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Organic Chemistry

25.1 Classification of Carbohydrates

Organic Chemistry25.1 Classification of Carbohydrates

25.1 • Classification of Carbohydrates

Carbohydrates are generally classified as either simple or complex. Simple sugars, or monosaccharides, are carbohydrates like glucose and fructose that can’t be converted into smaller sugars by hydrolysis. Complex carbohydrates are made of two or more simple sugars linked together by acetal bonds (Section 19.10). Sucrose (table sugar), for instance, consists of one glucose linked to one fructose. Similarly, cellulose is made up of several thousand glucose units linked together. Enzyme-catalyzed hydrolysis of a complex carbohydrate breaks it down into its constituent monosaccharides.

Enzyme-catalyzed hydrolysis decomposes sucrose (a disaccharide) into glucose and fructose. Similarly, cellulose (a polysaccharide) decomposes into approximately three thousand glucose units.

Monosaccharides are further classified as either aldoses or ketoses. The -ose suffix designates a carbohydrate, and the prefixes aldo- and keto- identify the kind of carbonyl group in the molecule, whether aldehyde or ketone. The number of carbon atoms in the monosaccharide is indicated by the appropriate numerical prefix tri-, tetr-, pent-, hex-, and so forth, in the name. Putting it all together, glucose is an aldohexose, a six-carbon aldehyde sugar; fructose is a ketohexose, a six-carbon keto sugar; ribose is an aldopentose, a five-carbon aldehyde sugar; and sedoheptulose is a ketoheptose, a seven-carbon keto sugar. Most of the common simple sugars are either pentoses or hexoses.

The structures of glucose (an aldohexose), fructose (a ketohexose), ribose (an aldopentose), and sedoheptulose (a ketoheptose), presented in Fischer projection.
Problem 25-1
Classify each of the following monosaccharides:
The structure of D threose, an aldose monosaccharide. The molecular formula is C 4 H 8 O 4, with H and O H groups on opposite sides of stereogenic centers.
The structure of D ribulose which is a ketopentose with the chemical formula C 5 H 10 O 5. It is a structural isomer of ribose.
The structure of D tagatose which is a ketohexose. The molecular formula is C 6 H 12 O 6.
The structure of 2-deoxyribose. It is an aldopentose with 5 carbon atoms and an aldehyde functional group.
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