Table 5.2. Some examples of hard and soft acids and bases. (R represents an alkyl
group.)
H⊕ is a hard acid because it has no electrons and has a high
positive charge density. The Hc- ion is a soft base because it has a pair
of electrons and only one proton, so it holds the electrons rather
loosely. Thus, it is quite polarizable and soft.
Exercise 5.5
Classify each of the following chemical species as a hard, soft, or
borderline acid or base.
a) (CH3)3B
b) CH3CH2Oc-
c) (CH3)3Al
d) AsH3
e) FeCl3
f) CH3OH
g) (CH3)3C⊕
h) (CH3)3Cc-
i) c- SeH
j) (CH3)3N
k) CH3NH2
l) SnCl2
Sample solution
b) Oxygen is small and nonpolarizable and has a high
electronegativity. Therefore, it is a hard base, making the alkoxide ion
a hard base.
An important rule concerning acid-base reactions is that hard
acids prefer to bond with hard bases, and soft acids prefer to bond
with soft bases. This rule, often called the HSAB Principle, has
nothing to do with acid or base strength, but merely states that a bond
between a particular acid and a particular base has extra stability if
both are either hard or soft. The HSAB Principle also helps to predict
the outcome of an acid-base reaction. For example, the acyl group
(RCO⊕) is a hard Lewis acid and forms stable combinations with hard
Lewis bases such as c- NH2, ROc- , and Clc- . In contrast, it forms
marginally stable or even unstable compounds with soft Lewis bases
such as RSc- and Ic- .
Exercise 5.6
According to the HSAB Principle, which of the following chemical
compounds would you expect to be stable (or only moderately reactive)
and which would you expect to be unstable (or very reactive)?
a) AlI3 b) CH3COSH c) NaH
d) Mg(SH)2 e) Hg(OH)2 f) CH3Cl
g) AgF h) CuCH3 i) CuI
j) HgCO3 k) CsOH l) KCH3
Sample solution
a) From Table 5.2, note that Al3⊕ is a hard acid, and Ic- is a soft base.
Thus, AlI3 is either unstable or highly reactive.
Perhaps the most important application of the HSAB Principle
is in determining whether a particular compound will act as a base or
as a nucleophile. Generally, a soft base is a good nucleophile, and a
hard base is a better base. Chapters 12 through 14 show this rule of
thumb in action. The statement was made earlier that a nucleophile
generally reacts with a positive or partially positive carbon, and a base
generally reacts with a positive or partially positive hydrogen. This
statement is a simplified form of the HSAB principle: H⊕ is a much
harder acid than C⊕, so it tends to react with a harder base than C⊕
does. For example, chlorocyclohexane reacts with a hydroxide ion to
remove a proton from the carbon adjacent to the carbon bearing the
chlorine. This reaction forms a double bond.
On the other hand, chlorocyclohexane reacts with cyanide ion (c- CN)
to form a product containing a nitrile group.