Organic Chemistry

Section 16.5 The Effect of Substituents on pKa 639

The more deactivating (electron

withdrawing) the substituent, the more

it increases the acidity of a COOH, an OH,

or an group attached to a benzene

ring.

NH 3

Take a minute to compare the influence a substituent has on the reactivity of a ben-
zene ring toward electrophilic substitution with its effect on the of phenol. Notice
that the more strongly deactivating the substituent, the lower the of the phenol;
and the more strongly activating the substituent, the higher the of the phenol. In
other words,electron withdrawal decreases reactivity toward electrophilic substitu-
tion and increases acidity, whereas electron donation increases reactivity toward elec-
trophilic substitution and decreases acidity.
A similar substituent effect on is observed for substituted benzoic acids and
substituted protonated anilines.

PROBLEM 13

Which of the compounds in each of the following pairs is more acidic?

a. e.

b. f.

c. g.

d. h.

FCl

COOH

or

COOH

OCH 3

COOH

or

COOH

FCH 2 COH or

O

ClCH 2 COH

O

CH 3 CH 2 COH or

O

H 3 NCH 2 COH

O

+

HCOH or

O

CH 3 COH

O

O 2 NCH 2 COH or

O

O 2 NCH 2 CH 2 COH

O

HOCCH 2 COH or

O O

−OCCH

2 COH

O O

CH 3 COH or

O

ClCH 2 COH

O

++++++

NH 3

OCH 3

NH 3

CH 3

NH 3 NH 3

Br

NH 3

HC O

NH 3

NO 2

pKa = 5.29 pKa = 5.07 pKa = 4.58 pKa = 3.91 pKa = 1.76 pKa = 0.98

OCH 3

COOH COOH COOH COOH COOH COOH

CH 3 Br CH 3 C O NO 2

pKa = 4.47 pKa = 4.34 pKa = 4.20 pKa = 4.00 pKa = 3.70 pKa = 3.44

pKa

pKa

pKa

pKa

Tutorial:

Effect of substituents on pKa

pKa = 10.20 pKa = 10.19 pKa = 9.95 pKa = 9.38 pKa = 7.66 pKa = 7.14

OH

OCH 3

OH

CH 3

OH OH

Cl

OH

HC O

OH

NO 2

phenol