Question

Which one of the following is lewis acid?
A. ${N_2}{H_4}$
B. $R - OH$
C. $R - O - R$
D. $AlC{l_3}$

Answer 1

According to lewis concept, acids are those which accept a pair of electrons to form a coordinate bond. In other words, we can say Lewis acid is an electron-pair acceptor.

Complete step by step answer: In the case of ${N_2}{H_4}$ i.e. hydrazine, every nitrogen atom has connected or bonded with three bonding atoms and one lone pair of electrons. Thus, the availability of lone pairs in hydrazine makes it basic and due to two lone pairs of the electron in hydrazine, it is dibasic. In the case of $R - OH$ i.e., alcohol hydroxyl oxygen has lone pairs of the electron. Due to the availability of electron-pair to a strong acid to produce a protonated alcohol it acts as a Lewis base. In the case of $R - O - R$ i.e., ether, oxygen possesses two lone pairs of the electron, since the oxygen atom is capable of donating electron pairs to electron-deficient species it is termed as lewis base.
In the case of aluminium chloride i.e., $AlC{l_3}$, the central atom $Al$ has vacant d orbital. So it can accept a pair of electrons, so it acts as lewis.
So, the correct answer is “Option D”.

Additional Information: There is some limitation of lewis concept:
It does not explain the relative strength of acids and bases.
The process of neutralization between common acids and the base is instantaneous whereas the coordinate bond formation is a slow process.
It fails to explain the catalytic activity of protic acids and it also fails to explain the amphoteric character of ${H_2}O$, $Al{(OH)_3}$ etc.

Note: Lewis concept is more useful than the Bronsted concept. This is because many species which are acid and bases according to Bronsted and Arrhenius concepts are also covered by lewis concept.