Considering the basic strength of amines in an aqueous solution, which one has the smallest pK_b value?

A. (CH₃)₃N

B. C₆H₅NH₂

C. (CH₃)₂NH

D. CH₃NH₂

To determine which amine has the smallest pK_b value in an aqueous solution, we need to consider the basicity of each amine. The pK_b value is a measure of how well a compound can accept protons (H⁺) in water; the lower the pK_b, the stronger the base. Let's analyze each option provided.

Understanding the Amines

Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. The basicity of amines is influenced by the electron-donating or electron-withdrawing effects of these substituents.

• (CH₃)₃N (Trimethylamine): This is a tertiary amine. The three methyl groups are electron-donating, which enhances the basicity by stabilizing the positive charge on the nitrogen when it accepts a proton.
• C₆H₅NH₂ (Aniline): This is an aromatic amine. The phenyl group is electron-withdrawing due to resonance, which decreases the basicity of the nitrogen atom, making it less likely to accept a proton.
• (CH₃)₂NH (Dimethylamine): This is a secondary amine. It has two methyl groups that provide some electron-donating effects, making it more basic than aniline but less than trimethylamine.
• CH₃NH₂ (Methylamine): This is a primary amine. The single methyl group enhances its basicity, but it is generally less basic than tertiary amines.

Comparing Basicity

To summarize the basicity of these amines:

• Trimethylamine (Tertiary) - Strong base, likely the highest basicity.
• Dimethylamine (Secondary) - Moderate basicity, less than trimethylamine.
• Methylamine (Primary) - Basic, but less than dimethylamine.
• Aniline (Aromatic) - Weak base due to resonance stabilization of the lone pair on nitrogen.

pK_b Values

The pK_b values generally follow this trend:

• Trimethylamine: Lowest pK_b (strongest base)
• Dimethylamine: Higher pK_b than trimethylamine
• Methylamine: Higher pK_b than dimethylamine
• Aniline: Highest pK_b (weakest base)

Based on this analysis, the amine with the smallest pK_b value, indicating the strongest basicity, is (CH₃)₃N (Trimethylamine). Therefore, the answer to your question is A. (CH₃)₃N.