Question

What is the order of basic character of ethyl amine (I), diethyl amine (II) and triethyl amine (III)?

• A. I > II > III
• B. II > III > I
• C. III > II > I
• D. III > I > II

Hint:

In water, basicity of amines depends on both inductive effect and solvation of the protonated amine.

Answer 1

The Correct Option is B

Step 1: In aqueous solution, basicity of amines depends on three factors: electron donation by alkyl groups (+I effect increases basicity), steric hindrance around nitrogen, and hydrogen bonding of the protonated ammonium ion with water.

Step 2: A primary amine like ethylamine has only one alkyl group, so the +I effect is weak, though it has two N-H bonds available for solvation.

Step 3: A secondary amine like diethylamine has two alkyl groups giving a stronger +I effect, moderate steric bulk, and still one N-H bond left for hydrogen bonding, so it is the most basic of the three in water.

Step 4: A tertiary amine like triethylamine has the strongest +I effect but no N-H bond and suffers the most steric hindrance, lowering its basicity below the secondary amine but still above the primary amine.

Step 5: The observed order of basicity in water is diethylamine > triethylamine > ethylamine, that is, II > III > I.

\[\boxed{\text{II} > \text{III} > \text{I}}\]