A Guidebook to Mechanism in Organic Chemistry

The Strengths of Acids and Bases

to be accurately measured. Both the neutral molecule and the cation,

H 2 N=C(NH:J) 2 , resulting from its protonation, are stabilised by
delocalisation

r:NH 2 ANH 2 NH 2

©HI f). 0 I e

HN—C NH 2 «-» HN—C^NH, -» HN—C=NH 2 Neutral Molecule

H®

H 2 N=rC—NH 2 <- H 2 N—C-^NH 2 fl 2 N—C=NH 2 Cation

r:NH 2 fNH 2 NH 2

4==C—NH 2 «- H 2 N—C-^-NH 2 <-* H 2 N—(

but in the cation the positive charge is spread symmetrically by the

contribution to the hybrid of three exactly equivalent structures of

equal energy. No comparably effective delocalisation occurs in the

neutral molecule (in which two of the contributing structures involve

separation of charge) with the result that the cation is greatly stabi­

lised with respect to it, thus making protonation 'energetically profit­

able' and guanidine an extremely strong base.

(iii) Aromatic basts

Thaaexact reverse 06 the above is seen with aniline which is a very

weak base, having a pK 6 of 9 • 38 compared to 4 • 75 for ammonia and

3-46 for methylamine. In aniline, the unshared electron pair on

nitrogen can interact with the delocalised n orbitals of the nucleus:

If the aniline is protonated, any such interaction, with resultant

stabilisation, is prohibited, as the electron pair on nitrogen is no

longer unshared:

H,N:H

(VII)