Organic Chemistry Laboratory Techniques, 2016a

6.4.B FLOWCHARTS
In some teaching labs, a combination of spectroscopy and chemical
tests are used in determination of an unknown. If available, an infrared
spectrometer (Figure 6.39) is very useful in determining possible
functional groups present in an unknown. The following flowcharts
summarize key signals present in an IR spectrum, and chemical tests
that can be used to support or narrow down
structural identification.
Figure 6.39: Student using an
Infrared spectrometer.
1. O-H Bond
Figure 6.40: IR spectra of: a) cyclohexanol (alcohol), b) heptanoic acid (carboxylic acid).
An O-H bond strongly absorbs infrared radiation over a broad range of wavenumbers (2400-3400 cm –1 ),
and has a characteristic shape in IR spectra. The specific range of absorption can be used to identify whether
the O-H is part of an alcohol or carboxylic acid functional group: a broad absorption centering around 3300
cm -1 corresponds to an alcoholic O-H bond (Figure 6.40a), while a broader absorption centering around
3000 cm -1 corresponds to a carboxylic acid O-H bond (Figure 6.40b).
Chemical tests (pH and bicarbonate tests) can be done to support the identification of a carboxylic acid.
Specific structural features of alcohols (1˚, 2˚, or 3˚) can be determined using a variety of chemical tests, as
summarized by the flowchart in Figure 6.41.
OH
(Alcohol)
Chromic
Acid Test
Lucas Test
Iodoform
Test
Positive:
1˚ ROH,
2˚ ROH
Negative:
3˚ ROH
Positive:
3˚ ROH or
allylic/benzylic
ROH
Negative:
2˚ ROH (slow),
1˚ ROH
Positive:
2˚ ROH
adjacent to a
methyl
Negative:
Other ROH
Figure 6.41: Flowchart summarizing chemical tests that support identification of an alcohol.
Organic Chemistry Laboratory Techniques | Nichols | Page 339