Tutorial

Spectroscopy & Chromatography

1. Consider the following reaction scheme:

C6H6 + CH3COCl C6H5COCH3 + HCl A

C6H5COCH3 C6H5CH(OH)CH3

B

(a) Give the reagents and conditions needed for step 1 and step 2

(i) Step 1 Aluminium chloride / AlCl3 / iron (III) chloride / FeCl3

(1)

(ii) Step 2 LiAlH4

Dry ether(2)

(b) The IR spectrum for compounds A and B are shown below and the data of the spectrum is given in table 1.

Bond Assigment Wavenumber / cm-1

C - H AlkanesAlkenes, arenes

2850 – 29503000 - 3100

Benzene ring Arenes 1450 - 1650C = O Aldehydes, ketones, esters,

carboxylic acids1680 - 1750

O – H FreeHydrogen bonded in alcohols or phenolsHydrogen bonded in carboxylic acids

3580 – 36703230 – 3550

2500 - 3300

Step 1

Step 2

(i) Using suitable data from Table 1, give evidence from the spectra which shows that compound A has been reduced. You will need to comment on both spectra.

A spectrum shows bond due to C=O at 1680 – 1700 cm-1 (1)B spectrum shows bond due to OH at 3230 – 3550 cm-1 (1)A has no OH or no bond at 3230 – 3550 cm-1 (1)

(ii) Compound B is chiral. The IR spectra of the two optical isomers of B are identical. Suggest why this is so.

IR spectra due to bonds present (1)Same bonds / functional groups in both isomers (1)

2. An alcohol, C3H8O was analyzed by low resolution NMR spectroscopy. The spectrum is given below.

The chemical shift of some hydrogen nuclei are given in the following table (where R represents an alkyl group)

group δ / ppm group δ / ppm

CH3-R 0.8 – 1.2 R - OH 1.0 – 6.02.0

R – CH2 - R 1.1 – 1.5 R – CH2 - OH 3.3 – 4.0

CH – R3 1.5 R2 – CH -OH 3.2 – 4.1

Show that the alcohol must be propan-2-ol

3 types of H ratio 6:1:1 or some correct reference to height peaks (1)This related to structure of propan-2-ol in shift data (1)

3. Compounds A and B shown below are used as intermediates in the manufacture of ibuprofen.

(i) Suggest how infra-red spectroscopy could be used to identify an impurity in a sample of ibuprofen which is known to be one of these compounds. A table of infrared absorbencies is given below.

Bond Assigment Wavenumber / cm-1

C - H AlkanesAlkenes, arenes

2850 – 29503000 - 3100

Benzene ring Arenes 1450 - 1650C = O Aldehydes, ketones, esters,

carboxylic acids1680 - 1750

O – H FreeHydrogen bonded in alcohols or phenolsHydrogen bonded in carboxylic acids

3580 – 36703230 – 3550

2500 - 3300

A spectrum will include absorbance at 1700 cm-1 but not 3600 cm-1

B will include 3600 cm-1

Identification of groups to note (1)Link to peaks in spectrum (1)How to distinguish (1)

(3)

4. The two organic compounds of C and D contain the elements carbon, hydrogen and oxygen only. Compound C were found to give yellow precipitate with 2, 4-dinitrophenylhydrazine and red precipitate with Fehling solution. Compound D produced

a pleasant smell when reacted with ethanol and concentrated sulphuric acid. Mass spectrum of compounds C and D are given below.

State m/e and suggest a full structural formula for both compounds.

Compound C

m/e = 44 CH3 – CHO

Compound D

m/e = 74 CH3 – CH2-COOH

Mass Spectrum of compound C

Mass Spectrum of compound D