Answer
a)2<3<1
b)3<2<1
Work Step by Step
Factors that affect the basicity of amines are as follows.
Hybridization:
If the percentage of s-character increases, then basicity decreases.
Inductive effect:
If a nitrogen atom is attached to electron donating group, then basicity increases.
Resonance:
If a lone pair on nitrogen involves in delocalization through resonance, then basicity decreases.
Aromaticity:
If a lone pair on nitrogen participates in delocalization to get aromatic character, then basicity decreases.
(a)Isoniazid has three nitrogen atoms they are labelled as 1, 2, 3. Now we have to decide
which nitrogen atom is most basic.
The lone pair electron on nitrogen-1 is localized and it is hybridized. So, nitrogen-1 is most basic.
The lone pair on nitrogen-2 is delocalized with the adjacent carbonyl group through resonance which leads to decrease in electron density on the nitrogen and makes it least basic.
The lone pair on nitrogen-3 is not participating in aromatic pi system (i.e. localized). However, nitrogen-3 is hybridized (Increasing percent of s character leads to decrease in basicity). So, nitrogen-3 is less basic than nitrogen-1, and more basic than nitrogen-2.
Therefore, the increasing order of basicity is, .2<3<1
b)It has three nitrogen atoms they are labelled as 1, 2, 3. Now we have to decide which nitrogen atom is most basic
The lone pair electron on nitrogen-1 is localized and it is hybridized. So, nitrogen-1 is most basic.
The lone pair on nitrogen-2 is not participating in delocalization to get the aromatic character (i.e. localized). However, nitrogen-2 is hybridized (Increasing percent of s character leads to decrease in basicity). So, nitrogen-2 is less basic than nitrogen-1.
The lone pair on nitrogen-3 is participating in delocalization to get the aromatic character. It leads to decrease in electron density on the nitrogen-3 and makes it least basic.
Therefore, the increasing order of basicity is, 3<2<1
