Compare Boiling Points of Halogenoethane

Boiling point increases from chloroethane to iodoethane which is proportionate to molar mass and electron cloud size.

Hence dominant intermolecular forces will be instantaneous dipole - induced dipole (id-id) attractions.

Let's go through the statements to see which are correct and relevant.

1. the electronegativity difference between the halogen and carbon increases from C-Cl to C-I

Cl is more electronegative than I.

Hence C-Cl should have a bigger electronegativity difference than C-I.

Electronegativity difference decrease from C-Cl to C-I instead, hence this statement is false.

2. the strength of permanent dipole-permanent dipole attraction increases from C-Cl to C-I

C-Cl bond is more polar hence pd-pd attraction for C-Cl should be stronger.

Strength of pd-pd attraction should decrease from C-Cl to C-I, this statement is false.

3. the strength of instantaneous dipole-induced dipole attraction increases from CH₃CH₂Cl to CH₃CH₂I

From chloroethane to iodoethane, electron cloud size gets bigger and more polarisable.

Instantaneous dipole-induced dipole attraction gets stronger, more energy needed to overcome hence boiling point increases.

This is consistent with the data hence statement 3 is correct.

4. the bond energy of C-X bond decreases from C-Cl to C-I

Bond energy decreases from C-Cl to C-I but this affects the reactivity of halogenoethanes and not their physical properties, such as boiling point.

This statement is true but not relevant for this question.

Therefore the answer to this question is option C (only statement 3 is correct).

Topic: Intermolecular Forces, Physical Chemistry, A Level Chemistry, Singapore

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