Organic Chemistry

Section 20.12 Oxidation of Hydroquinones and Reduction of Quinones 871

Similarly,ortho-benzenediols are oxidized to ortho-quinones.

Overall, the oxidation reaction involves the loss of two hydrogen atoms and the re-

duction reaction involves the gain of two hydrogen atoms. In Section 9.8, we saw that

phenols are used as radical inhibitors because of their ability to lose a hydrogen atom.

Coenzyme Q (CoQ) is a quinone found in the cells of all aerobic organisms. It is

also called ubiquinone because it is ubiquitous (found everywhere) in nature. Its func-

tion is to carry electrons in the electron-transport chain. The oxidized form of CoQ ac-

cepts a pair of electrons from a biological reducing agent such as NADH and

ultimately transfers them to

In this way, biological oxidizing agents are recycled: oxidizes a compound,

thereby forming NADH, which is oxidized back to by oxygen, via coenzyme

Q, which is unchanged in the overall reaction. Biological redox reagents and their re-

cycling are discussed further in Sections 25.2 and 25.3.

++H+

NAD+ +

NAD+

Substratereduced Substrateoxidized

NADH

+ NADH + H+

21 O^2 + H^2 O

NAD+

NAD+

O 2.

O

hydroquinone

mechanism for hydroquinone oxidation–quinone reduction

O H

O H

semiquinone para-benzoquinone

O

O H O H

H

O

O

ortho-benzoquinone

an ortho-quinone

OH

OH

(KSO 3 ) 2 NO

NaBH 4

oxidation

reduction

CH 3

CH 3 O R

R = (CH 2 CH

n = 1 − 10

CCH 2 )nH

CH 3 O

++

O

O

coenzyme Q

oxidized form

OH

OH

H+

CH 3

CH 3 O R

CH 3 O

OH

OH

coenzyme Q

reduced form

NADH + NAD+

CH 3

CH 3

CH 3 O R

CH 3 O

O 2 +

O

O

CH 3

CH 3 O R

CH 3 O

21 + H^2 O

3-D Molecules:

Coenzyme Q (oxidized form);

Coenzyme Q (reduced form)

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