Edexcel A-Level Chemistry: Analytical Techniques

A liquid compound W contains only carbon, hydrogen and oxygen and has the molecular formula C₄H₈O ($M_r = 72$Mr​=72). It does not decolourise bromine water and gives no reaction with Tollens reagent. The following spectroscopic data were recorded.

Infrared spectrum (selected absorptions)

Wavenumber / cm⁻¹	Appearance
2960	medium
1715	strong, sharp

There is no broad absorption between 2500 and 3600 cm⁻¹.

¹H NMR spectrum

δ / ppm	Integration ratio	Splitting
2.4	2	quartet
2.1	3	singlet
1.0	3	triplet

Mass spectrum (selected peaks)

m/z	Relative abundance / %
72	24
57	30
43	100
29	55

Deduce the structure of W, explaining how each piece of evidence supports your answer. (6 marks)

A volatile organic compound Y contains carbon, hydrogen and one other element. Its low-resolution mass spectrum shows the peaks below. (Use ³⁵Cl = 35 and ⁷⁹Br = 79.)

m/z	Relative abundance / %	Note
78	30
80	10
43	100	base peak
27	35

A small peak is also seen at m/z 79, with an abundance of about 3.3 % of the m/z 78 peak.

(a) Use the peaks at m/z 78 and 80 to identify the other element present, and state the species responsible for the m/z 78 peak. (2 marks)

(b) Use the m/z 79 peak to determine the number of carbon atoms in Y. (1 mark)

(c) Deduce the molecular formula of Y, calculate its degree of unsaturation, and hence identify the fragment ions at m/z 43 and m/z 27. (3 marks)

A halogenoalkane T has the molecular formula C₃H₇Br. Its high-resolution ¹H NMR spectrum, recorded in CDCl₃, is summarised below.

δ / ppm	Integration ratio	Splitting
3.4	2	triplet
1.9	2	sextet
1.0	3	triplet

Interpret this spectrum to deduce the structure of T. Assign each signal and explain how the integration ratio and the splitting patterns establish the connectivity. (5 marks)

Propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol ((CH₃)₂CHOH) are isomers with the molecular formula C₃H₈O. Both show a broad O–H absorption near 3350 cm⁻¹ in their infrared spectra, so IR alone cannot distinguish them.

Explain how their ¹³C NMR spectra would allow you to tell the two isomers apart. In your answer, state the number of peaks each isomer produces and justify your reasoning. (5 marks)

A student uses thin-layer chromatography (TLC) on a silica plate to analyse a mixture of two organic compounds, P and R, using a non-polar solvent as the mobile phase. After development, the solvent front had travelled 8.0 cm from the origin. Compound P had travelled 6.0 cm and compound R had travelled 2.0 cm.

Spot	Distance moved / cm
P	6.0
R	2.0
Solvent front	8.0

(a) Calculate the Rf value of each compound. (2 marks)

(b) State and explain which compound, P or R, is the more polar. (2 marks)

2-Methylpropan-2-ol has the structure (CH₃)₃COH.

State how many peaks would appear in its ¹³C NMR spectrum, and explain your answer. (3 marks)