Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Qualitative Analysis I Notes
Application and History
Many times a chemist is faced with an unknown material and is asked to identify
the composition. Determining some physical properties (melting point, boiling
point, conductivity, viscosity, refractive index, density, etc.) of the material is one
of the most straightforward methods to help reduce the possible identity options.
Infrared Analysis quickly identifies the presence and absence of many functional
groups and further clarifies identity options.
Today each of you will be given an unknown material. You will take a boiling point, an
index of refraction and an infrared on your material. This information will be
utilized together to identify your material from 30 possible compounds.
The possible compounds are listed below. Most of these compounds you should
already be familiar with or the structures are easily deduced from the IUPAC
name. The structures for those materials with which you may not be familiar are
provided. (If a material ends with “one” it means that material is a ketone. The
number designates the carbon upon which the functional group is attached.) In
your notebook write all of the structures and all of the names of all of the
compounds. (Yes, draw all of the structures, even the ones where the structure is
not given. Figure them out and draw them ALL.) Also list the indexes of refraction
(RI) and boiling points (B.P.).
Review the notes associated with refractive index, infrared, and fractional distillation. A
boiling point analysis is very similar to a fractional distillation but without the
column packing (copper mesh) and conducted on a pure material instead of a
mixture.
Possible Unknowns for Qualitative Analysis 1
1
2
3
4
5
6
7
8
9
IUPAC NAME (common
name)
Select Structures
B. P.,
o
C
Refractive
Index
Ethoxy ethane (diethyl
ether)
2-Methyl-2methoxypropane
(MTBE)
2-Propanone (acetone)
Methanol
Diisopropyl ether
Hexane
Ethanal (acetaldehyde)
Butanal
Ethyl ethanoate (ethyl
acetate)
CH3CH2OCH2CH3
35
1.3526
55
1.3690
56
66
68
69
70
75
77
1.3588
1.3288
1.3679
1.3751
1.3316
1.3843
1.3723
78
80
1.3611
1.3788
10 Ethanol
11 2-Butanone (MEK)
(CH3)2CHOCH(CH3)2
CH3CH2CH2CH=O
12 Cyclohexane
13 2-Propanol
14 2-Methyl-2-propanol (tbutanol)
15 Cyclohexene
16 1,2-Dimethoxyethane
17 Trans- 1,2dimethycyclopentane
18 Heptane
19 3-Pentanone
20 Propyl ethanoate (propyl
acetate)
81
83
83
1.4266
1.3776
1.3878
83
85
92
1.4465
1.3770
1.4120
98
102
102
1.3878
1.3924
1.3842
21 Toluene
111
1.4961
22 Butyl ethanoate (butyl
acetate)
116
1.3941
117
118
127
141
149
1.3842
1.3716
1.4004
1.3809
1.4023
155
159
160
164
1.4507
1.4178
1.4641
1.3980
23
24
25
26
27
Butanenitrile
Acetic Acid
2-Hexanone
Propanoic Acid
Pentyl ethanoate (pentyl
acetate)
28
29
30
31
Cyclohexanone
1-hexanol
Cyclohexanol
Butanoic acid
CH3-O-CH2CH2-O-CH3
CH3-CH2-CH2-C≡N
CH3CO2H
Measurement Error and Identification
Each of the analysis methods has a different associated error. To correctly identify the
unknown, it is important to understand the accuracy and precision of the three methods.
If the sample is not contaminated, and if good technique is followed when obtaining an
IR spectrum, then the results of an IR are absolute. The IR does not lie. This means that
as long as the acetone from cleaning the plates was removed before the sample was
added, if there is no strong absorbance observed in the carbonyl region of the sample
spectrum, then the unknown does not contain a carbonyl functional group. Alternatively,
if a carbonyl is present in the spectrum, all compounds without a carbonyl can be
eliminated as possible candidates. Often the absence of a functional group is just as
useful as the presence of a functional group in identification.
When carrying out a small scale boiling point, as long as the placement of the
thermometer is correct and the thermometer is intact, it is much easier to get an
artificially low boiling point than an artificial high value. Therefore the observed boiling
points are often lower than the literature boiling points. It is not uncommon to obtain a
boiling point 15oC or more below the literature value. The obtained boiling points are
rarely more than 5o-10 o above the literature value.
The refractometers are calibrated against water regularly. Some error is inherent in
discerning when the horizon line is exactly in the middle and there is some error in
reading the scales accurately. Refractive Index measurements are rarely more than
±0.0050 away from literature values.
To correctly determine the identity of your unknown all three pieces of information have
to be used. It may be easiest to first use the boiling point to narrow down the options,
second to use IR, and finally use RI. It is often necessary to subtract the RI of the
sample from the two most promising candidates, and the candidate with the smallest
deviation is most likely the correct choice.
Revised: November 27, 2015, S. L. Weaver