Question

Which of the following compound will have highest boiling point?

• A. CH\(_3\)F
• B. CH\(_3\)CH\(_2\)F
• C. CH\(_3\)Cl
• D. CH\(_3\)I
• E. CH\(_3\)Br

Hint:

Boiling point trend for alkyl halides: RI $>$ RBr $>$ RCl $>$ RF.

Answer 1

The Correct Option is D

Step 1: Understanding Factors Affecting Boiling Point:
The boiling point of a substance is determined by the strength of its intermolecular forces. For non-ionic compounds like haloalkanes, the primary intermolecular forces are dipole-dipole interactions and London dispersion forces (a type of van der Waals force).
Step 2: Analysis of Intermolecular Forces:
1. Molecular Mass and Size: London dispersion forces increase with the size of the molecule and the number of electrons (which correlates with molecular mass). Larger molecules have more polarizable electron clouds, leading to stronger temporary dipoles and stronger attraction.
2. Dipole Moment: While halomethanes are polar, the effect of the dipole moment on the boiling point is generally less significant than the effect of London dispersion forces, especially as the halogen atom gets larger.
Let's compare the molecular masses (approximate, in g/mol):
(A) CH\(_{3}\)F: 12 + 3 + 19 = 34
(B) CH\(_{3}\)CH\(_{2}\)F: 24 + 5 + 19 = 48
(C) CH\(_{3}\)Cl: 12 + 3 + 35.5 = 50.5
(E) CH\(_{3}\)Br: 12 + 3 + 80 = 95
(D) CH\(_{3}\)I: 12 + 3 + 127 = 142
Among the methyl halides (CH\(_{3}\)X), the molecular mass increases significantly from F to I. This leads to a dramatic increase in the strength of London dispersion forces. CH\(_{3}\)I has the largest molecular mass and the largest, most polarizable electron cloud. Therefore, it experiences the strongest London dispersion forces among all the given options. Even though CH\(_{3}\)CH\(_{2}\)F has more carbons, its molecular mass is much lower than that of CH\(_{3}\)Br and CH\(_{3}\)I. The boiling point trend for methyl halides is: CH\(_{3}\)I>CH\(_{3}\)Br>CH\(_{3}\)Cl>CH\(_{3}\)F.
Note: The provided answer key states (E) CH\(_{3}\)Br is the correct answer. This is factually incorrect based on established chemical principles and experimental data (Boiling point of CH\(_{3}\)I is 42.4 \(^\circ\)C, while for CH\(_{3}\)Br it is 3.6 \(^\circ\)C). The correct answer should be CH\(_{3}\)I due to its significantly higher molecular mass leading to stronger intermolecular dispersion forces. The solution follows the chemically correct reasoning.
Step 3: Final Answer:
CH\(_{3}\)I has the highest molecular mass and size, leading to the strongest London dispersion forces and thus the highest boiling point. This corresponds to option (D).