Organic Chemistry

Section 15.11 Nitration of Benzene 609

THYROXINE

Thyroxine is a hormone that regulates the meta-

bolic rate, causing an increase in the rate at which

fats, carbohydrates, and proteins are metabolized. Humans ob-

tain thyroxine from tyrosine (an amino acid) and iodine. We get

iodine primarily from the iodized salt in our diet. An enzyme

Unlike the reaction of benzene with or the reaction of an alkene with
or does not require a Lewis acid (Section 4.7). An alkene is more reactive than
benzene because an alkene has a smaller activation energy, since carbocation forma-
tion is not accompanied by a loss of aromaticity. As a result, the or
bond does not have to be weakened to form a better electrophile.

PROBLEM 18

Why does hydration inactivate

Electrophilic iodine is obtained by treating with an oxidizing agent such as
nitric acid.

Once the electrophile is formed, iodination of benzene occurs by the same mechanism
as bromination and chlorination.

I

+ H

I

mechanism for iodination

+ I+ B + HB+

I 2 2 I+

iodination

I

++I+ H+

iodobenzene

oxidizing agent

1 I+ 2 I 2

FeBr 3?

Br¬Br Cl¬Cl

Cl 2

Br 2 Cl 2 , Br 2

2 Fe + 3 Br 2 2 FeBr 3

2 Fe + 3 Cl 2 2 FeCl 3

15.11 Nitration of Benzene

Nitration of benzene with nitric acid requires sulfuric acid as a catalyst.

NO 2

++HNO 3 H 2 O

H 2 SO 4

nitration

nitrobenzene

called iodoperoxidase converts the we ingest to the elec-

trophile needed to place an iodo substituent on the benzene ring.

Low thyroxine levels can be corrected by hormone supplements.

Chronically low levels of thyroxine cause enlargement of the

thyroid gland, a condition known as goiter.

I- I+,

tyrosine thyroxine

CH 2 CHCO

NH 3

O

HO HO

II

+ II

− CH

2 CHCO

NH 3

O

O

+

−