Question

How can you identify the nucleophile and the electrophile in the following acid-base reaction?
$\text{AlC}{{\text{l}}_{3}}+\text{N}{{\text{H}}_{3}}\to \text{AlCl}_{3}^{-}-\text{NH}_{3}^{+}$

Answer 1

The nucleophile means nucleus loving and the electrophile means electron loving. So, the nucleophiles always tend to donate electrons to an electrophile and the electrophiles always tend to accept electrons from a nucleophile.

Complete step by step answer: To identify the nucleophile and electrophile in the given acid-base reaction first we need to understand what is meant by the terms nucleophile and electrophile.
-Any chemical species which is rich in electrons or having lone pairs to donate to another species is called a nucleophile. Whereas any chemical species which is electron deficient in order to complete its octet or duplet is called an electrophile.
Now, in the given acid-base reaction we have two different species - $\text{AlC}{{\text{l}}_{3}}$ and $\text{N}{{\text{H}}_{3}}$. To check the number of electrons present in each species we first need to write down their electronic configuration.
-$\text{AlC}{{\text{l}}_{3}}$ is a covalent compound in which aluminum is $\text{s}{{\text{p}}^{3}}$ hybridized to form three covalent bonds with chloride ions and one p-orbital remains vacant.
$\text{Al}\to \left[ \text{Ne} \right]3{{\text{s}}^{2}}3{{\text{p}}^{1}}$

From this, we can infer that it is electron deficient in nature and therefore, it will act as an electrophile.
-On the other hand, in $\text{N}{{\text{H}}_{3}}$ nitrogen forms 4 equivalent energy orbitals of $\text{s}{{\text{p}}^{3}}$ hybridization for making bond with hydrogens. We can see from below that one of the four $\text{s}{{\text{p}}^{3}}$ hybridized orbital having a pair of electrons does not participate in bonding. Thus, it remains as a lone pair on nitrogen making $\text{N}{{\text{H}}_{3}}$ rich in electrons. So, it will act as a nucleophile.
$\text{N}\to \left[ \text{He} \right]2{{\text{s}}^{2}}2{{\text{p}}^{3}}$

Hence, in the given acid-base reaction, $\text{AlC}{{\text{l}}_{3}}$ acts as an electrophile while $\text{N}{{\text{H}}_{3}}$ acts as a nucleophile.

Additional information: According to Lewis’s definition of acids and bases, Lewis acids are electron acceptor and Lewis bases are electron donor species. Thus, all electrophiles act as Lewis acids and all nucleophiles act as Lewis bases.

Note: The reaction between a Lewis acid and a Lewis base results in the formation of a coordinate covalent compound known as adduct in which Lewis acid or the electrophile accepts a pair of electrons in its vacant orbital and Lewis base or the nucleophile donates its lone pair. It is not always necessary that only negatively charged species act as a nucleophile. The nucleophile can be negatively charged as well as a neutral molecule. For example - $\text{N}{{\text{H}}_{3}}$.