Organic Chemistry

Section 1.16 An Introduction to Acids and Bases 39

Born in Denmark,Johannes

Nicolaus Brønsted (1879–1947)

studied engineering before he

switched to chemistry. He was a pro-

fessor of chemistry at the University

of Copenhagen. During World War II,

he became known for his anti-Nazi

position, and in 1947 he was elected

to the Danish parliament. He died

before he could take his seat.

The dipole moment of chloromethane is greater (1.87 D) than the dipole
moment of the bond (1.5 D) because the dipoles are oriented so that
they reinforce the dipole of the bond—they are all in the same relative direc-
tion. The dipole moment of water (1.85 D) is greater than the dipole moment of a sin-
gle bond (1.5 D) because the dipoles of the two bonds reinforce each
other. The lone-pair electrons also contribute to the dipole moment. Similarly, the di-
pole moment of ammonia (1.47 D) is greater than the dipole moment of a single
bond (1.3 D).

PROBLEM 24

Account for the difference in the shape and color of the potential maps for ammonia and

the ammonium ion in Section 1.12.

PROBLEM 25

Which of the following molecules would you expect to have a dipole moment of zero? To

answer parts g and h, you may need to consult your answers to Problem 23 a and b.

a. c. e. g.

b. d. f. h.

1.16 An Introduction to Acids and Bases

Early chemists called any compound that tasted sour an acid (from acidus, Latin for
“sour”). Some familiar acids are citric acid (found in lemons and other citrus fruits),
acetic acid (found in vinegar), and hydrochloric acid (found in stomach acid—the sour
taste associated with vomiting). Compounds that neutralize acids, such as wood ashes
and other plant ashes, were called bases, or alkaline compounds (“ash”in Arabic is al
kalai). Glass cleaners and solutions designed to unclog drains are alkaline solutions.
The definitions of “acid”and “base”that we use now were provided by Brønsted
and Lowry in 1923. In the Brønsted–Lowry definitions, an acidis a species that do-
nates a proton, and a baseis a species that accepts a proton. (Remember that positive-
ly charged hydrogen ions are also called protons.) In the following reaction, hydrogen
chloride (HCl) meets the Brønsted–Lowry definition of an acid because it donates a
proton to water. Water meets the definition of a base because it accepts a proton from
HCl. Water can accept a proton because it has two lone pairs. Either lone pair can form
a covalent bond with a proton. In the reverse reaction, is an acid because it do-
nates a proton to Cl-,and Cl-is a base because it accepts a proton from H 3 O+.

H 3 O+

H 2 C“O NH 3 H 2 C“CHBr BF 3

CH 3 CH 3 CH 2 Cl 2 H 2 C“CH 2 BeCl 2

H H

Cl

C

H

H H

H

H

H

O

chloromethane

μ = 1.87 D

water

μ = 1.85 D

ammonia

μ = 1.47 D

N

N¬H

O¬H O¬H

C¬Cl

C¬Cl C¬H

(CH 3 Cl)

CCCl

Cl

Cl

carbon dioxideμ = 0 D Cl

carbon tetrachloride

μ = 0 D

O O

Thomas M. Lowry (1874–1936)

was born in England, the son of an

army chaplain. He earned a Ph.D. at

Central Technical College, London

(now Imperial College). He was head

of chemistry at Westminster Training

College and, later, at Guy’s Hospital

in London. In 1920, he became a

professor of chemistry at Cambridge

University.