A Guidebook to Mechanism in Organic Chemistry

Substituted Aliphatic Acids

former are little more acidic than the hydrogens in ethane, whereas
those of acetylene are sufficiently acidic to be readily replaceable by
a number of metals!

(v) Substituted aliphatic acids

The effect of introducing electron-withdrawing substituents into fatty
acids is more marked. Thus halogen, with an inductive effect acting in
the opposite direction to alkyl, would be expected to increase the
strength,of an acid so substituted, and this is indeed observed as
fKa values show: ^
CI
CH 3 -v-COaH
4-76

F-<-CH 2 -+-C0 2 H

2-66

Cl-<-CH 2 --£C0 2 H

2-86

Br-«-CH 2 -«-C0 2 H

2-90

i-<-CH 2 -<-C0 2 H

§•16

CI

\

( CH-^C0 2 H

1-29
CI
I
Cl-^C-<-C0 2 H
t
CI
0-65
The action of this effect results in increased Stabilisation of the
substituted anion, by spreading of its charge, m»

O

F-«-CH 2 -«-C

O

o

CH 8

o

and the halogenated acids are thus stronger than their unsubstituted
analogues. The relative effect of the different halogens is in the
expected order, fluorine being the most electronegative (electron-
withdrawing) and producing a hundredfold increase in strength of
fluoracetic acid as compared with acetic acid itself. The effect is very
much greater than that produced, in the opposite direction, by the
introduction of an alkyl group, and the introduction of further
halogens still produces large increases in acid strength: trichloracetic
is thus a very strong acid.

The introduction of a halogen further away than in the a-position
to the carboxyl group has much less effect, its inductive effect quickly
dying in a saturated chain as the following pK„ values show: