The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 44

of electron transfer is governed by (η A + η B ) and a small value of the summation is
favorable for maximum covalent bonding.
The concept of electronegativity provides a measure of the intrinsic strength of
an acid or base [64, 74, 75]. A strong Lewis acid is a good electron acceptor and has
high electronegativity/low chemical potential. A weak Lewis acid has a lower electro-
negativity than a strong Lewis acid, but a higher electronegativity than a Lewis base.
A strong Lewis base readily donates electrons and has a lower electronegativity than a
weak Lewis base. These relations are summarized by Ayers [64] as follows:

χ(strong acid)> χ(weak acid)> χ(weak base)> χ(strong base)>0 (93)

The perfect electron donor has χ = 0. One can reify the electron-accepting abilities
of real molecules by imagining how they would react with a perfect electron donor.
Thus, the electronegativity concept plays a dominating role in the principle of Hard
and Soft Acid and Base.
One of the most important questions connected with the problem of reactivity of
molecules in different environmental conditions is the prediction and interpretation of
the preferred direction of a reaction and the product formation. Sekhon [25] examined
metathesis reaction of the type AB + CD = AD + BC in terms of the equalized elec-
tronegativity values of various species involved in the double exchange reaction. The
conclusion was made by him from the study that an exchange reaction proceeds from
left to right if the total sum of the equalized electronegativity value of the products is
greater than that of reactants.

THE CONCEPT OF GROUP ELECTRONEGATIVITY
One important application of the electronegativity concept is in the estimation of the
electron-withdrawing ability of chemical groups. The idea of group electronegativity
is important because the electronegativity concept evolved largely from the desire of
organic chemists to understand reaction mechanism in terms of the inductive effects
of various functional groups. For several years' synthesis chemists have expressed a
desire for a group electronegativity scale. This application requires the ability to ac-
count for charges on group.
Garner-O' Neale, Bonamy, Meek, and Patrick [76] extended the original Pauling
concept of electronegativity for defi ning group electronegativity as the power of a
group in a molecule to attract electron to it. They pointed out that the groups have a
better ability to donate or accept charges than atom and therefore be considered as res-
ervoir of enhanced charged capacity. Hence, a group of atoms as a unit is potentially
able to donate or withdraw considerable amounts of charge with a very little effect on
itself.
The ability to dissipate charge over several atoms increases as the number of at-
oms which constitute the group increases. Upon bond formation between two atoms,
charge is transferred from one atom to the other. In the case of a chemical group charge
is transferred to the central atom because of its bond. The electronegativity of the
group will then be the orbital electronegativity of the central atom suitably modifi ed
to account for its charge [77].