Question

Question 5 Which one has the largest pkb (weak base)? Op-Methoxyaniline Op-Nitroaniline Triethylamine Methylamine Piperidine

Answer 1

The structure of the molecules given on the question are as follows:

NH ₂ dl NH, b) a) O 0 NO, c) OCH 1600 ar M 10 d) CH Nik H - N - CHS GHS e) #

Generally the base with highest pKb value is the weakest base. A lewis base is a specie which has a tendency to donate its lone pair to a proton. More easily it donates the lone pair, stronger the base it is. Tendency of a base to donate its lone pair depends on its electron density. Groups which increases the electron density around the donor atom (i.e. electron releasing groups), makes it more basic while groups which decreases the electron density around the donor atom (i.e. electron withdrawing groups) decreases its basic strength.

In amines donor atom is nitrogen. Generally, aliphatic amines are more basic than aromatic amines because in aromatic amines the lone pair present on the nitrogen is busy in resonance with benzene ring and hence is not available for donation. That means options (c), (d) and (e) can be eliminated because they all are examples of aliphatic amines and hence will be comparatively more basic.

Among remaining options, one is p-methoxyaniline which has an electron releasing -OCH3 group attached to the ring which increases the electron density on the nitrogen hence making it more basic.

While second option is p-nitroaniline, which has a very strong electron withdrawing -NO2 group attached to the benzene ring which decreases the electron density on the nitrogen hence decreasing the basic strength of the aniline.

​​​​​​So, the correct choice is option (b) i.e. p-nitroaniline.

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