Question

Given an example of a base which is both a weak Bronsted base and a weak Lewis base

Answer 1

Bronsted base

• Bronsted-Lowery theory describes acid base interaction in terms of proton transfer.
• Bronsted base are those which are capable accepting a protons.
• Bronsted base should have at lest one lone pair to form bond with the proton
• Example: $\underset{\mathrm{Bronsted}\mathrm{acid}}{\mathrm{HCl}}+\underset{\mathrm{Bronsted}\mathrm{base}}{{\mathrm{NH}}_3}\rightleftharpoons \underset{\mathrm{Conjugate}\mathrm{acid}}{{\mathrm{NH}}_4^{+}}+\underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{Cl}}^{-}}$
• Hydrochloric acid is the Brønsted-Lowry acid as it donates a hydrogen ion.
• Ammonia is the Brønsted-Lowry base as it accepts the hydrogen ion.
• From the above reaction we have, the ammonium ion is the conjugate acid of the base ammonia and the chloride ion is the conjugate base of hydrochloric acid.

Diagram representing Bronsted-Lowery theory

Lewis base

• Lewis base is a species which can donate lone pair of electrons.
• They act as a nucleophiles.
• They have electron rich $\mathrm{\pi }$systems(such as benzene, ethyne, and ethene).
• Example: ${\mathrm{CH}}^{3–}$ and ${\mathrm{OH}}^{–}$ are identified as Lewis bases because they are capable of donating electron pair.

Diagram representing Lewis concept of acid and base.

Example of weak Lewis base and Bronsted base

• An example of a weak Lewis base and Bronsted base${\mathrm{Cl}}^{-}$ions
• Chlorine ion is a weak Lewis base because it do not donate lone pair of electrons easily due to completion of its octet, it also acts a a weak Bronsted base because it's forms strong Bronsted acid( Hydrochloric acid)

Therefore, ${\mathrm{Cl}}^{-}$ions have little tendency to gain protons as their octet is complete, therefore, they act as weak Lewis base and Bronsted base.