23

Evidence for benzyne mechanism:

(i)

indiscriminate amide attack on symmetrical benzyne
(ii)

has no ortho hydrogen to be lost

(iii)

recovered starting material contains more deuterated bromobenzene than
hydrogenous bromobenzene. This isotope effect shows that the ortho hydrogen is
involved andis part of the r.d.s.

Benzynes can be trapped in a polymer matrix, to ascertain them as intermediatess.

5. Aromatic Radical Substitution

5.1.

Radical Substitution Reactions

Sandmeyer Reaction

Iodination Reaction

Iodination can be carried out in the presence of cupric salts. The cupric salt appears
to act both as a Lewis acid catalyst and as an oxidant for converting the iodide to the
iodine.

Cl

NH2

+NaNH2

liquid NH3
-33oC

13C

13C

13C

1:1 mixture

MeO

Br

+NaNH2

liquid NH3
-33oC

no reaction

Br

H (D)

+NaNH2

< 1 equiv.

1:1 mixture

2 N2

+

N
NH2

N

N

N
N

N

N

+

O