Question

Assertion (A) : Boiling point of (C₂H₅)₂NH is lower than that of n-C₄H₉NH₂.

Reason (R) : Hydrogen bonding is much more extensive in n-C₄H₉NH₂ as compared to (C₂H₅)₂NH.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Both amines are isomers of \(\mathrm{C_4H_{11}N}\). Primary amines have two N-H bonds and form more hydrogen bonds than secondary amines, so they boil higher.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The boiling point of a liquid depends on how strongly its molecules attract each other. For amines the main attraction is hydrogen bonding through the N-H bond. Molar mass and shape also matter, but only when the compounds differ in them.

Step 2: Compare the two amines.
Diethylamine \(\mathrm{(C_2H_5)_2NH}\) and n-butylamine \(\mathrm{n\text{-}C_4H_9NH_2}\) are isomers. Both have the formula \(\mathrm{C_4H_{11}N}\) and the same molar mass of 73 g/mol. So the van der Waals forces are nearly the same, and the difference in boiling point must come from hydrogen bonding.

Step 3: Count the N-H bonds.
n-Butylamine is a primary amine. Its nitrogen carries two N-H bonds. Diethylamine is a secondary amine. Its nitrogen carries only one N-H bond.
A molecule with two N-H bonds can form more hydrogen bonds with its neighbours, so the network in n-butylamine is more extensive.

Step 4: Check the Assertion (A).
Because hydrogen bonding is weaker overall in diethylamine, it needs less energy to boil. The boiling points are about 329 K (56 \(^{\circ}\)C) for diethylamine and 351 K (78 \(^{\circ}\)C) for n-butylamine. So (A) is TRUE.

Step 5: Check the Reason (R).
The statement says that hydrogen bonding is much more extensive in n-butylamine than in diethylamine. This is correct, as shown in Step 3. So (R) is TRUE.

Step 6: Link between (A) and (R).
The lower boiling point of diethylamine is caused exactly by the less extensive hydrogen bonding. So (R) is the correct explanation of (A). Options (B), (C) and (D) are wrong because each of them either denies the truth of a statement or denies the link between them.

Final Answer:
Both (A) and (R) are true and (R) explains (A). The correct option is (A) of the paper, the first option. \[ \boxed{\text{Option 1 (A)}} \]