2020 P1 Q17 - Describing Constitutional Isomers

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 17.

2020 p1 q17 question

We want to determine which statement is correct to describe constitutional isomers.

Let's have a recap of constitutional or structural isomerism.

2020 p1 q17 structural isomerism overview

Constitutional or structural isomers have the same molecular formula but different structural formula.

In syllabus we have 3 types of structural isomers to consider.

1. Chain isomers - different carbon chain arrangements, eg butane and 2-methylpropane

2. Positional isomers - same functional group on a different carbon, eg propan-1-ol and propan-2-ol

3. Functional group isomers - different functional groups, eg ethanoic acid and ester methyl methanoate

Check out this video lesson to learn more about structural isomerism.

Let's run through each statement and see which is the best answer.

A. C4H10 has 3 constitutional isomers.

2020 p1 q17 option A number of isomers

We have only 2 chain isomers possible for C4H10, hence statement A is not true.

B. But-1-ene and pent-1-ene are examples of constitutional isomers.

2020 p1 q17 option B butene vs pentene

The number of carbons for butene and pentene are different hence they will not have the same molecular formula.

They are not isomers, hence statement B is not true.

C. Constitutional isomers may differ in their chemical properties.

2020 p1 q17 option C different chemical properties

Isomers are different compounds with different physical and/or chemical properties.

For example, functional group isomers ethanoic acid and methyl methanoate differ greatly in their reactivity since the functional groups are different.

Positional isomers propan-1-ol and propan-2-ol are oxidised to different products acid and ketone respectively so their chemical properties differ too.

Hence statement C is true and should be the best answer.

D. Constitutional isomers may have different molecular formulae.

2020 p1 q17 option D different molecular formula

Constitutional isomers must have the same molecular formula, ie same number of carbon, hydrogen, oxygen, etc.

The difference is in their structural formula.

Hence statement D is not true.

Therefore the answer to this question will be option C.

Topic: Intro to Organic Chemistry, Organic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2020 A Level H2 Chemistry Paper 1.

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