Do Our sruaenrs nearry unaersrana me nammen Equation? Adrian L. Schwan Department of Cnemcstry and Biochem stry Jn verslry of G~elpn Gde pn. ON N1G 2W1 Canada I n senior undereraduate courses or in early eraduate courses, organic s e m i s t r y students often arei&oduced to the Hammett equation. They learn that each substituent has a ovalue depending whither it is in the para or the m e t a position, a n d they learn t h a t these values may change if there is "through resonance" in the reaction. The students also are taught to interpret the meaning of the p value and offer a mechanism for the reaction. Some general physical organic textbooks ( 1 3 )offer problems that begin with experimental data and ask the student to plot a curve, determine p and suggest a mechanism. I n my experience, the students oRen can proceed throueh those auestions satisfactorilv. vet thev do not eain a full;ndersta;lding of all aspects o f i i r ~ a m m e t scalyt sis. Studcnts can ocrform the auestions without undcrstanding the significance of the parameters involved. For this reason I present a question I have developed that ensures that students appreciate the magnitude and sign of the o values and the meaning of other data that is used or obtained. The question requires that the student has had s simole substituent and resa n education that b e ~ n with onance effects and iscarried t h G g h to mechanisms that involve through resonance. I have used this question for exams and in class discussions.
Question Consider the following to be a bad report of a physical organic investigation into a certain reaction. The report contains a number of mistakes and oversights. Can you find them? A Hammett a p analysts was carried out on the solvolysis nf substituted curnyl uruylatrs. The solvent was 70% ncetnne i n H20 except for the nitro-substituted compounds. In those instances, for solubility reasons, 70% EtOH in H20 was employed as the reaction solvent. The results are summarized in the table. Linear regression was carried out on the data as the equation indicates.
b.1
log 2 =po
Substituent
o
Rate mnstant
m-NO2
4.71
pOCH3 mCH3
4.27
51200
4.17
11300
pCH3 H &I PC1 pC02Et mBr
4.07
34100
0.10
312
0.19 0.23
0.791 517
0.37
19.9
0.39
63.2
0.78
pN02
0.0892
0.00231
The slope was found to be 1.63 and is the p value. The correlation coefficient was -0.729. The positive p value indicates that the there is buildup of positive charge in the product. Its absolute value indicates that there is much less substituent involvement than in the case of thr dissoeinrion of phrnylnertie neida in wntrr at 20 'C, which 1s the accepted standard fur Hammett analyses.
Solution The following "mistakes" are deliberate. They are divided into two types: those that are procedural and interpretive and hence are key to the student's understanding and those that are of a data handling nature. The instructor may chose to ignore the latter points because they require the student to have a strong grasp of some subtle issues. Procedural and Interpretive 1. Solvent should never be changed. 2. The o for H is 0 not 0.10. 3. o-Chloro should not be used. No ortho substituents should be used. 4. The equation is incorred-k, and k, are interchanged. 5. The slope should be negative 1.63. 6. The positive p value is misinterpreted. 7. The magmitude of p is misinterpreted. 8. Phenylacetic acids are not the standard. Benzoic acids are the accepted standard; the temperature is 25 'C. 9. The positive charge buildup that occurs is not in the prod-
uct but in the transition state. 10. The reaction is one where through resonance can be invoked, at least for electron donating substituents, so o+ values should be used rather than simply a values.
Volume 70 Number 12 December 1993
1001
11. No temperature is given for the reaction.
15. The regression correlation coefficient should be same sign as the slope.
Data Handling 12. The o for the m-nitro group has the wrong sign. 13' The for the m- and pups are interchanged. 14' The "lue is actual1y the formcarboethoxy. The student should recognize a mistake since P-COZE~ is a stranger elwtron withdrawing PUP than m-Br.
1002
Journal of Chemical Education
Literature Cited 1. Car= F A.: Sundbelg, R. J.AduonrPd Org.nk Ckmishy, P a r A, 3rd ed.; Plenum: New York,1990: Chaptu 4. 2. h w , T H.;Richadsm,KS.Meehonismond~oryinOgnnkChmisby, Sd.ed.: Harper & ~ o w : ~ e u ~ o 1981; r k . chapter 2. 3. I ~N S. ~Phrsiml ~ orgo"& ~ chemistry; , hugman snentse~r T ~ C MWOW, ~ 1987: chapter 4.
