Organic Chemistry

352 CHAPTER 9 Reactions of Alkanes • Radicals

a radical intermediate. The radical that abstracts the hydrogen is formed as a result of

the interaction between Fe(III) and an amino acid at the active site of the enzyme.

Unwanted radicals in biological systems must be destroyed before they have an op-

portunity to cause damage to cells. Cell membranes, for example, are susceptible to

the same kind of radical reactions that cause butter to become rancid (Section 26.3).

Imagine the state of your cell membranes if radical reactions could occur readily. Rad-

ical reactions in biological systems also have been implicated in the aging process.

Unwanted radical reactions are prevented by radical inhibitors—compounds that de-

stroy reactive radicals by creating unreactive radicals or compounds with only paired

electrons. Hydroquinone is an example of a radical inhibitor. When hydroquinone

traps a radical, it forms semiquinone, which is stabilized by electron delocalization

and is, therefore, less reactive than other radicals. Furthermore, semiquinone can trap

another radical and form quinone, a compound whose electrons are all paired.

Two examples of radical inhibitors that are present in biological systems are vita-

min C and vitamin E. Like hydroquinone, they form relatively stable radicals. Vitamin

C (also called ascorbic acid) is a water-soluble compound that traps radicals formed in

the aqueous environment of the cell and in blood plasma. Vitamin E (also called

) is a water-insoluble (hence fat-soluble) compound that traps radicals

formed in nonpolar membranes. Why one vitamin functions in aqueous environments

and the other in nonaqueous environments should be apparent from their struc-

tures and electrostatic potential maps, which show that vitamin C is a relatively polar

compound, whereas vitamin E is nonpolar.

a-tocopherol

hydroquinone

OH

+

OH

R R

semiquinone quinone

reactive

radical

OH+

O

RH O +

O

RH

a ribonucleotide

HO

RO RO RO

OH

N

O

a radical intermediate a deoxyribonucleotide

HO H

N

O

HO OH

N

O

several steps

3-D Molecules:

Vitamin C; Vitamin E

CH 3

O

CH 3

HO

H 3 C

CH 2 OH

OH

O O

H

H

HO OH

vitamin E

α-tocopherol

vitamin C

ascorbic acid