Question

Given below are two statements labelled as Assertion (A) and Reason (R).

Assertion (A): The boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF.

Reason (R): The van der Waals forces of attraction decrease in the order RI > RBr > RCl > RF.

Select the correct answer from the codes (A), (B), (C) and (D) given below.

• 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:

For the same alkyl group, a heavier halogen gives stronger van der Waals forces, so the boiling point rises from RF to RI. Both statements are true and R explains A.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Boiling point tells us how much energy is needed to pull molecules apart in the liquid. For haloalkanes with the same alkyl group, the main attraction between molecules is the van der Waals force (dipole-dipole plus dispersion force).

Step 2: Key Formula or Approach:
1. Compare the size and mass of the halogen atoms: \(\mathrm{I} > \mathrm{Br} > \mathrm{Cl} > \mathrm{F}\).
2. A larger halogen has more electrons and a bigger, more polarisable electron cloud. This gives stronger van der Waals forces and a higher boiling point.

Step 3: Check the Assertion (A).
For the same alkyl group R, the boiling points of the halides rise as the halogen gets heavier. So the order is \(\mathrm{RI} > \mathrm{RBr} > \mathrm{RCl} > \mathrm{RF}\), which means the boiling points decrease from RI to RF. For example, \(\mathrm{CH_3I}\) boils at about 316 K, \(\mathrm{CH_3Br}\) at 277 K, \(\mathrm{CH_3Cl}\) at 249 K and \(\mathrm{CH_3F}\) at 195 K. So (A) is TRUE.

Step 4: Check the Reason (R).
The iodine atom is the largest and most polarisable, so the van der Waals forces in RI are the strongest. The forces get weaker through RBr and RCl, and are weakest in RF. So the order \(\mathrm{RI} > \mathrm{RBr} > \mathrm{RCl} > \mathrm{RF}\) is correct and (R) is TRUE.

Step 5: Is R the explanation of A?
Yes. The boiling point order exists because the intermolecular forces decrease in the same order. The reason directly causes the assertion.

Step 6: Why the other options are wrong.
Option (B) says R does not explain A, but it does. Option (C) says R is false, but R is true. Option (D) says A is false, but A is true.

Final Answer:
Both statements are true and R explains A. This is option (A).
\[ \boxed{\text{(A)}} \]