Organic Chemistry

Section 22.2 The Dand LNotation 923

A monosaccharidecan be a polyhydroxy aldehyde such as D-glucose or a polyhydroxy
ketone such as D-fructose. Polyhydroxy aldehydes are called aldoses(“ald”is for alde-
hyde; “ose”is the suffix for a sugar), whereas polyhydroxy ketones are called ketoses.
Monosaccharides are also classified according to the number of carbons they contain:
Monosaccharides with three carbons are trioses, those with four carbons are tetroses,
those with five carbons are pentoses, and those with six and seven carbons are hexosesand
heptoses, respectively. A six-carbon polyhydroxy aldehyde such as D-glucose is an aldo-
hexose, whereas a six-carbon polyhydroxy ketone such as D-fructose is a ketohexose.

PROBLEM 1

Classify the following monosaccharides:

22.2 The Dand LNotation

The smallest aldose, and the only one whose name does not end in “ose,”is glycer-
aldehyde, an aldotriose.

Because glyceraldehyde has an asymmetric carbon, it can exist as a pair of enantiomers.

Emil Fischer and his colleagues studied carbohydrates in the late nineteenth cen-
tury, when techniques for determining the configurations of compounds were not
available. Fischer arbitrarily assigned the R-configuration to the dextrorotatory iso-
mer of glyceraldehyde that we call D-glyceraldehyde. He turned out to be correct:
D-Glyceraldehyde is -glyceraldehyde, and L-glyceraldehyde is -
glyceraldehyde (Section 5.13).

HC

CH 2 OH

O

HOH

HC

CH 2 OH

O

HO H

D-glyceraldehyde L-glyceraldehyde

(R)-(+) (S)-(-)

HOCH 2 CHCH

OH

O

glyceraldehyde

CH 2 OH

HO H

HO H

HOH

HOH

HC O

CH 2 OH

CH 2 OH

HO H

HOH

HOH

HOH

C O

CH 2 OH

HC O

HOH

HOH

HOH

D-ribose D-sedoheptulose D-mannose

a sugar subunit

MMMMMMMMM x M

hydrolysis

polysaccharide monosaccharide

A carbon to which four different groups

are attached is an asymmetric carbon.

HC

CH 2 OH

O

HOH

HC

CH 2 OH

O

HO H

(R)-(+)-glyceraldehyde (S)-(−)-glyceraldehyde

CH O

(R)-(+)-glyceraldehyde

CH 2 OH

HO H

C

CH O

(S)-(−)-glyceraldehyde

perspective formulas Fischer projections

HOCH 2

H OH

C