Question

The descending order of basicity of following amines is :

Choose the correct answer from the options given below :

• A. \(B>E>D>A>C\)
• B. \(E>D>B>A>C\)
• C. \(E>D>A>B>C\)
• D. \(E>A>D>C>B\)

Hint:

When comparing the basicity of amines, consider whether they are aliphatic or aromatic. For aliphatic amines, more alkyl groups generally increase basicity. For aromatic amines, the presence and nature of substituents on the ring significantly affect basicity. Electron-donating groups increase basicity, while electron-withdrawing groups decrease it.

Answer 1

The Correct Option is B

To determine the descending order of basicity of the given amines, we should consider the electronic effects of substituents and the structure of the amines.

1. Aromatic amines generally have lower basicity than aliphatic amines because the lone pair on the nitrogen in aromatic amines gets delocalized into the aromatic ring, reducing its availability for protonation.
2. Methoxy substituent (—OCH₃) in compound (B) is an electron-donating group. It increases the electron density on the nitrogen, making compound (B) more basic than other aromatic amines without any electron-donating groups.
3. Nitro substituent (—NO₂) in compound (C) is a strong electron-withdrawing group, which decreases the basicity significantly by withdrawing electron density from the ring and hence from the nitrogen as well.
4. Dimethylamine (E) is an aliphatic amine and is usually more basic due to direct availability of the lone pair on the nitrogen atom for protonation, unaffected by resonance.
5. Methylamine (D) is also an aliphatic amine and is generally more basic than aromatic amines.

Based on this understanding, we can order the given amines in terms of decreasing basicity:

• (E) (CH₃)₂NH - most basic due to direct lone pair availability.
• (D) CH₃NH₂ - basic due to being an aliphatic amine.
• (B) Methoxy Aniline - increased basicity due to electron-donating group.
• (A) Aniline - basicity due to lone pair delocalization.
• (C) Nitro Aniline - least basic due to strong electron-withdrawing group.

Thus, the correct descending order of basicity is E>D>B>A>C, matching the correct answer choice given in the question.

Answer 2

Step 1: Understand the factors affecting the basicity of amines.
The basicity of an amine depends on the availability of the lone pair of electrons on the nitrogen atom for protonation. 
Factors that increase the electron density on the nitrogen atom increase basicity, while factors that decrease it decrease basicity. 
These factors include:
Alkyl groups: Electron-donating alkyl groups (+I effect) increase the electron density on the nitrogen atom, making the amine more basic.
Aryl groups: The lone pair of electrons on the nitrogen atom in aromatic amines is delocalized into the benzene ring through resonance, making them less basic than aliphatic amines.
Electron-donating groups on the aryl ring: These groups (+I or +M effect) increase the electron density on the nitrogen atom, increasing the basicity of aromatic amines.
Electron-withdrawing groups on the aryl ring: These groups (-I or -M effect) decrease the electron density on the nitrogen atom, decreasing the basicity of aromatic amines.
Steric factors: In some cases, steric hindrance around the nitrogen atom can affect protonation and thus basicity. 
Step 2: Analyze the basicity of each amine.
(A) Aniline (\( C_6H_5NH_2 \)): The lone pair on nitrogen is delocalized into the benzene ring, making it weakly basic. 
(B) p-Methoxy aniline (\( p-MeOC_6H_4NH_2 \)): 
The methoxy group (\( -OCH_3 \)) is an electron-donating group (+M and -I effect, with +M dominating). It increases the electron density on the nitrogen atom through resonance, making p-methoxy aniline more basic than aniline. 
(C) p-Nitro aniline (\( p-NO_2C_6H_4NH_2 \)): The nitro group (\( -NO_2 \)) is a strong electron-withdrawing group (-M and -I effect). It decreases the electron density on the nitrogen atom through resonance, making p-nitro aniline much less basic than aniline. 
(D) Methylamine (\( CH_3NH_2 \)): Methyl group is an electron-donating alkyl group (+I effect), making methylamine more basic than aniline. 
(E) Dimethylamine (\( (CH_3)_2NH \)): Two electron-donating methyl groups (+I effect) increase the electron density on the nitrogen atom further compared to methylamine. In the gas phase, dimethylamine is more basic than methylamine. In aqueous solution, solvation effects also play a role, but generally, secondary aliphatic amines are more basic than primary aliphatic amines. 
Step 3: Arrange the amines in descending order of basicity.
Based on the above analysis:
Dimethylamine (E) is the most basic due to two electron-donating methyl groups and favorable solvation in aqueous solution.
Methylamine (D) is more basic than aniline due to one electron-donating methyl group.
p-Methoxy aniline (B) is more basic than aniline due to the electron-donating methoxy group.
Aniline (A) is less basic due to the delocalization of the lone pair into the benzene ring.
p-Nitro aniline (C) is the least basic due to the strong electron-withdrawing nitro group.
The descending order of basicity is: E>D>B>A>C. 
Step 4: Match the order with the given options.
The order E>D>B>A>C matches option (2).