Analytical Techniques: Chemistry 9701 (Cambridge International AS & A Level)

A mass spectrometer is used to analyse a small, volatile organic compound, J. The mass spectrum shows a molecular ion peak at $m/z = 108$ and a second peak, of almost identical height, at $m/z = 110$. No peak of comparable height is seen at $m/z = 106$ or at $m/z = 112$.

Which conclusion is correctly supported by this data?

Compound L is a colourless, fruity-smelling liquid used as a flavouring in the food industry.

In its mass spectrum, compound L shows a molecular ion peak at $m/z = 88$ and a prominent fragment ion peak at $m/z = 57$.

Its infrared spectrum shows a strong, sharp absorption at $1735\ \text{cm}^{-1}$, a further strong absorption at $1200\ \text{cm}^{-1}$, and no absorption anywhere in the ranges $2500$–$3000\ \text{cm}^{-1}$ or $3200$–$3600\ \text{cm}^{-1}$.

You may use the following characteristic infrared absorption ranges:

(a) State the relative molecular mass, $M_r$, of compound L. [1]

(b) Identify the functional group present in compound L using the infrared data above, and explain how this data rules out both a carboxylic acid and an alcohol as possibilities. [3]

(c) The peak at $m/z = 57$ is formed by the loss of a single neutral fragment from the molecular ion. Calculate the relative mass of the fragment lost, and suggest its formula. [3]

(d) Suggest a structural formula for compound L that is fully consistent with all of the data above, and give its name. [2]

A student separates a mixture of three carboxylic acids, P, Q and R, first by thin-layer chromatography (TLC) and then by gas-liquid chromatography (GLC).

In the TLC experiment, a polar silica stationary phase and a non-polar mobile solvent are used. After development, the solvent front has travelled $8.0\ \text{cm}$ from the baseline. Spot P has travelled $6.4\ \text{cm}$, spot Q has travelled $4.0\ \text{cm}$, and spot R has travelled $1.6\ \text{cm}$.

(a) Calculate the $R_f$ value of each of P, Q and R. [3]

(b) State which of P, Q or R interacts most strongly with the stationary phase in this TLC experiment. Explain your answer. [2]

The same mixture is then analysed by GLC, using a high-boiling-point non-polar liquid (on a solid support) as the stationary phase and an unreactive gas as the mobile phase. Three peaks are obtained, with retention times of $1.8$ minutes for P, $3.5$ minutes for Q and $6.2$ minutes for R, and with peak areas in the ratio $3 : 8 : 9$ for P : Q : R.

(c) (i) Calculate the percentage composition, by moles, of the original mixture. [2]

(c) (ii) Explain, in terms of interaction with the stationary phase, why R has the longest retention time in this experiment. [2]

Compound M has molecular formula $\text{C}_5\text{H}_{10}\text{O}_2$ ($M_r = 102$) and is a colourless liquid with a fruity smell.

Its carbon-13 NMR spectrum shows five distinct peaks (five different carbon environments).

Its proton ($^1\text{H}$) NMR spectrum shows four signals, with the data below:

You may use the following typical chemical shift ranges:

(a) A structural isomer of compound M, isopropyl ethanoate, $\text{CH}_3\text{COOCH}(\text{CH}_3)_2$, shows only four carbon-13 environments rather than five. Explain why isopropyl ethanoate gives one fewer carbon-13 environment than compound M, and state what the five separate environments observed for M tell you about whether M's carbon skeleton is branched in the same way. [3]

(b) Using the chemical shift ranges, integrations and splitting patterns given, assign each of the four $^1\text{H}$ signals to a specific proton environment in compound M. Hence deduce the structural formula of compound M, and give its name. [5]

(c) Explain, using the n + 1 rule, why the signal at $\delta = 1.65$ appears as a multiplet rather than as a simple triplet or quartet. [2]

In the proton ($^1\text{H}$) NMR spectrum of a compound, one signal is a quartet integrating for 2H, and a second signal is a triplet integrating for 3H. No other signals are present, and there is no evidence of an exchangeable O–H or N–H proton.

Which structural fragment is consistent with this pattern?

The mass spectrum of an organic compound, X, containing only carbon and hydrogen, shows a molecular ion peak at $m/z = 128$ with relative abundance 100, and a smaller peak at $m/z = 129$ (the $[\text{M}+1]^+$ peak) with relative abundance 9.9. No peak of significant height appears at $m/z = 130$.

