CAIE A-Level Chemistry AS 22.1 Infrared Spectroscopy Questions

Practise matching infrared absorptions to bonds and functional groups and using missing peaks to reject structures.

Exam points

match a significant absorption range to both its bond and the functional group containing it
combine several diagnostic peaks such as C=O, C–O, O–H, C=C or C≡N before identifying a compound
use the absence of an expected diagnostic absorption as evidence against a proposed structure

Covered syllabus points

22.1.1—Analyse an infrared spectrum of a simple

[Maximum number: 3]

Organic compounds can be distinguished using chemical tests and analytical techniques.

(a)

Organic compound E contains three carbon atoms. E reacts with cold dilute acidified $\mathrm{KMnO}_{4}(\mathrm{aq})$ to form a single compound F with $M_{\mathrm{r}}=154.9$ .
Fig. 3.1 shows the infrared spectrum of E.
Fig. 3.2 shows the infrared spectrum of F.

Fig. 3.1

Fig. 3.2

Table 3.2

Both spectra show absorptions between 2850 and $2950 \mathrm{~cm}^{-1}$ owing to C-H bonds in each molecule.

[ 3 ]

(i)

Use the two infrared spectra and Table 3.2 to identify the functional group present only in E.
Explain your answer, referring only to absorptions at frequencies greater than $1500 \mathrm{~cm}^{-1}$ .
functional group explanation

[ 1 ]

C=C (bond) / alkene

AND (absorption within) $\mathbf{1 5 0 0} \boldsymbol{-} \mathbf{1 6 8 0 ~} \mathbf{c m}^{-1}$ (present in E's IR spectrum only)

(ii)

Use the infrared spectrum of F to identify the functional group formed when E reacts with cold dilute acidified $\mathrm{KMnO}_{4}(\mathrm{aq})$ .
Explain your answer, referring only to absorptions at frequencies greater than $1500 \mathrm{~cm}^{-1}$ .
functional group explanation

[ 1 ]

hydroxyl / alcohol

AND (broad absorption within) $3200-3650 \mathrm{~cm}^{-1}$ (present in F's IR spectrum only)

(iii)

Use the information in 3(c) to suggest a structure for E.

[ 1 ]

[Maximum number: 1]

$4\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCHO}$ is used in the synthesis of some antibiotics.

X, Y and Z are all isomers of $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCHO}$ .

A summary of some of the reactions and properties of X, Y and Z is shown in the table.

Table for Question 2 — CAIE A-Level Chemistry AS

Complete the table with the bond responsible for each of the principal absorptions seen in the infra-red spectrum of Z.

principal absorptions in the infra-red spectrum	bond responsible
3200–3600 cm−1	RO–H / O–H
1630 cm−1	C=C
1050 cm−1	C–O

[Maximum number: 3]

Fig. 4.1 shows how propane, $\mathrm{C}_{3} \mathrm{H}_{8}$ , can be converted to propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ .

Fig. 4.1

$\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ reacts with an unsaturated alcohol R to form unsaturated ester S.

The infrared spectrum of S is shown in Fig. 4.2.

Fig. 4.2

Three absorptions in the infrared spectrum in Fig. 4.2 confirm that S is an ester and is unsaturated.
- Write 1, 2 or 3 on Fig. 4.2 against each of these three absorptions.
- Complete Table 4.1 to show which bond is responsible for each absorption that you have identified in Fig. 4.2.

Table 4.1

Table 4.2

M1 absorption labelled at approx. $1750 \mathrm{~cm}^{-1}$ identified as C=O

M2 absorption labelled at approx. $1650 \mathrm{~cm}^{-1}$ identified as C=C

M3 absorption labelled at approx. $1170 \mathrm{~cm}^{-1}$ identified as C-O

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Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question 2

Question 3