OCR A-Level Chemistry: Carbonyls, Carboxylic Acids, Polymers & Spectroscopy

Two unlabelled bottles each contain a colourless liquid. One is pentanal (an aldehyde) and the other is pentan-3-one (a ketone). A technician carries out a sequence of simple chemical tests to identify which bottle is which.

Describe and explain how the technician can first confirm that both liquids contain a carbonyl group, and then use further reagents to distinguish the aldehyde from the ketone. Your answer should name the reagents, give the observation expected with each compound, and explain why the results differ in terms of what is happening to the carbonyl group.

(6 marks)

A student prepares the ester ethyl ethanoate by refluxing ethanoic acid with an excess of ethanol in the presence of a few drops of concentrated sulfuric acid as catalyst. The equation for the reaction is:

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$CH3​COOH+C2​H5​OH⇌CH3​COOC2​H5​+H2​O

The student uses 9.00 g of ethanoic acid with the ethanol in excess. After distillation and purification, 6.60 g of ethyl ethanoate is collected.

Quantity	Value
Mass of ethanoic acid used	9.00 g
Ethanol	in excess
Mass of ethyl ethanoate collected	6.60 g
$M_r$Mr​(ethanoic acid, $CH_3COOH$CH3​COOH)	60.0
$M_r$Mr​(ethyl ethanoate, $CH_3COOC_2H_5$CH3​COOC2​H5​)	88.0

Calculate the percentage yield of ethyl ethanoate. Give your answer to three significant figures.

(6 marks)

An unknown organic liquid X has the molecular formula $C_4H_8O_2$C4​H8​O2​ and is found to be an ester. It was analysed by mass spectrometry, infrared spectroscopy and ¹H NMR spectroscopy. The data are summarised below.

Mass spectrum: molecular ion peak at $m/z = 88$m/z=88.

Infrared spectrum: a strong absorption at 1735 cm⁻¹; no broad absorption in the 2500–3300 cm⁻¹ region.

¹H NMR spectrum (relative to TMS):

Chemical shift δ / ppm	Relative integration	Splitting pattern
1.1	3	triplet
2.3	2	quartet
3.7	3	singlet

Deduce the structure of X. Justify your deduction by interpreting each piece of data, making clear use of the integration ratio and the n+1 splitting rule.

(5 marks)

A condensation polymer P is manufactured for use in a heat-resistant fibre. Its repeat unit is:

$-[ \text{NH} - (\text{CH}_2)_5 - \text{NH} - \text{CO} - (\text{CH}_2)_4 - \text{CO} ] -$−[NH−(CH2​)5​−NH−CO−(CH2​)4​−CO]−

(a) Name the type of linkage that joins the units in P, and state whether P is a polyester or a polyamide. (1 mark)

(b) P can be broken down by hydrolysis. Deduce the structures (displayed or in shorthand) of the two monomers obtained when P is hydrolysed. (2 marks)

(c) Hydrolysis can be carried out using hot aqueous sodium hydroxide or hot aqueous hydrochloric acid. State how the products containing the original diamine differ between these two conditions, and explain why. (2 marks)

(5 marks)

The ester ethyl methanoate, $HCOOCH_2CH_3$HCOOCH2​CH3​, gives a characteristic set of NMR spectra.

(a) State the number of peaks expected in the ¹³C NMR spectrum of ethyl methanoate, and explain your answer. (2 marks)

(b) Predict the ¹H NMR spectrum: state how many proton environments there are and, using the n+1 rule, give the splitting pattern expected for each. (2 marks)

(4 marks)

This question is about some key ideas in organic analysis and stereochemistry.

(a) The amino acid alanine, $CH_3CH(NH_2)COOH$CH3​CH(NH2​)COOH, exists as a pair of optical isomers. State what is meant by a chiral centre. (1 mark)

(b) Two amino acids join together to form a dipeptide. Name the linkage formed and state the small molecule that is eliminated when it forms. (1 mark)

(c) Tetramethylsilane (TMS) is added to a sample before it is run in an NMR spectrometer. State one reason why TMS is used as the reference standard. (1 mark)

(3 marks)