Carbon-13 NMR Spectroscopy (Cambridge (CIE) A Level Chemistry): Exam Questions

Tetramethylsilane (TMS) is added as a standard reference chemical when obtaining an NMR spectrum.

i) State the structural formula of TMS.

[1]

ii) State two reasons why TMS is suitable for use as a standard reference chemical in NMR spectroscopy.

[2]

State the number of peaks in the ¹³C NMR spectrum of 1,3-dichlorobenzene.

i) Deduce the number of peaks that would be present in the ¹³C NMR spectrum of ethylbenzene, shown in Fig. 1.1.

Fig. 1.1

[1]

ii) Use the Data Booklet to state the ¹³C chemical shift range, δ / ppm, for the carbon atom labelled with an asterisk (*) in Fig. 1.1.

[1]

Compound A is an isomer of dinitrobenzene, C₆H₄N₂O₄.

Part of the mass spectrum of A is shown in Fig. 2.2.

Fig. 2.2

State the formula of the fragment ion responsible for the peak at m/e = 76 in Fig. 2.2.

Draw the skeletal formulae of the three structural isomers of dinitrobenzene.

The ¹³C NMR spectrum of compound A contains exactly four peaks.

Deduce which of the three isomers drawn in (b) is compound A. Explain your reasoning by stating the number of peaks expected in the ¹³C NMR spectra of the other two isomers.

An unknown alcohol was analysed and found to contain 64.9% carbon, 13.5% hydrogen and the rest oxygen.

The molar mass of the alcohol is 74.12 g mol^-1.

Calculate the empirical formula and the molecular formula of the unknown alcohol. Show your working.

Using your answer to part (a), the unknown alcohol can exist as four possible structural isomers.

Complete the table below. Sketch the skeletal formula of each isomer and state the number of peaks expected in the ¹³C NMR spectrum.

The ¹³C NMR spectrum of one of the isomers is shown in Fig. 3.1.

Fig. 3.1

Deduce which isomer produced the spectrum shown in Fig. 3.1. Use the Data Booklet to explain your answer.

Three hydrocarbons, L, M and N, have the molecular formula C₈H₁₀. Information about the number of peaks seen in the carbon-13 (¹³C) NMR spectrum of the three isomers is shown in Table 1.1.

Table 1.1

Deduce the structures of compounds L, M and N.

Complete Table 1.2 to give details of the proton NMR spectra for isomers L, M and N.

Table 1.2

Explain which of the three isomers, L, M or N has the highest melting point.

A chemist analyses a naturally occurring compound, K.

The percentage composition by mass is carbon 70.58%; hydrogen 5.92% and oxygen 23.50%.

The mass spectrum of the compound is shown in Fig. 5.1.

Fig. 5.1

Determine the molecular formula of the compound K. Show your working.

The results of qualitative tests performed on compound K are shown in Table 5.1.

Table 5.1

Identify the functional groups present in compound K. Explain your answer.

The carbon-13 (¹³C) NMR of compound K is shown in Fig. 5.2.

Fig. 5.2

Suggest the structure of compound K using your answers to (a) and (b) along with information from Fig. 5.2. Use the Data Booklet to help you. Explain your answer.