Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

T is a saturated alcohol. It was examined by mass spectroscopy and NMR spectroscopy. In the mass spectrum, the molecular ion peak, M, occurred at an $m/e$ value of $74$, and the height ratio of the M and M+1 peaks was $20.4 : 0.9$.
(a(i))

Use the ratio between the M and M+1 peaks to work out the number of carbon atoms in one molecule of T.

(a(ii))[3]

State the molecular formula of T.

(b(i))

The NMR spectrum of T shown below contains four absorptions. The signal at $1.8\,\text{ppm}$ is a multiplet, whereas the one at $2.5\,\text{ppm}$ is a singlet. Use this information together with your answer to a(ii) to deduce the structure of T.

(b(ii))

Describe and explain which type of proton gives rise to each absorption.

(b(iii))

The absorption at $1.8\,\text{ppm}$ is a multiplet, and the one at $2.5\,\text{ppm}$ is a singlet. State and explain the splitting patterns for the remaining absorptions at $0.9$ and $3.4\,\text{ppm}$.

(b(iv))[9]

Describe and explain how the NMR spectrum of T in $\text{D}_2\text{O}$ would differ from the spectrum shown.

(ii)[3]

Describe and explain which type of proton gives rise to each absorption.

(iii)[3]

The absorption at $1.8\,\text{ppm}$ is a multiplet, and the one at $2.5\,\text{ppm}$ is a singlet. Describe and justify the splitting patterns for the other peaks at $0.9$ and $3.4\,\text{ppm}$.

(iv)[3]

Describe and explain how the NMR spectrum of $T$ in $\text{D}_2\text{O}$ would differ from the spectrum shown.

Worked solution & mark scheme

This 21-mark question has a full step-by-step worked solution and mark scheme. One marking point: Applying $M:M+1=100:(1.1n)$ gives $n=4$

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