Interpreting Infrared and Nuclear Magnetic Resonance Spectra of Simple Organic Compounds for the Beginner A. M. lngham and R. C. Henson University of London Goldsmiths' College, New Cross, London SE14 6NW. England
A great deal of information may he gleaned from an infrared or nuclear magnetic resonance spectrum by an experienced chemist. A beginner, however, sees only a piece of paper with a multitude of peaks. How is i t possible to "decode" the complicated spectrum? We have devised flow charts which successfully assist the beginner to become proficient. The use of a flow chart is not totally revolutionary but merely attempts to incorporate in a systematic method the approach which might he followed by an experienced chemist.' The charts are designed primarily for the beginner. They are comparatively simple in that more complex spectra will not he encountered by the user. The student can use the chart to help him or her ask the correct questions and look for various features in the spectrum, and then to draw some conclusion. He or she soon learns a method of "attacking" the interpretation. The chart will he discarded, or rather a new chart will be built up in the student's own mind. Infrared When studvinz infrared s ~ e d r aaccount . must he taken not only of the &ti& ofthe peaks hut also thpirsizes and shape. The flow chart in Figure 1 enables this to he done. Examples can he selected to develop the student's experience of interpreting these features. The spectrum of propan-2-01 is shown in Figure 2. For this compound the sepdrum above 2700 cm-' shows a broad intense peak (due to -OH stretch) and a sharp peak (due to aliphatic -CH s t r ~ t c h ) . Students have no difficulty in answering questions 2-4 of the flow chart (Fig. 1). No information can he obtained in answer toauestion 5, and this is a limitation to which students' attention may be drawn.
Tahle I shows a typical response in following the flow chart for this soectrum. The overall conclusion that a student will come to is that the following groups are present: OH, CHa and C-0 or C-C. We use infrared in coniunction with other methods of identification. Students are kncouraged to make use of all available information at~nutthe compound when drawing conclusions. Table 2. Flow Chart Responses for NMR Spectrum of pmethoxybenzaldehyde Reference oeak at 6 = 0 identified Molecular formulaC8H8O2 Number of hydrogens = 8 NO peaks removed by D20 From integrated tram calculate number of hydragens at different chemical shfits. Note splitting pattern for each absorption. Chemical shift 9.8 6 7.9 6 7.0 6 3.8 6
Number of hydrogens 2 2 3
(luestian Region
Box
Answer
-OH -OH
Answer Yes
2
yes
Inference -OH
and -COOH absent).
eliminated .: group must be CHO).Subbacting CHO from CsH~O~leavesC~H~O. Need to go round the loop again. There are 2 doublets in the region 8.0-5.6 6 of 2 hyrogen each, so these are likely to be different
Ouestlon 1
w -COOH group (but
ArH. OH. COOH or CHO group present (but OH. -GOOH already
Table 1. Typical Student Response for Spectrum ol Llquld Contalnlng Carbon, Hydrogen, and Oxygen Bax
AWH, NH absent
Consider 9.8 6 peak first.One hydrogen not in region 8.0-6.6 6 so unlikely to be attached to aromatic ring. Possible CHO.
(1979).
above 2700 cm-'
Inference -OH.
' Henson, R. C., and Stumbles, A. M., S d i w l Sci. Rev., 80,212,446
Region
Splining singlet doublet doublet singlet
1
mesent
hydmgansanachedloabenrene
ring, i.e.,a disubstituted ring. 4
yes
5
not known (broad
2000-1500 cm-'
6
absorption obscures this region) no
150o-ttoOcm-'
10
11 12 below 900 cm-'
704
13 14 15
Yes Yes Yes Ye5 Yes no
aliphatic CH present
C=O, aromatic. NH.
alkene groups absent C-N, or cc present CHs present CH2 or CHI present
present. pdisubstitution with different subsliiuents. Subtracting GsHl fromC,H,O leaves CHIO.
ArH
6.6-4.5 6 Above 4.5 6
yes Isolated CH3 group. All peaks accounted for.This must be CH30-.
LO.
absent. aromatic,alkene or mnOchiorO C-*I possible
Conclusion: Groups found CHSO-,
-CHI-
Journal of Chemical Education
M compound is:
e;' CHO
CHO
0 START
YES
,
NO
NO
YES
NO
YES
NO
NO
C-C.
C-0,C-N
CH2 or CHI
t
Absence ofC-O
C-N,
CHa, C H ~ '
4 * h k
at region
Summarize findings
.Ald~hydes: CH sUeVh in the CHO group near 2720an-I may hedoublet
Flgwe 1. Flow chart tor IR interpretation.
Volume 61
Number 8
August 1984
705
Figure 3 shows the proton NMR flow chart. This is somewhat longer and more comprehensive than the IR chart, hut the approach is the same. The NMR spectrum is examined region by region starting a t low field (high 6 number). From the integrated intensities and splitting patterns within each reeion the student learns to recoenize the vatterns which mav heattributed t o definite groupings of atoms within the moiecule. The exact values of chemical shifts are not used at this stage but can be used later in confirming the structure. The flow chart does not require a detailed knowledge of chemical shift, hut students are encouraged t o refer totables of chemical shifts to confirm or to modify their overall conFigure 4 shows the NMR spectrum of p-methoxybenzaldehyde and Table 2 responses to the flow chart questions for this compound. Uslng the Flow Charts Both the IR and NMR charts have been used as part of our teaching for four years. Compared to the groups who had not used flow charts, those who use them are
Figure 2. Spe*rum ot propan-2-01.
(1) impressively quicker, (2) more competent, (3) more confident, (4) and progress more quickly and more confidently with these
topics than with others of comparable difficulty where flow charts have not been used
Flow charts offer the advantages of enjoyment while using them and the opportunity to work a t one's own pace. The teacher is free to spend more time dealing with individual problems.
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