This question is about molecules with molecular formula C3H6O2. (a) Give the structural formulae of the pair of chain isomers with the formula C3H6O2 that are carboxylic acids. (b) (i) Give the structural formulae of a pair of positional isomers with the formula C3H6O2 that are esters. (ii) Give the reagents and conditions to produce one of your esters in (i). (c) The infra-red spectra of one of the esters and of another isomer, X, are shown: X decolourises bromine water and is not an ester or an acid. Explain the differences between these two spectra, with particular reference to the peaks with wavenumbers above 1500 cm-1.

A-Level CIE Chemistry 3.6 Organic Analysis: This question is about molecules

This question is about molecules with molecular formula C3H6O2 - CIE - A-Level Chemistry - Question 4 - 2016 - Paper 1

Question 4

This question is about molecules with molecular formula C3H6O2. (a) Give the structural formulae of the pair of chain isomers with the formula C3H6O2 that are carbo... show full transcript

Worked Solution & Example Answer:This question is about molecules with molecular formula C3H6O2 - CIE - A-Level Chemistry - Question 4 - 2016 - Paper 1

Step 1

Give the structural formulae of the pair of chain isomers with the formula C3H6O2 that are carboxylic acids.

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Answer

The structural formulae for the carboxylic acids with the molecular formula C3H6O2 can be represented as:

Propanoic acid: CH3CH2COOH
2-Methylpropanoic acid: (CH3)2CHCOOH

Step 2

Give the structural formulae of a pair of positional isomers with the formula C3H6O2 that are esters.

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Answer

The structural formulae for the positional isomers that are esters include:

Methyl propanoate: CH3COOCH3
Ethyl acetate: CH3COOCH2CH3

Step 3

Give the reagents and conditions to produce one of your esters in (i).

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Answer

To produce methyl propanoate (from (b)(i)), the reagents and conditions are:

React propanoic acid with methanol,
Use concentrated sulfuric acid as a catalyst,
Heat under reflux.

Step 4

Explain the differences between these two spectra, with particular reference to the peaks with wavenumbers above 1500 cm-1.

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Answer

In the infra-red spectra:

The ester shows a peak at 1710-1750 cm-1 due to the C=O stretch.
The isomer X shows a peak at 1500-1600 cm-1 indicating the presence of C=C (alkene).
Additionally, isomer X shows a peak around 3200-3600 cm-1 due to the alcohol O-H stretch, which is absent in the ester.

This question is about molecules with molecular formula C3H6O2 - CIE - A-Level Chemistry - Question 4 - 2016 - Paper 1

Worked Solution & Example Answer:This question is about molecules with molecular formula C3H6O2 - CIE - A-Level Chemistry - Question 4 - 2016 - Paper 1

Give the structural formulae of the pair of chain isomers with the formula C3H6O2 that are carboxylic acids.

Give the structural formulae of a pair of positional isomers with the formula C3H6O2 that are esters.

Give the reagents and conditions to produce one of your esters in (i).

Explain the differences between these two spectra, with particular reference to the peaks with wavenumbers above 1500 cm-1.

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