Arrange the following in decreasing order of basic strength.

(A) \(C_6H_5NH_2\)

(B) \(C_6H_N(CH_3)_2\)

(C) \((C_2H_5)_2NH\)

(D) \(CH_3NH_2\)

Choose the correct answers from the options given below:

C, D, B, A

The correct answer is option 2. C, D, B, A.

The basicity of an amine is determined by how easily the nitrogen atom can donate its lone pair of electrons to accept a proton (H⁺). The more available the lone pair on nitrogen is, the stronger the base.

Several factors influence the basicity of amines:

Inductive Effect: Alkyl groups are electron-donating through the inductive effect (+I effect). They increase the electron density on the nitrogen atom, making the lone pair more available for protonation, which increases basicity.

Resonance Effect: In aromatic amines (amines where nitrogen is attached to a benzene ring), the lone pair on nitrogen can be delocalized into the benzene ring. This delocalization reduces the availability of the lone pair for protonation, decreasing basicity

Steric Effect: Bulky groups around the nitrogen atom can hinder the approach of protons, which can reduce the basicity.

Analysis of the Compounds:

(C) \((C_2H_5)_2NH\) (Diethylamine):

Inductive Effect: The ethyl groups are electron-donating, increasing the electron density on the nitrogen, making the lone pair more available for protonation.

Basicity: High basicity due to the strong electron-donating effects of the two ethyl groups.

(D) \(CH_3NH_2\) (Methylamine):

Inductive Effect: The methyl group is electron-donating, increasing the electron density on the nitrogen, making the lone pair more available for protonation.

Basicity: Less basic than diethylamine because it has only one alkyl group donating electron density, but still more basic than aromatic amines.

(B) \(C_6H_5N(CH_3)_2\) (N,N-Dimethylaniline):

Inductive Effect: The two methyl groups are electron-donating, increasing the electron density on nitrogen. However, this effect is somewhat offset by the resonance effect.

Resonance Effect: The lone pair on nitrogen can participate in resonance with the benzene ring, reducing its availability for protonation.

Basicity: The presence of two methyl groups increases basicity relative to aniline (A), but the resonance effect reduces it relative to aliphatic amines (C and D).

(A) \(C_6H_5NH_2\) (Aniline):

Resonance Effect: The lone pair on nitrogen is delocalized into the benzene ring through resonance, making it less available for protonation.

Basicity: The least basic among the given compounds because the resonance effect strongly reduces the availability of the lone pair.

Order of Basic Strength: Given the effects discussed, the order of decreasing basic strength is as follows:

(C) \((C_2H_5)_2NH\): The strongest base due to the two electron-donating ethyl groups.

(D) \(CH_3NH_2\): Slightly less basic than diethylamine due to having only one electron-donating methyl group.

(B) \(C_6H_5N(CH_3)_2\): Less basic than the aliphatic amines because of resonance with the benzene ring, but more basic than aniline due to the electron-donating methyl groups.

(A) \(C_6H_5NH_2\): The weakest base because the lone pair on nitrogen is significantly delocalized into the benzene ring.

Conclusion: The correct order of decreasing basic strength is C > D > B > A.

Thus, the correct option is Option 2: C, D, B, A.