i) Explain why the region between 1000cm-1 and 1550cm-1 is described as the
‘fingerprint region’.
The region between 1000 cm-1 and 1550 cm-1 is known as the fingerprint region because it
consists of bands that are unique to each molecule. It is specific to each molecule and is full
of small peaks showing that IR absorption results from vibrations of the whole molecule
(Thornton 2009).
ii) Identify the functional groups that could be responsible for the peaks in the
non-fingerprint region of the spectrum.
a) alkyl C−H group
b) carboxylic acid C=O group
c) ketone C=O group
The non-fingerprint region of the IR spectrum generally has five zones namely Zone 1 (4000-
3200 cm-1), zone 2 (3200-2600 cm-1), zone 3 (2400 -2000 cm-1), zone 4 (1850-1650 cm-1) and
zone 5 (1680-1550 cm-1). Each zone has distinct peaks corresponding to the possibility of
presence different molecular functional groups.
Performing the five-zone analysis, the peaks found in zone 2 only occur after 3000 cm-1 hence
there are alkyl C−H functional group in this region.
The peak around 3000 cm-1 as well as the peak occurring at around 1720 cm-1 indicates that
carboxylic acid C=O group is present.
The strong peak occurring at around 1720 cm-1 indicates that ketone C=O group is present.
iii. Is this the spectrum of butan-2-ol or butanone? Explain your answer.
This is butanone spectrum since butanone has ketone group which are present.
Butan-2-ol has alcohol group which is absent (Academics.eckerd.edu 2020).
3. Explain how IR spectroscopy can be used to distinguish between alcohols,
carboxylic acids and esters.
The five-zone analysis of the non-fingerprint regions of IR spectrum is used (Meislich 2014).
Alcohol (O=H ) groups demonstrate strong and broad peaks between 3650-3200 cm-1.
Carboxylic acid ( C=O) groups demonstrate broad peaks that are very strong between
3000-2500 cm-1 and also similar peaks between 1725- 1700 cm-1 (Ibarra, Valencia and Pérez
2015).
Ester group demonstrates strong peaks between 1750-1735 cm-1.
LO3- Use nuclear magnetic resonance (NMR) spectra to understand the features of molecular
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