Interpreting infrared and nuclear magnetic resonance spectra of

Aug 1, 1984 - Flowcharts to help the beginner become proficient in interpreting infrared and nuclear magnetic resonance spectra of simple organic ...
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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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