydrates have the following basic composition: the formula of carbohydrates Cn(H2O)n can be expressed as hydrates of carbon Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. Disaccharides - 2 monosaccharides covalently linked. Oligosaccharides - a few monosaccharides ( three to ten monosaccharides)covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide (ten or more monosaccharides) or disaccharide units.

According to nature of carbonyl group: Carbohydrates are described as aldoses if they have aldehydic group at C-1 and described as ketoses if they have ketonic group on C-2 According to the number of carbon atoms in a given aldose or ketose: If a sugar is made of three atoms it is described as triose and if it has four atoms it is tetrose. The full name of a sugar must has a prefix indicating the nature of the carbonyl group, a Latin word indicating the number of carbons present and a suffix –ose.

Fischer projection and D and L notation According to the configuration of the hydroxyl group attached to the last chiral carbon atom in a given aldose or ketose drawn as fischer projection Fischer projection and D and L notation For carbohydrates the convention is to put the carbonyl group at the top for aldoses and closest to the top for ketoses, then the carbon atoms are numbered from the top to the bottom. The simplest aldose is glyceraldehyde (aldotriose), it is the simplest chiral molecule in nature with one stereogenic centre thus it can be represented as a pair of enantiomers. L- Glyecraldehyde D- Glyecraldehyde As shown in the above the isomer of glyeraldehyde with the OH group on the chiral carbon is pointing to the right is described to have D configuration (for dextro- because it rotates the plan polarized light to right (+)) while the isomer with the OH pointing to left it is assigned the L-configuration (for laevo-because it was (-)-enantiomer)

Aldoses (e.g., glucose) have an aldehyde group at C-1 When there is more than one chiral centre in a carbohydrate, look at the chiral carbon farthest from the carbonyl group in Fischer projection and assign its configuration as D or L as described Aldoses (e.g., glucose) have an aldehyde group at C-1 Ketoses (e.g., fructose) have a keto group, usually at C-2.

Hemiacetal & hemiketal formation An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal.

CONVENTIONS For WRITING CYCLIC MONOSACCHRIDE STRUCTURES React the OH group at the last chiral carbon (C-5 ) in the Fischer projection withthe carbonyl group CHO i.e. in aldolses. This hydroxyl will become hemiacetal (anomeric carbon C-1 has the OH at the right in the form α form and at the left in the β form for the D-monosaccharides (i.e. in D-glucose).

Cyclic Structure of Monosaccharides Hemiketal Formation React the OH group at the last chiral carbon (C-5 ) in the Fischer projection with the carbonyl group (ketonic group in ketoses This hydroxyl will become hemiketal (anomeric carbon C-2 has the OH at the right in the form α form and at the left in the β form for the D-monosaccharides (i.e. in D-Fructose).

Reactions of Monosaccharides 1) Reduction of Monosaccharides The carbonyl group of aldoses and ketoses can be reduced by various reagents to give polyols. Example, catalytic hydrogenation or reduction with sodium borohydride (NaBH4) converts D-glucose to D-glucitol (sorbitol).

2) Oxidation of Monosaccharides Aldoses can be oxidized easily but the product of oxidation depends on the oxidizing agent used. Although ordinary ketones resist oxidation, ketoses can be oxidised specially in the basic medium due to they will isomerize to aldoses thus being oxidized easily. Oxidation by Tollen’s reagent Tollen’s reagent can be used to differentiate between simple aldehydes and ketones where simple aldehydes give +ve result (sliver mirror due to reduction of Ag+1 ions in the reagent by the aldehyde to Ag0) while ketones give –ve result. However this reagent can not be used to differentiate between aldoses and ketoses since both of them give +ve result this due to ketoses isomerize to aldoses by this reagent which is basic in nature. Sugars which give +ve results with Tollen’s called reducing sugars.

2.1. With Mild Oxidizing Agents These aldehyde groups can be easily oxidized to acids which are called aldonic acids. For example, D-glucose is easily oxidized to D-gluconic acid. The oxidation of aldoses is so easy that they react with such mild oxidizing agents as Tollens’ reagent (Ag+ in aqueous ammonia), Fehling’s reagent (Cu2+ complexed with tartrate ion), or Benedict’s reagent (Cu2+ complexed with citrate ion).

2) Oxidation of Monosaccharides 2.2. With Strong Oxidizing Agents Stronger oxidizing agents, such as aqueous nitric acid, oxidize the aldehyde group and the primary alcohol group, producing dicarboxylic acids called aldaric acids. For example, D-glucose gives D-glucaric acid.