Organic Chemistry

40 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

According to the Brønsted–Lowry definitions, any species that has a hydrogen can

potentially act as an acid, and any compound possessing a lone pair can potentially act

as a base. Both an acid and a base must be present in a proton-transfer reaction,be-

cause an acid cannot donate a proton unless a base is present to accept it. Acid–base

reactionsare often called proton-transfer reactions.

When a compound loses a proton, the resulting species is called its conjugate base.

Thus, is the conjugate base of HCl, and is the conjugate base of When

a compound accepts a proton, the resulting species is called its conjugate acid. Thus,

HCl is the conjugate acid of and is the conjugate acid of

In a reaction involving ammonia and water, ammonia is a base because it ac-

cepts a proton, and water is an acid because it donates a proton. Thus, is the con-

jugate base of and is the conjugate acid of In the reverse

reaction, ammonium ion is an acid because it donates a proton, and hydroxide

ion is a base because it accepts a proton.

Notice that water can behave as either an acid or a base. It can behave as an acid be-

cause it has a proton that it can donate, but it can also behave as a base because it has a

lone pair that can accept a proton. In Section 1.17, we will see how we know that water

acts as a base in the first reaction in this section and acts as an acid in the second reaction.

Acidityis a measure of the tendency of a compound to give up a proton. Basicityis

a measure of a compound’s affinity for a proton. A strong acid is one that has a strong

tendency to give up its proton. This means that its conjugate base must be weak be-

cause it has little affinity for the proton. A weak acid has little tendency to give up its

proton, indicating that its conjugate base is strong because it has a high affinity for the

proton. Thus, the following important relationship exists between an acid and its con-

jugate base:The stronger the acid, the weaker is its conjugate base. For example, since

HBr is a stronger acid than HCl, we know that is a weaker base than

PROBLEM 26

a. Draw the conjugate acid of each of the following:

1. 
   
   
   
   2. 
      
      
      
      3. 
         
         
         
         4. b. Draw the conjugate base of each of the following:
         
         
         
         
      
      
      
      
   
   
   
   


2. 
   
   
   
   2. HBr 3. 4.
   
   
   
   

PROBLEM 27

a. Write an equation showing reacting as an acid with and an equation

showing it reacting as a base with HCl.

b. Write an equation showing reacting as an acid with and an equation showing

it reacting as a base with HBr.

1.17 Organic Acids and Bases; and pH

When a strong acid such as hydrogen chloride is dissolved in water, almost all the

molecules dissociate (break into ions), which means that products are favored at equi-

librium. When a much weaker acid, such as acetic acid, is dissolved in water, very few

molecules dissociate, so reactants are favored at equilibrium. Two half-headed arrows

are used to designate equilibrium reactions. A longer arrow is drawn toward the

species favored at equilibrium.

pKa

NH 3 HO-

CH 3 OH NH 3

NH 3 HNO 3 H 2 O

NH 3 Cl- HO- H 2 O

Br- Cl-.

NH 3 +

a base

H 2 O

an acid

+ HO −

a base

+NH

4

an acid

(HO-)

(+NH 4 )

H 2 O, (+NH 4 ) (NH 3 ).

HO-

(NH 3 )

Cl-, H 3 O+ H 2 O.

Cl- H 2 O H 3 O+.

HCl +

an acid

H 2 O

a base

+ H 3 O+

an acid

Cl −

a base

The stronger the acid, the weaker is its
conjugate base.