A Guidebook to Mechanism in Organic Chemistry

THE STRENGTHS OF ACIDS AND BASES

MODERN electronic theories of organic chemistry have been highly
successful in a wide variety of fields in relating behaviour with
structure, but nowhere has this been mWe marked than fl£accounting
for the relative strengths of organic acids and bases. According to the
definition of Arrhenius, acids are compounds that yield hydrogen
ions, He, in solution while bases yield hydroxide ions, ®OH. Such
definitions are reasonably adequate if reactions in water only are to
be considered, but the acid/base relationship's proved so useful in
practice that the concepts of both acids and bases have become con­
siderably more generalised. Thus Brensted defined acids as sub*
stances that would give up protons, i.e. proton donors, while bases
were proton acceptors. The first iqnisation of sulphuric acid in
aqueous solution is then looked upbn as:
HsS0 4 +HsO HgO® + HS0 4 ©
Acid Base Con- Con­
jugate jugate
acid | base
Here water is acting as a base by accepting a proton and is thereby
converted into its so-called conjugate acid, HaO®, while the acid,
H 2 S0 4 , by donating a proton is converted into its conjugate base,
HS0 4 e.
The more generalised picture provided by Lewis who denned acids
as molecules or ions capable of co-ordinating with unshared electron
pairs, and bases as molecules or ions which have such unshared
electron pairs available for co-ordination, has already been referred
to (p. 26). Lewis acids include such species as boron trifluoride (I)
•which reacts with trimethylamine to form a solid salt (m.p. 128°):

Me F MeF

1

Me—N:

1 1 el le

Me—N: B—F

l

^ Me—N:B—F i i

|

Me

1

F

1 1

MeF

(D