Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Aniline is less basic than (a) 4-nitroaniline (b) 4-aminobenzaldehyde (c) anilinium hydrochloride (d) dimethyl amine

Step-by-Step Solution

Answer

Aniline is less basic than dimethylamine.

Step 1: Understand Basicity in Amines

Basicity in amines is determined by the availability of the lone pair of electrons on the nitrogen atom for donation. This availability can be affected by resonance and inductive effects.

Step 2: Analyze Aniline's Structure

Aniline (C6H5NH2) has a nitrogen atom bonded to a benzene ring. The lone pair on nitrogen can delocalize into the benzene ring due to resonance, reducing its availability to donate protons, thus decreasing aniline's basicity.

Step 3: Evaluate 4-Nitroaniline

In 4-nitroaniline, the nitro group is an electron-withdrawing group through resonance and induction, which further reduces the basicity compared to aniline due to its stronger withdrawing nature.

Step 4: Evaluate 4-Aminobenzaldehyde

In 4-aminobenzaldehyde, the aldehyde group is an electron-withdrawing group, but through resonance, it is less effective than the nitro group. Aniline's basicity is more influenced by the electron-withdrawing character of such groups.

Step 5: Evaluate Anilinium Hydrochloride

Anilinium hydrochloride is already a salt where the aniline molecule is protonated, making it less basic than aniline itself, as the ion does not have a free lone pair.

Step 6: Evaluate Dimethylamine

Dimethylamine (CH3)2NH is an aliphatic amine with no resonance with an aromatic ring and has two methyl groups which donate electrons by induction, increasing its basicity compared to aniline.

Step 7: Compare Basicities and Conclude

Compared to aniline, dimethylamine is more basic due to electron-donating groups and lack of resonance stabilization of the lone pair. Aniline is less basic than dimethylamine due to its resonance effect with the benzene ring.

Key Concepts

Resonance Effect in Aniline

Aniline, a compound with the formula \( C_6H_5NH_2 \), is a primary aromatic amine. Its structure includes a nitrogen atom bonded directly to a benzene ring. One of the hallmark features of aniline is the resonance effect, which plays a significant role in its chemical behavior, particularly its basicity. In resonance, the lone pair of electrons on the nitrogen atom can delocalize into the benzene ring.

This delocalization leads to a decrease in the electron density on the nitrogen. As a result, the lone pair becomes less available for protonation, reducing the basicity of aniline. This resonating characteristic is unique to aromatic amines like aniline where the -electron cloud extends to the nitrogen atom.

Resonance in aniline is a stabilizing factor; however, it comes at the cost of reducing basicity. While the resonance effect introduces stability, it makes it more energetically costly for aniline to act as a base, compared to aliphatic amines without such interactions.

Inductive Effect in Amines

The inductive effect refers to the transmission of charge through a chain of atoms in a molecule, resulting from a polar bond's electronegativity differences. This effect can either be electron-donating or electron-withdrawing and significantly influences the basicity of amines.

In aliphatic amines, alkyl groups can donate electrons to the nitrogen atom through the +I (positive inductive effect). This electron-donating effect enhances the electron density on the nitrogen, making its lone pair more available for protonation, thus increasing basicity. A prime example is dimethylamine \((CH_3)_2NH\), which shows a higher basicity than aniline due to the presence of two methyl groups that donate electrons.

In contrast, in situations where electron-withdrawing groups are present, such as in nitroanilines, the -I (negative inductive effect) reduces the electron density on the nitrogen atom. This reduced electron density makes the lone pair less available, thus decreasing the basicity. This is evident in 4-nitroaniline, where the nitro group significantly impacts basicity through its inductive withdrawing effect.

Electron-Donating and Withdrawing Groups

Substituents on an amine can drastically alter its basicity due to their abilities to donate or withdraw electrons. These groups interact with the amine's nitrogen atom through either resonance or inductive effects.

Electron-donating groups (EDGs) increase the electron density of the nitrogen atom, enhancing the ability to donate a lone pair. Alkyl groups, such as those in dimethylamine \((CH_3)_2NH\), are classical examples of EDGs. They donate electrons through hyperconjugation and the inductive effect, making the amine more basic.

On the other hand, electron-withdrawing groups (EWGs), like the nitro group in 4-nitroaniline, pull electron density away from the nitrogen. This can occur through both inductive and resonance effects, leading to reduced basicity. EWGs like the nitro group create a significant decrease in the availability of the lone pair for protonation, further lowering the basicity compared to other amines without such substituents.

Understanding these effects is crucial in predicting the behavior and reaction outcomes of amines, as different groups play pivotal roles in defining the chemical properties and reactivity of these compounds.