2021 P1 Q18 - Compare Reactivity of Halogenoalkanes

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 18.

2021 p1 q18 question

We need to deduce which of the two statements explain why 2-bromopropane is more reactive than 2-chloropropane.

First let's take a look at a simple nucleophilic substitution using hydroxide.

2021 p1 q18 nucleophilic substitution

From the reaction we can tell that nucleophilic substitution involves breaking of the C-X bond to form X-.

Let's go through each statement and see which are correct.

1. The ionisation energy of bromine is less than the ionisation energy of chlorine.

Ionisation energy is the energy required to remove electron from halogen to form halogen cation.

2021 p1 q18 statement 1

During nucleophilic substitution, there is no loss of electron from halogen atom and no halogen cations are formed.

Therefore this statement is not relevant and not correct.

2. The shared pair of electrons in the carbon-halogen bond is more shielded from the halogen nucleus in 2-bromopropane than in 2-chloropropane.

The shared pair of electrons in the C-X bond is simply the bond pair of electrons.

For C-Cl bond:

2021 p1 q18 statement 2 C Cl bond

Chlorine has 2 inner shells and the bond pair experiences less shielding effect.

Hence there is stronger attraction on bond pair of electrons, C-Cl bond is stronger and more energy needed to break.

This makes 2-chloropropane less reactive.

For C-Br bond:

2021 p1 q18 statement 2 C Br bond

Bromine has 3 inner shells and the bond pair experiences greater shielding effect.

There is weaker attraction on the bond pair, C-Br bond is weaker and less energy required to break.

Hence 2-bromopropane is more reactive.

This means statement 2 is correct since it can explain why 2-bromopropane is more reactive.

3. The atomic radius of bromine is larger than the atomic radius of chlorine.

2021 p1 q18 statement 3

Bromine is larger in size which makes the C-Br bond longer and weaker.

This makes the bromo compound more reactive and statement 3 is correct.

4. There are more protons in the nucleus of a bromine atom than in the nucleus of a chlorine atom.

2021 p1 q18 statement 4

Bromine has more protons which makes its nuclear charge greater.

There is stronger attraction on bond pair of electrons which will make C-Br bond stronger and less reactive.

This does not explain why 2-bromopropane is more reactive hence statement 4 is not correct.

We can now go through the options and determine the answer to this question is option B (statements 2 and 3).

2021 p1 q18 answer

Topic: Halogenoalkanes, Organic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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