Section 16.10 Synthesis of Substituted Benzenes Using Arenediazonium Salts 647KCl and KBr cannot be used in place of CuCl and CuBr in Sandmeyer reactions; the
cuprous salts are required. This indicates that the cuprous ion is involved in the reac-
tion, most likely by serving as a radical initiator (Section 4.10).
Although chloro and bromo substituents can be placed directly on a benzene ring
by halogenation, the Sandmeyer reaction can be a useful alternative. For example, if
you wanted to make para-chloroethylbenzene, chlorination of ethylbenzene would
lead to a mixture of the ortho and para isomers.
However, if you started with para-ethylaniline and used a Sandmeyer reaction for
chlorination, only the desired para product would be formed.
PROBLEM 20Explain why a diazonium group on a benzene ring cannot be used to direct an incoming
substituent to the meta position.p-ethylaniline p-chloroethylbenzeneNaNO 2 , HCl
0 °CCH 2 CH 3NH 2CuClCH 2 CH 3 CH 2 CH 3N+ NCl− ClFeCl 3CH 2 CH 3++Cl 2CH 2 CH 3
ClCH 2 CH 3ethylbenzene o-chloroethylbenzene Cl
p-chloroethylbenzeneCuCN+
NCl−BrCN+ N 2
BrNm-bromobenzenediazonium chloride m-bromobenzonitrileCuBrNNBr−+
Br+ N 2CuClNNCl−+
ClCH 3 CH 3+ N 2Sandmeyer reactionsbenzenediazonium
bromidebromobenzenep-toluenediazonium
chloridep-chlorotolueneTraugott Sandmeyer (1854–1922)
was born in Switzerland. He received
a Ph.D. from the University of
Heidelberg and discovered the
reaction that bears his name in 1884.
He was a scientist at Geigy Co. in
Basel, Switzerland.