Question

Rank the compounds in each group in order of increasing basicity.

(a)

(b)

Short answer

The order of increasing basicity

(a)

(b)

Step-by-step solution

Trend in basicity

The greater the tendency of nitrogen to donate its electrons, the greater is its basicity.

If a compound containing nitrogen is involved in conjugation, then its ability to donate its lone pair of electrons decreases, making the compound less basic.

Order of increasing basicity

(a)

• The nitrogen on indole is the least basic nature because delocalized electron pair on nitrogen.

Delocalized lone pair of indole

• $s{p}^2$ hybridized nitrogen is more electronegative than $s{p}^3$ hybridized nitrogen, due to which compound containing $s{p}^2$ hybridized nitrogen is less basic than $s{p}^3$ hybridized one.

N atom hybridization

• Hence, the most basic amine is piperidine.

Therefore, the order of increasing basicity is as follows:

order of increasing basicity

(b)

• The substituents which are electron-donating in nature, increase the electron density on nitrogen, making them more basic and the substituents that are electron-withdrawing in nature, decrease the electron density on nitrogen, making them less basic.

• Among the substituents given, the nitro group is most electron-withdrawing in nature and it decreases the electron density on nitrogen, making it least basic.

Nature of nitro group

• Methyl substituent is electron-rich, so it makes the p-toluidine most basic.

Nature of methyl group

Therefore, the order of increasing basicity is as follows:

order of increasing basicity