Given that carbon-13 makes up approximately 1.1% of all naturally occurring carbon atoms, which of the following is the correct interpretation of this data?

Compound Y is a colourless liquid with a sharp, unpleasant odour, used industrially as a chemical intermediate.

In its mass spectrum, compound Y shows a molecular ion peak at $m/z = 116$ and a prominent fragment ion peak at $m/z = 99$.

Its infrared spectrum shows a strong, very broad absorption across the range $2500$–$3000\ \text{cm}^{-1}$, a strong, sharp absorption at $1715\ \text{cm}^{-1}$, and no absorption anywhere in the range $3200$–$3600\ \text{cm}^{-1}$.

You may use the following characteristic infrared absorption ranges:

(a) State the relative molecular mass, $M_r$, of compound Y. [1]

(b) Identify the functional group present in compound Y using the infrared data above, and explain how this data rules out an alcohol as a possibility. [3]

(c) The peak at $m/z = 99$ is formed by the loss of a single neutral fragment from the molecular ion. Calculate the relative mass of the fragment lost, and suggest its formula. [2]

(d) Given that compound Y has an unbranched carbon skeleton, suggest a structural formula for compound Y that is fully consistent with all of the data above, and give its name. [2]

The infrared spectrum of a colourless liquid, compound Z, shows two separate medium-intensity absorptions at $3300\ \text{cm}^{-1}$ and $3380\ \text{cm}^{-1}$, together with a C–H absorption in the range $2850$–$2960\ \text{cm}^{-1}$. There is no absorption anywhere in the range $1650$–$1750\ \text{cm}^{-1}$, and no absorption in the range $3200$–$3600\ \text{cm}^{-1}$ other than the two peaks already stated.

You may use the following characteristic infrared absorption data:

(a) Identify the functional group present in compound Z, and explain how the number of absorptions in the $3200$–$3600\ \text{cm}^{-1}$ region distinguishes it from both a secondary amine and an alcohol. [3]

(b) Explain why the absence of any absorption in the range $1650$–$1750\ \text{cm}^{-1}$ rules out compound Z being a primary amide, $\text{R-CONH}_2$, even though an amide also contains N–H bonds. [2]

(c) A second compound, X, gives only one absorption in the $3300$–$3500\ \text{cm}^{-1}$ region. Suggest why this single absorption alone is not, by itself, enough to decide whether X is a secondary amine or an alcohol, and state what further infrared evidence would help distinguish between them. [2]

The mass spectrum of a ketone, compound A, shows a molecular ion peak at $m/z = 86$ and a prominent fragment ion peak at $m/z = 71$. No peak of significant height is seen at $m/z = 68$, $m/z = 69$ or $m/z = 57$.

Which of the following is the best interpretation of the fragment ion peak at $m/z = 71$?

Compound V is a colourless liquid containing carbon, hydrogen, oxygen and chlorine only. Its mass spectrum shows three peaks close together at high $m/z$: at $m/z = 126$, $m/z = 128$ and $m/z = 130$, in the approximate height ratio $9 : 6 : 1$. No other peak of comparable height appears above $m/z = 130$.

Its infrared spectrum shows a strong, sharp absorption at $1715\ \text{cm}^{-1}$, and no absorption anywhere in the ranges $2500$–$3000\ \text{cm}^{-1}$ or $3200$–$3600\ \text{cm}^{-1}$.

You may use the following characteristic infrared absorption ranges:

(a) Using the abundances of the chlorine isotopes $^{35}\text{Cl}$ (approximately 76%) and $^{37}\text{Cl}$ (approximately 24%), explain why a molecule containing two chlorine atoms gives three peaks (M⁺, [M+2]⁺ and [M+4]⁺) in the approximate ratio 9:6:1, and hence state the number of chlorine atoms in compound V and its value of $M_r$. [3]

(b) Identify the functional group present in compound V using the infrared data, and explain how this data rules out both a carboxylic acid and an alcohol as possibilities. [2]

(c) A prominent fragment ion peak also appears at $m/z = 77$, formed by loss of a single neutral radical from the molecular ion. Calculate the mass of this radical and suggest its formula. [2]

(d) Given that compound V is symmetrical about its carbonyl carbon, suggest a structural formula for compound V that is fully consistent with all of the data above, and give its name. [2]