Question

Which of the following is the weakest Lewis base?
1. \[{\rm{O}}{{\rm{H}}^ - }\]
2. \[{{\rm{H}}^ - }\]
3. \[{\rm{HC}}{{\rm{O}}_{\rm{3}}}^ - \]
4. \[{\rm{C}}{{\rm{l}}^ - }\]

Answer 1

Hint: A Lewis base behaves as a donor of a pair of electrons. In simple words, a Lewis base defines a species that donates an electron pair to result in a covalent bond. Some of the names of Lewis base chloride ion, iodide ion, etc.

Complete Step by Step Solution:
The weakest Lewis base will be the one that does not donate electrons easily.
The hydroxide ion (\[{\rm{O}}{{\rm{H}}^ - }\]) is the electron sufficient because of the presence of an electron pair. So, it can act as a donor of an electron pair to an electrophile so that a covalent bond forms. Therefore, the hydroxide ion is not the weakest Lewis base.

The hydride ion (\[{{\rm{H}}^ - }\]) can also donate an electron pair. So, it is not the weakest Lewis base. So, option 2 is not true.

Let's understand the chloride ion. Chloride ion is the weakest Lewis base as it follows the octet rule. The octet rule says that the valence shell is fully filled if it is filled with eight numbers of electrons in the valence shell. The chloride ion possesses eight numbers of electrons at the outermost shell. Therefore, the donation of electron pairs from the chloride ion is difficult. Therefore, \[{\rm{C}}{{\rm{l}}^ - }\]is the weakest Lewis base.
Therefore, option 4 is right.

Note: A Lewis acid is a substance that can accept an electron pair. They are also called as the donor of protons. They act as electrophiles, that is, they are electron deficient in nature. Some of the Lewis acids are hydrogen ion (\[{{\rm{H}}^{\rm{ + }}}\]), \[{\rm{F}}{{\rm{e}}^{{\rm{2 + }}}}\] , \[{\rm{F}}{{\rm{e}}^{{\rm{3 + }}}}\] etc.