Question

Which among the following amines is most basic in aqueous medium?

• A. $(C_2H_5)_2NH$
• B. $(C_2H_5)_3N$
• C. $C_2H_5NH_2$
• D. $NH_3$
• A. \((CH_3)_2NH\)
• B. \(CH_3NH_2\)
• C. \((CH_3)_3N\)
• D. \(NH_3\)
• A. \((CH_3)_2NH\)
• B. \(H_3NH_2\)
• C. \((CH_3)_3N\)
• D. \(NH_3\)
• A. Conjugation factor
• B. Inductive effect
• C. Solvation effect
• D. Steric hindrance
• A. 2,4-dinitrophenol
• B. NaNO₂ + HCl mixture
• C. HNO₃ + H₂SO₄ mixture
• D. Benzenesulphonyl chloride

Answer 1

The Correct Option is C

Amines are basic because the nitrogen atom possesses a lone pair of electrons capable of accepting protons (H⁺). In aqueous solutions, amine basicity is influenced by alkyl group inductive effects, solvation, and steric hindrance.

Let's examine the provided amines:

• $(C_2H_5)_2NH$ (Diethylamine): A secondary amine with two ethyl groups. The inductive effect from these groups increases basicity. However, greater steric hindrance impedes water solvation.
• $(C_2H_5)_3N$ (Triethylamine): A tertiary amine with three ethyl groups. While the inductive effect is strong, significant steric hindrance severely limits solvation, reducing basicity.
• $C_2H_5NH_2$ (Ethylamine): A primary amine with one ethyl group. It exhibits a moderate inductive effect. Solvation is more effective than in secondary and tertiary amines due to minimal steric hindrance.
• $NH_3$ (Ammonia): Lacks alkyl groups, resulting in a low inductive effect. Its small size allows for excellent solvation.

Based on these factors, $C_2H_5NH_2$ (Ethylamine) demonstrates higher basicity in aqueous solution than the other compounds. Ethylamine achieves a favorable balance between the inductive effect of an alkyl group and efficient solvation without excessive steric hindrance.

Answer 2

The least basic amine among the given options in an aqueous medium is identified by examining the influence of alkyl groups on basicity:

• Methylamine (CH₃NH₂) : This primary amine exhibits considerable basicity due to minimal steric hindrance and efficient solvation.
• Dimethylamine ((CH₃)₂NH) : A secondary amine that is basic, but its basicity is somewhat impacted by increasing steric hindrance.
• Trimethylamine ((CH₃)₃N) : As a tertiary amine, it experiences significant steric hindrance, resulting in lower basicity compared to primary and secondary amines.
• Ammonia (NH3) : While lacking alkyl groups, ammonia serves as a baseline for basicity comparisons.

Consequently, trimethylamine ((CH₃)₃N) is determined to be the least basic in aqueous conditions because substantial steric hindrance impedes effective solvation and protonation.

Answer 3

In the gaseous phase, amine basicity is primarily governed by the lone pair availability on nitrogen. Solvation and steric effects, influential in aqueous solutions, are less significant here. The provided options are:

• Dimethylamine: $(CH_3)_2NH$
• Methylamine: $CH_3NH_2$
• Trimethylamine: $(CH_3)_3N$
• Ammonia: $NH_3$

Alkyl groups increase electron density at the nitrogen atom via the +I inductive effect. A greater number of alkyl groups enhances electron availability, thus increasing basicity. Consequently, tertiary amines, possessing three electron-donating groups, are generally more basic than secondary and primary amines. Considering the options:

• Methylamine ($CH_3NH_2$) features one methyl group.
• Dimethylamine ($(CH_3)_2NH$) contains two methyl groups.
• Trimethylamine ($(CH_3)_3N$) has three methyl groups.
• Ammonia ($NH_3$) lacks methyl groups.

Accordingly, trimethylamine ($(CH_3)_3N$) exhibits the highest basicity in the gaseous phase due to its three methyl groups maximizing the nitrogen lone pair's availability.

Therefore, the most basic amine in the gaseous medium is $(CH_3)_3N$.

Answer 4

Factors influencing the fundamental basicity of amines in water include:

1. Inductive Effect: Alkyl group electron donation or withdrawal alters the accessibility of the nitrogen's lone pair for proton acceptance.
2. Solvation Effect: Enhanced solvation by water molecules strengthens basicity through stabilization of the protonated amine.
3. Steric Hindrance: Bulky alkyl groups impede proton access to the nitrogen, thereby diminishing basicity.
4. Conjugation Factor: Conjugation, involving pi bonds, has minimal impact on amine basic strength as it does not directly influence protonation.

Therefore, conjugation is not a determinant of amine basicity.

Answer 5

Benzenesulphonyl chloride, known as Hinsberg's reagent, is employed in organic chemistry for the differentiation of primary, secondary, and tertiary amines. The key to this separation lies in the distinct reactions observed:

• Primary amines: React to form a sulphonamide that is soluble in alkaline solutions due to its acidic nature, allowing it to dissolve in aqueous NaOH.
• Secondary amines: Produce a sulphonamide that is insoluble in alkali, leading to the formation of a precipitate that remains undissolved in NaOH owing to the lack of an acidic hydrogen.
• Tertiary amines: Exhibit no reaction with benzenesulphonyl chloride, remaining unaltered in the reaction mixture.

Consequently, this differential reactivity enables the effective identification and separation of amine classes based on their interactions with Hinsberg's reagent, benzenesulphonyl chloride.