Question

Which of the following is Lewis acid?
A.${{\text{C}}_2}{{\text{H}}_5}{\text{OH}}$
B.${\text{B}}{{\text{F}}_3}$
C.${\text{C}}{{\text{l}}^ - }$
D.${\text{N}}{{\text{H}}_3}$

Answer 1

Lewis acids are the electron pair acceptor. Atom will accept electrons if it is either deficient in electrons, that is, its octet is incomplete or it has a vacant d orbital to accept electrons.

Complete step by step solution:
There are various definitions for acid and bases, according to Arrhenius an acid is a substance which gives hydrogen ions in aqueous solution. A base is a substance which gives hydroxide ion upon dissociation in an aqueous medium. The option A is an alcohol which gives hydrogen ion when dissociated in water. Hence ethanol is an Arrhenius acid and not a Lewis acid. The solvent for dissociation was taken water only aqueous by Arrhenius.
A Lewis acid is an electron pair acceptor. As we know boron has atomic number 5 and has 3 electrons in its valence shell. Even after the formation of 3 bonds with the fluorine it will have only 6 electrons in its valence shell. Hence its octet is still incomplete. So boron trifluoride takes electrons from the other species to complete its octet. So it is Lewis acid.

Hence, the correct option is B.

Note: Now both chloride ion and ammonia are Lewis bases. Chloride ion has a negative charge on it and hence can easily donate its lone pair electron. An electron donor is termed as base according to Lewis acid base concept. Nitrogen has 5 electrons in its valence shell. Even after formation of 3 bonds with hydrogen in ammonia, it still has the lone pair of electrons to donate. Ammonia is also a Lewis base.