Example 2
The spectra shown in Figs 2(a)−(d)are for a liquid, boiling at 141°C and soluble
in water. The elemental composition was C 48.6%, H 8.1%.(i) Empirical formula: C 3 H 6 O 2 ; RMM = 74(ii) DBE = 1(iii) 2a: IR (liquid film) The most notable features of the spectrum are the broad
band around 3000 cm−^1 and the strong carbonyl band at 1715 cm−^1.
3000 cm−^1 H-O-, hydrogen bonded stretch of acid
2900 H-C- aliphatic str
1715 C=O str of acid
1450 CH 2 and CH 3 bend
1380 CH 3 bend
1270 C-O- str
This strongly suggests a carboxylic acid.(iv) UV: No significant UV absorption above 220 nm, therefore aliphatic.(v) 2b: MS(EI)
m/z
74 M+•
57 M- 17 possibly M–OH
45 −COOH
27, 29 C 2 H 5 present?
The fragment ions suggest an aliphatic carboxylic acid(vi) 2c: 1-H NMR (200 MHz, CDCl 3 solution)
d/ppm Relative integral Multiplicity Assignment
1.2 3 3 CH 3 −CH 2 −
2.4 2 4 −CO−CH 2 −CH 3
11.1 1 1, broad HOOC−
The fact that the proton at d= 11.1 ppm exchanges with D 2 O suggests an
acid.
2d: 13-C NMR (50.0 MHz, CDCl 3 solution)
d/ppm Multiplicity Assignment
9.5 4 CH 3 −C
28.2 3 −CO−CH 2 −C
180.0 1 −CO−(vii) The compound is propanoic acid, CH 3 -CH 2 -COOH. This is in agreement
with all the spectrometric data, and with the boiling point.290 Section F – Combined techniques