Question

Which of the following statement is correct -

• A. Decreasing order of density of alkyl halides is RI > RBr > RCI > RF
• B. The stability order of alkyl halides is RF > RCI > RBr>RI
• C. Among isomeric alkyl halides the decrease in boiling point 1°>2°>3°
• D. All are correct

Answer 1

Answer: (D)

All are correct

Answer 2

Let's analyze each statement:

(1) Decreasing order of density of alkyl halides is RI > RBr > RCI > RF. For a given alkyl group R, as the atomic mass of the halogen increases, the molar mass of the alkyl halide increases significantly. Although the volume also increases, the increase in mass dominates, leading to an increase in density. The atomic masses of halogens are F (19 u), Cl (35.5 u), Br (79.9 u), and I (126.9 u). Thus, the molar mass of RI > RBr > RCl > RF. Consequently, the density generally follows the same order: $\rho(\text{RI}) > \rho(\text{RBr}) > \rho(\text{RCl}) > \rho(\text{RF})$. This statement is correct.

(2) The stability order of alkyl halides is RF > RCI > RBr > RI. The stability of alkyl halides is related to the strength of the carbon-halogen (C-X) bond. Bond strength is inversely proportional to bond length. As the size of the halogen atom increases down the group, the C-X bond length increases, and the bond dissociation energy decreases. The order of bond dissociation energies for C-X bonds is C-F > C-Cl > C-Br > C-I. Therefore, the stability order of alkyl halides is RF > RCl > RBr > RI. This statement is correct.

(3) Among isomeric alkyl halides the decrease in boiling point 1° > 2° > 3°. Boiling point depends on the strength of intermolecular forces, primarily Van der Waals forces (London dispersion forces) in alkyl halides. For isomeric alkyl halides (same molecular formula, different structures), the extent of branching affects the surface area. A more branched molecule has a smaller surface area, leading to weaker Van der Waals forces and a lower boiling point. Primary alkyl halides (1°) are less branched than secondary (2°), which are less branched than tertiary (3°). Thus, the order of boiling points among isomeric alkyl halides is generally 1° > 2° > 3°. This statement is correct.

(4) All are correct. Since statements (1), (2), and (3) are all correct, this statement is also correct.