A Guidebook to Mechanism in Organic Chemistry

Other Nucleophilic Displacement Reactions

widespread in organic chemistry and embraces very many more types
than those of which passing mention has been made. Thus other
typical examples include:
(i) The formation of a tetralkylammonium salt

R 3 N: + RBr R 3 N:R+Br®

where the unshared pair of electrons on nitrogen attack carbon with

the expulsion of a bromide ion; and also the breakdown of such a

salt

• Br® + R:NR 3 ->• BrR + :NR 3

in which it is the bromide ion that acts as a nucleophile and :NR 3 that

is expelled as the leaving group. An exactly similar situation is, of

course, met with in the formation of sulphonium salts

R„S: + RBr ->• RaS:R + Br®

and in their breakdown.

(ii) The alkylation of reactive methylene groups etc. (cf. p. 221):

e

EtO® + CHa(COaEt), -> EtOH + CH{COaEt),

(EtO,C)aHC® + RBr ->• (EtOjQjHf • R+Br® ^

Here, and in the next two examples, a carbanion or a source of negative

carbon is acting as the nucleophile.

(iii) Reactions of acetylene in the presence of strong base, e.g.

NaNHgin liquid NH 3 :

HaN© + HGECH - H 3 N + GEECH

HC^C® + RBr -* HC=C • R + Br®

(iv) Reaction of Grignard reagents:

BrMgR+R'Br -»• Br 3 Mg+RR'

(v) Decomposition of diazonium salts in water:

HaO: + PhNa® -*• HO:Ph+Na

H

HO:Ph ^-HO:Ph + H®

H