Chapter 6: Sugars – Structure and Chemistry of Monosaccharides

Functional Groups in Monosaccharides

Alcohols in Monosaccharides

Monosaccharides are simple carbohydrates that contain several functional groups, including alcohols. Alcohols are organic compounds characterized by the presence of a hydroxyl group (-OH) attached to a carbon atom. The classification of alcohols depends on the number of alkyl groups attached to the carbon bearing the hydroxyl group.

• Primary (1°) alcohol: One alkyl group attached to the alcoholic carbon.

• Secondary (2°) alcohol: Two alkyl groups attached.

• Tertiary (3°) alcohol: Three alkyl groups attached.

• Monosaccharides contain both primary and secondary alcohols.

Example: Ethanol, a simple alcohol, is produced by fermentation of sugars and is present in alcoholic beverages and fuel blends.

Aldehydes in Monosaccharides

Aldehydes are organic compounds with a carbonyl group (C=O) bonded to a hydrogen atom and an alkyl or aromatic group. In monosaccharides, the presence of an aldehyde group at the end of the molecule classifies the sugar as an aldose.

• Structure: R-CHO, where R is an alkyl or aromatic group.

• Common examples include benzaldehyde and formaldehyde.

• Monosaccharides may have multiple hydroxyl groups in addition to the aldehyde.

Ketones in Monosaccharides

Ketones are similar to aldehydes but have the carbonyl group bonded to two alkyl or aromatic groups. Monosaccharides with a ketone functional group are called ketoses.

• Structure: R-CO-R', where both R and R' are alkyl or aromatic groups.

• Biologically relevant ketones include pyruvate and butanedione (butter flavor).

Classification of Monosaccharides by Functional Groups and Carbon Number

Monosaccharides are classified based on the type of carbonyl group and the number of carbon atoms.

• Aldose: Contains an aldehyde group.

• Ketose: Contains a ketone group.

• Triose: Three carbon atoms.

• Tetrose: Four carbon atoms.

• Pentose: Five carbon atoms.

• Hexose: Six carbon atoms.

Example: An aldose with four carbons is called an aldotetrose; a ketose with six carbons is a ketohexose.

Type	Functional Group	Number of Carbons	Example Name
Aldose	Aldehyde	4	Aldotetrose
Ketose	Ketone	6	Ketohexose

Additional info: The classification of monosaccharides is fundamental for understanding their chemical behavior and biological roles. The presence and position of functional groups determine the reactivity and the type of reactions these sugars can undergo, such as oxidation, reduction, and ring formation.