J. W. Blunt and D. A. I?. Happer University of Canterbury Christchurch. New Zealand
I I
"C NMR Spectra of Styrene Derivatives An undergraduate experiment involving the application of the Hammett equation
I t has been our experience that many students encounter considerable difficultv in understanding the Hammett equation and its undeilying principles unless instruction is reinforced by problems involving supplied experimental data. This approach is less effective, however, than one in which the students obtain and apply the experimental data themselves. IJnfortunately reactions for this purpose are few and far between. A suitable system should satisfy three criteria; (1) the necessarv -~~~~~~~ romrwunds jhould be readilv accessible. (2) the data should d e i a s i y measured, and (3) the precision the data should be high enough that the student can use them with confidence. Of these it is (2) and (3) that cause the most difficult~. when measurements are made under con..esneciallv . ditions prevailing in teaching laboratories. The recent ohservationl that for meta- and para-substituted styrene derivatives the magnitude of the 'C shielding value for the 3-carhon is linearly related to a for the ring substituent means that a very accurite method is now available for the determination of a values. We have used this as the basis of a senior undergraduate experiment in our organic laboratories. The exneriment is best undertaken as a class ~ , .r o.i e c twith each student preparing and examining one or more compounds of a selected series. Series suitable for study include the styrenes, P,P-dimethylstyrenes, 8-nitrostyrenes, cinnamic acids and esters, stilbenes, chalcones, and benzylidene malononitriles, although care should be taken when dealing with the latter as some are skin irritants. Which series is chosen may often depend on the degree of difficulty desired for the preparation. Most are prepared via the aldehydes hy either base catalyzed condensation reactions or the Wittig reaction, although in the case of the &&dimethylstyrenes benzyl halides are better starting materials. In view of the cost of some of the aldehydes, their preparation as part of the exneriment mav he advantaeeous. The 13C n;hr spectra are obtained from moderately concentrated solutions in chloroform or dimethvl sulfoxide. While the former is more convenient the latter is the better solvent. Spectra from 1M solutions may be determined quite rapidly but for certain substituents (-NHAc, -NOz, -I, -OH for examole) this concentration cannot be achieved. '?he spectra may be &signed hy the application of the usual substituent additivity effects to the parent ~ o m p o u n dand .~ by consideration of intensity differences arising from sym-
of
.~ ~
metry andlor differences in relaxation times hetween protonated and non-protonated carbons. Ambiguities in assignments mav occur.. esoeciallv where more than one aromatic . ring is present, but in only very few cases has the assignment for the C, resonance been in doubt. I t is our experience that students find this introduction to '3C nmr spectral interpretation more eniovable than the usual situation involving - sets of provided spectra. The best a~proachto the analvsis of the data is to construct a graph of 6 1 i ~ 8 against U , . ~ . This graph may then be used to read off a values for para-substituents. The standard a values, based as they are on reactions in protic media, are unsuitable in certain cases (see table) and reasons for this may be discussed (they appear to he due to a solvent dependence of a,). The derived a,.,, values may then be compared with literature a+, a (or a"),and a- values (although in the latter instance a shortaee of -R substituents is a ~roblem).E x ~ l a n a tions for the observed n,.,. values may now he considered. It is observed that both t R and -H substituents require exalted a values and suhstituents capable of both + R a i d -R interaction eive intermediate values. At t k s point it should have occurred to the hetterstudents that the svstemdifkrsauite substantially from the more usual situation i n which reaction rates or equilibrium constants are being determined. ~ L i anomaly s may be overcome by discussing the results in terms of a hypothetical reaction. For instance, the results for the 8-nitrosiyrenes may be compared with literature results for the styrenes and the change accompanying the replacement of a hydrogen by an NOz group may be discussed as if it were a case of going from a starting material to a transition state. or as an eauilibrium. Students should then become aware that exaltation of o values is based on differences in electronic interactions. This a ~ ~ r o a can c h greatly facilitate their understanding of the difference between a and v'' and whv ~ h e n oreactions l can correlate with a- in spite of there b e k g substantial resonance interaction in both starting material and product. ~~~~
~~~
Suggested Solvent Correctlonsa to o values Sub-
Happer, D. A. R., McKerrow, S. M., and Wilkinson,A. L.,Aust. J. Chem., 30,1715 (1977). Levy, G. C., and Nelson, G. L., "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists," Wiley Interscience, New York, 1972. p. 81.
56 1 Journal of Chemical Education
stituent
NMe2
OH
OMe
CF.
CN
3 (CDCI,) A (MeISO)
+0.06 0
0 -0.17
-0.04 -0.04
+0.10 +010
+O.OB 0
NO. +010 0
mese cwmstmnr appb to boh metb a-d psr%substituems. ~o mnecfim is necesJay for t k OC&.
F. CI. Br. I. OAC. C&.
3,qCH14 and alkyl groups.
