Question

Which of the following is a Lewis acid?
a.) $B{\left( {C{H_3}} \right)_3}$
b.) NaH
c.) $N{F_3}$
d.) $P{H_3}$

Answer 1

Hint: In this question, firstly we will learn about the term Lewis acid and the concept behind it including all the required information. Then we will give the explanation of the answer that is, why it is a Lewis acid. With this information, we will approach our answer.

Complete step by step solution: Lewis in 1923 wrote that an acid is that substance which employs a lone pair of an electron from another molecule which completes its own atom’s stable group. The acid-base theory of Bronsted-Lowry was also published in 1923. These are distinct theories but complimentary.
One pair of electrons is accepted by the Lewis acid also known as an electrophile. They have vacant orbits in it. Lewis acids are electrophilic which means that electrons are attracted by them. Lowest unoccupied molecular orbital or LUMO is used by an acid when they bond with a base.
(A) $B\left( {C{H_3}} \right)3$ Is a Lewis acid because it can accept a lone pair as we know that Lewis acids can accept the lone pair and have vacant orbital. In the outermost shell of Boron, it has 6 electrons and in triethyl borane- 3 electrons are shared by the borane with the methyl group of 3 carbons. Hence, Boron trichloride is stronger Lewis acid than the triethyl borane because the electron density is withdrawn more towards themselves that’s why chlorine atoms are electronegative. By the mutual sharing of electrons, they form a covalent bond between boron and carbon.

Note: If there is an incomplete octet of electrons in an atom, ion or a molecule they act as a Lewis acid. Lewis found that an electron pair which acts as an acceptor is called acid and an electron pair which acts as a donor is called a base.