Information Technology Reference
In-Depth Information
whether the enhanced polarizability of soft reagents supports the HSAB
principle requires determining how the polarizability affects the interac-
tion energy between acids and bases.”
Now let us consider the mechanism of acid-base adduct formation.
When an acid and a base approach each other, the charge on the acid polar-
izes the base, and vice versa. This is commonly known as charge-induced
dipole interaction. The energy change due to this charge-induced dipole
interaction was given by Ayers [3] as follows:
ΔE
qα
= −{(α
B
q
A
2
)/2(
r
A
+
r
B
)
4
} − {(α
A
q
B
2
)/2(
r
A
+
r
B
)
4
}
(87)
where α
A
and α
B
denote the polarizability of acidic and basic reactive sites.
The charge-induceddipole is often the largest polarizability-dependent
contribution to the reaction energy and is only signifi cant when one or
both of the reagents are highly charged. In such case, the charge-charge
interaction or the electrostatic interaction will be the dominant factor.
On the other hand, in most cases where the charge-induced dipole in-
teraction energy is large, the reactivity preferences will be determined by
an even larger charge-charge interaction. Thus, while the charge-induced
dipole interaction in Eq. (86) is not quantitatively small, it is usually quali-
tatively unimportant.
This is supportive because the charge-induced dipole contribution to
the reaction energy is not especially favorable for the HSAB principle.
Now when neutral atoms react with each other, the reactive sites of the
acid and base are uncharged, or at least small, the dominant electrostatic
interaction is the London dispersion interaction:
ΔE
L
= -(3ε
A
ε
B
α
A
α
B
)/{2(ε
A
+ ε
B
) (
r
A
+
r
B
)
6
}
(88)
ε
A
and ε
B
arise from the closure approximation and represent the average
excitation energy of the acid and the base, respectively. The chemical
hardness is approximately equal to the first excitation energy [72]; there-
fore, it is reasonable to infer that
ΔE
L
≈ -(3η
A
η
B
α
A
α
B
)/{2(η
A
+ η
B
) (
r
A
+
r
B
)
6
}
(89)
From the review of Ayers [3], we may concur with his comment that “the
HSAB principle is a robust rule for predicting the products of chemical
reactions” [3]. He also noted that the double-exchange reactions always
Search WWH ::
Custom Search