Organic Chemistry

54 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

Lewis base: Have pair, will share.
Lewis acid: Need two from you.

PROBLEM 45

a. Indicate whether a carboxylic acid (RCOOH) with a of 5 will be mostly charged or

mostly neutral in solutions with the following pH values:

b. Answer the same question for a protonated amine with a of 9.

c. Answer the same question for an alcohol (ROH) with a of 15.

PROBLEM 46

For each compound in 1 and 2, indicate the pH at which

a. 50% of the compound will be in a form that possesses a charge.

b. more than 99% of the compound will be in a form that possesses a charge.

1.

2.

PROBLEM 47

For each of the following compounds, shown in their acidic forms, draw the form in which

it will predominate in a solution of

a. f.

b. g.

c. h.

d. i.

e. j.

1.21 Lewis Acids and Bases

In 1923, G. N. Lewis offered new definitions for the terms “acid”and “base.”He de-

fined an acid as a species that accepts a share in an electron pair and a base as a

species that donates a share in an electron pair. All proton-donating acids fit the

Lewis definition because all proton-donating acids lose a proton and the proton ac-

cepts a share in an electron pair.

The Lewis definition of an acid is much broader than the Brønsted–Lowry defini-

tion because it is not limited to compounds that donate protons. According to the

Lewis definition, compounds such as aluminum chloride boron trifluoride

and borane are acids because they have unfilled valence orbitals and thus

can accept a share in an electron pair. These compounds react with a compound that

has a lone pair just as a proton reacts with ammonia, but they are not proton-donating

acids. Thus, the Lewis definition of an acid includes all proton-donating acids and

some additional acids that do not have protons. Throughout this text, the term “acid”is

used to mean a proton-donating acid, and the term “Lewis acid”is used to refer to

non-proton-donating acids such as or All bases are Lewis basesbecause

they have a pair of electrons that they can share, either with an atom such as aluminum

or boron or with a proton.

AlCl 3 BF 3.

(BF 3 ), (BH 3 )

(AlCl 3 ),

H+ NH 3 NH 3

acid base

H

+

+

accepts a

share in a pair

of electrons

donates a share in

a pair of electrons

CH 3 CH 2 O HBr (pKa=-9)

+

H 2 (pKa=-2.5)

CH 3 CH 2 OH (pKa=15.9) HNO 3 (pKa=-1.3)

H 3 O+ (pKa=-1.7) HNO 2 (pKa=3.4)

CH 3 CH 2 N HC‚N (pKa=9.1)

+

H 3 (pKa=11.0)

N

+

CH 3 COOH (pKa=4.76) H 4 (pKa=9.4)

pH=7:

CH 3 N

+

H 3 (pKa=10.7)

CH 3 CH 2 COOH (pKa=4.9)

pKa

(RN pKa

+

H 3 )

pH= 3 pH= 7 pH= 11

pH= 1 pH= 5 pH= 9 pH= 13

pKa