Question 3

Option A Spectroscopy Question 3

Identify compound C from the data (a – d) given and explain how you arrived at your answer

a) Elemental analysis of compound C

Element

Percentage by mass

Carbon

66.61

Hydrogen

11.20

Oxygen

22.19

b) Mass spectrum of compound C

c) Infrared spectrum of compound C


d) Information about compound C from 1H NMR spectrum

Position of peak / δ ppm

Integration trace

0.9

3

2.0

3

2.3

2

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Answer

a) From the elemental analysis

Element

Amount / mol

Simplest ratio

Carbon

66.61/12.01 = 5.55

4

Hydrogen

11.20 / 1.01 = 11.1

8

Oxygen

22.19 / 16.00 = 1.39

1

the empirical formula is C4H8O

b) The M+ peak at 72 is evidence that the molar mass of compound C is 72 g mol-1 and hence its molecular formula is the same as its empirical formula, C4H8O. The fragment at 57 is due to loss of –CH3 leaving C3H5+ and the fragment at 43 is due to loss of -C2H5 leaving C2H3O+. The fragment at 29 is due to C2H5+ (it could also be due to CHO+).

c) The peaks at approximately 3000 cm-1 are due to C–H. The absorption at 1705 cm-1 is due to the presence of a carbonyl group, C=O.

d) The 1H NMR spectrum shows that the hydrogen atoms are in three different chemical environments. None of them indicate a single hydrogen atom on its own so the compound cannot be an aldehyde. It suggests it is a ketone with two –CH3 groups and one -CH2 group.

All this information taken together confirms that compound C is butanone, CH3CH2COCH3.

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