Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

Compound F is made up only of carbon, hydrogen and oxygen. Every carbon-carbon bond in F is single. Its structure was examined using mass spectrometry together with infra-red and NMR spectroscopy.
(a(i))[2]

The mass spectrum indicates an $m/e$ value of 90 for the M peak. The peak heights for M and M+1 are in the ratio $22.1 : 0.7$. Use this ratio to work out how many carbon atoms are present in one molecule of F.

(a(ii))[1]

Suggest the molecular formula for compound F.

(b)[2]

The infra-red spectrum of F was recorded. Use the Data Booklet and your knowledge of infra-red spectroscopy to identify the bond type and functional group responsible for the three absorptions at $3350\,\text{cm}^{-1}$, $2680\,\text{cm}^{-1}$ and $1725\,\text{cm}^{-1}$.

(a)

In the mass spectrum, the M peak has $m/e$ value 90. The peak-height ratio for M to M+1 is $22.1 : 0.7$.

(c)

F was dissolved in deuterated trichloromethane, $\text{CDCl}_3$, and the proton NMR spectrum of the resulting solution was obtained.

(c(i))[4]

Use the Data Booklet together with your response to (a)(ii) to fill in Table 1 for the proton NMR spectrum of F. The four chemical-shift values for F have already been provided.

(c(ii))[1]

Describe and explain how the absorption at $\delta = 1.4$ is split.

(c(iii))[1]

F was dissolved in $\text{D}_2\text{O}$ and the proton NMR spectrum of the new solution was obtained. Two of the absorptions in Table 1 were absent from this spectrum. Which absorptions were absent?

(c(iv))[1]

Suggest the structural formula of F.

(d)

Cycloheptadiene, $\text{C}_7\text{H}_{10}$, is made up of a seven-membered ring containing two carbon-carbon double bonds.

(d(i))[1]

Complete the skeletal formulae for two isomers of cycloheptadiene.

(d(ii))[2]

Predict how many peaks will appear in the carbon-13 NMR spectra of P and Q.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: The mole ratio calculation gives approximately $2.87$-$2.9$.

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