Organic Chemistry

Section 1.18 The Effect of Structure on pKa 45

The weaker the base, the stronger is its

conjugate acid.

Stable bases are weak bases.

The more stable the base, the stronger is

its conjugate acid.

PROBLEM 31

a. Which is a stronger base, or (The of is 4.8; the

of HCOOH is 3.8.)

b. Which is a stronger base, or (The of is 15.7; the of

is 36.)

c. Which is a stronger base, or (The of is the of

is )

PROBLEM 32

Using the values in Section 1.17, rank the following species in order of decreasing

base strength:

PROBLEM 33

Calculate the equilibrium constant for the acid–base reactions between the following pairs

of reactants.

a. c.

b. d.

1.18 The Effect of Structure on

The strength of an acid is determined by the stability of the conjugate base that is
formed when the acid gives up its proton. The more stable the base, the stronger is its
conjugate acid. A stable base is a base that readily bears the electrons it formerly
shared with a proton. In other words, stable bases are weak bases—they don’t share
their electrons well. So we can say,the weaker the base, the stronger is its conjugate
acid, or the more stable the base, the stronger is its conjugate acid.
The elements in the second row of the periodic table are all about the same size, but
they have very different electronegativities. The electronegativities increase across the
row from left to right. Of the atoms shown, carbon is the least and fluorine is the most
electronegative.

If we look at the bases formed when hydrogens are attached to these elements, we see
that the stabilities of the bases also increase from left to right because the more elec-
tronegative atom is better able to bear its negative charge.

The stronger acid is the acid that forms the more stable conjugate base, so HF is the
strongest acid and methane is the weakest acid (Table 1.9).

relative acidities: CH 4 NH 4 H 2 OHF

strongest

acid

<<<

• CH
  3
  
  
  • NH
    2 HO
    relative stabilities: – F–
  
  
  
  

most

stable

< < <

relative electronegativities: C < N< O< F

most

electronegative

pKa

CH 3 N

+

CH 3 COOH+H 2 O H 3 +H 2 O

HCl+H 2 O CH 3 NH 2 +H 2 O

CH 3 NH 2 CH 3 NH− CH 3 OH CH 3 O− CH 3 CO−

O

pKa

CH 3 OH 2 + -2.5.

H 2 O CH 3 OH? pKa H 3 O+ -1.7; pKa

HO- -NH 2? pKa H 2 O pKa NH 3

pKa

CH 3 COO- HCOO-? pKa CH 3 COOH