Question

Which of the following is a Lewis acid?
A. \[O{H^ - }\]
B. \[{H^ + }\]
C. \[{F^ - }\]
D. \[N{H_3}\]

Answer 1

Hint: Lewis acid is a substance that can accept a pair of non-bonding electrons. Lewis acid is an electron-pair acceptor.

Complete step by step answer:
Lewis acid is a chemical species that has a vacant orbital which accepts an electron pair from a Lewis base. So, Lewis acid is an electron-pair acceptor. The resulting product formed is called a Lewis adduct. Lewis base is a chemical species that has a filled orbital containing an electron which is not involved in bonding, but can form a bond with a Lewis acid to form a Lewis adduct. So, a Lewis base is an electron-pair donor.
From the above options, we can observe that \[{H^ + }\] is a Lewis base, as it can accept a pair of electrons from an electron donor (Lewis base). The term nucleophile and electrophile are more or less used interchangeably with Lewis base and Lewis acid respectively. However, nucleophilicity and electrophilicity emphasize on the kinetic aspect of reactivity, whereas Lewis basicity and Lewis acidity emphasize on the thermodynamic aspect of the formation of Lewis adduct.
The major advantage of Lewis acid-base theory is that it expands the range of acids and bases, and therefore the number of acid-base reactions.
The interaction between the Lewis base and Lewis acid is indicated by an arrow from the electrons of a Lewis base towards the Lewis acid. For example:\[{H_3}N \to B{F_3}\]
Hence, the correct answer is (B) \[{H^ + }\].

Note: A student can confuse between Lewis acid and Lewis base. Make sure to remember that a Lewis acid is an electron-acceptor while a Lewis base is an electron-donor.