Chemistry Reference
In-Depth Information
of the elixir is obtained. It is then a simple matter to read the ephedrine
concentration off the standard curve. Ephedrine has a simple benzenoid
absorbance (k
max
263 nm; see structure in Chapter 1) which appears sharp
and clear against the background absorbance when the D
2
spectrum is
obtained. A sample D
2
trace obtained in this assay is shown in Figure 7.19.
Infrared spectroscopy
Infrared (IR) spectroscopy is a very useful technique for the identification of
unknown compounds, e.g. products from a synthesis or urinary metabolites
from an animal experiment, especially when used in conjunction with other
structure elucidation techniques such as nuclear magnetic resonance and
mass spectrometry. The infrared region of the electromagnetic spectrum
refers to light of wavelength 2.5 to 15 lm (i.e. 2.5
10
-6
m)
and the absorption of this light by the molecule causes changes in the vibra-
tional energy of the molecule in its ground state. As stated previously, vibra-
tional transitions are always associated with changes in the rotation of
atoms about chemical bonds. This is analogous to electronic transitions in
the absorption of ultraviolet energy, which also result in vibrational and
rotational transitions. The usefulness of IR stems from the fact that each
peak on the spectrum can be assigned to a particular bond or functional
group in the molecule. This often means that IR spectra are complex, with
perhaps as many as 20 or 30 peaks on one spectrum.
Identification of chemical unknowns is made easier, however, because
certain functional groups always appear in the same region of the IR spec-
trum. Single bonds (e.g. O
10
-6
to 15
H) absorb in the high-
frequency part of the spectrum (approximately 4000-2100 cm
-1
). This is
because the low mass of the hydrogen atom allows vibrations to occur at
high frequency. Triple bonds (e.g. in organic nitriles, R
a
H, N
a
H, C
a
a
C
c
N) absorb at
approximately 2100-1900 cm
-1
, while double bonds (e.g. C
C)
absorb at approximately 1900-1500 cm
-1
. The region of the IR spectrum
corresponding to wavenumbers less than approximately 1500 cm
-1
is due
to stretching of the molecule as a whole and the peaks in this region are
more difficult to assign accurately. This region of the spectrum is called the
fingerprint region
, since the pattern of peaks occurring in this region is
characteristic of the compound in question
and no other
. Use is made of
this property in the
British Pharmacopoeia
where two samples are said to
be identical when the IR spectra, obtained under identical conditions, coin-
cide completely - i.e. the same peaks are present in the same positions with
the same intensities. Reference IR spectra of authentic samples of a drug are
published in the BP to verify the identity of unknown samples.
b
O, C
b
