1. For each of the compounds shown below, how many peaks do you expect in the c
ID: 590320 • Letter: 1
Question
1. For each of the compounds shown below, how many peaks do you expect in the c NMHR spectrum? How many peaks do you expect in the H NMR spectrum? What will the relative Br Br Br CH3 H3C 2. How could you differentiate cinnamaldehyde and cinnamic acid by each of the following methods: (a) IR spectroscopy (b) Mass spectrometry (c) 1C NMR spectroscopy For each be specific. In other words, if you were given two spectra and told one was cinnamaldehyde and the other was cinnamic acid, what would you look for in each spectrum to tell which is which?Explanation / Answer
1. a) 2-bromopentane
13C NMR shows 3 signals
1H NMR shows 3 signals in 1 : 2 : 3 ratio
b) 1-bromo-3-methylpentane
13C NMR shows 4 signals
1H NMR shows 4 signals in 2 : 2 : 1 : 6 ratio
c) 1-bromo-2,2-dimethylpropane
13C NMR shows 3 signals
1H NMR shows 2 signals in 2 : 9 ratio
d) Benzene
13C NMR shows only one signal
1H NMR shows only 1 signal for all 6 protons.
e) Toluene
13C NMR shows 5 signals
1H NMR shows 4 signals in 1 : 2 : 2 : 1 ratio
f) p-xylene
13C NMR shows 3 signals
1H NMR shows 2 signals in 2 : 3 ratio
2.
IR spectroscopy- Cinnamaldehyde has an aldehyde -CHO group. So, it would show two peaks in the range 2700 - 2830 cm-1 for carbonyl C-H stretch. It would show a peak for carbonyl group around 1680 cm-1. Also, cinnamic aid would show the peak for carbonyl group around 1680 cm-1.But, cinnamic acid would not show 2 medium peaks in the range 2700 - 2830 cm-1. Rather, it shows a broad absorption band near 2500 - 3600 cm-1 for O-H stretch which would be absent in the IR of cinnamaldehyde and would help to distinguish the 2 compounds.
Mass spectrometry would give a peak at m/z = 132(molecular ion) and 103 (loss of -CHO group) for cinnamaldehyde; whereas the molecular ion peak is observed at m/z = 148 for cinnamic acid and another peak is observed at 103 for loss of -COOH group. Molecular ion peak can be used to distinguish the two compounds in mass spectrometry.
13C NMR will show almost similar peaks for cinnamaldehyde and cinnamic acid. The only difference can be observed in the peak for carbonyl carbon. In cinnamaldehyde, the carbonyl C shows peak around 200 ppm whereas that in cinnamic acid shows the peak around175 ppm.
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