Organic Chemistry

870 CHAPTER 20 More About Oxidation–Reduction Reactions

NADH reduces a compound by donating a hydride ion from the 4-position of the

six-membered ring. Thus, NADH, and all act as reducing agents in

the same way—they donate a hydride ion.

C

R R

N+

R

O

CNH 2

NAD+

+ +

N

R

O

CNH 2

NADH

H H

O

H

C

O−

OH

R R

H

R C R

H+

NaBH 4 , LiAlH 4

CH 3 OH

methanol

O

+ NAD+ HCH + NADH + H+

formaldehyde

alcohol

dehydrogenase

AN UNUSUAL ANTIDOTE

Alcohol dehydrogenase, the enzyme that cat-

alyzes the oxidation of ethanol to acetaldehyde,

catalyzes the oxidation of other alcohols as well. For example,

it catalyzes the oxidation of methanol to formaldehyde.

Methanol itself is not harmful, but ingestion of methanol can

be fatal because formaldehyde is extremely toxic. The treat-

ment for methanol ingestion consists of giving the patient intra-

venous injections of ethanol. Alcohol dehydrogenase has 25

times the affinity for ethanol that it has for methanol. Thus, if

the enzyme can be kept loaded with ethanol, methanol will be

excreted before it has the opportunity to be oxidized.

FETAL ALCOHOL SYNDROME

The damage done to a human fetus when the

mother drinks alcohol during her pregnancy is

known as fetal alcohol syndrome. It has been shown that the

harmful effects—growth retardation, decreased mental func-

tioning, and facial and limb abnormalities—are attributable

to the acetaldehyde that is formed from the oxidation of

ethanol, which crosses the placenta and accumulates in the liver

of the fetus.

20.12 Oxidation of Hydroquinones

and Reduction of Quinones

Hydroquinone, a para-benzenediol, is easily oxidized to para-benzoquinone. Al-

though a wide variety of oxidizing agents can be used, Fremy’s salt (dipotassium ni-

trosodisulfonate) is the preferred oxidizing agent. The quinone can easily be reduced

back to hydroquinone.

O

O

hydroquinone para-benzoquinone

a para-quinone

OH

OH

oxidation

NaBH 4

(KSO 3 ) 2 NO

reduction

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