Robert D. Flsher University of Nevada, Las Vagas Las Vegas. 89154
I
Mechanistic Study of a Reaction An experimental course offering
The Chemistry Department at the University of Nevada, I.as Veras has attempted in its upper division elective courses to teach an integrated (organic,iiorganic, physical, etc.) approach to chemistry. The course herein described has been offered experimentally and appears to be ideally suited to this integrated approach. The probable reason for this is that reaction mechanisms do not seereeate themselves into senarate fields. The reaction type (x&eiphilic substitution) studied in the first offering of this course has e x a m ~ l ein s all fields of chemistry. ~ e p e n & n gon faculty interests oiher reaction types would nrohahlv serve as well (nroton transfers. redox reactions, ;tc.). The course is for two semester credits and involves ahout 30 (2 X 15weeks) 50-min lecture periods. The initial five weeks of the course introduce the student to those studies which yield mechanistic information (11(see course outline, 0. The amount of time spent in this area co~lldhe varied depending on student backgrounds. Examples of the use of these methl ods need not be restricted to the reaction type being studied. A second part of the course involves an in depth study of nucleophilic substitution a t saturated carbon (2). Carbon is a good starting place, since i t is familiar to the students and has been thoroughly studied. Optional topics such as nonclassical carhonium ions and secondary deuterium isotope effects, included in this offering, could be omitted, allowing more time for treatment of substitutions a t other atoms. This would depend on faculty and student interest. The final section of the course extends the methods, now well understood, to nucleophilicsubstitutionsa t other atoms. Silicon was studied first due to its similarity with carbon. The importance of d-orbitals in substitutions at non-first row elements was introduced. There is an excellent source for this . study of substit~~tions at phosphorus subject (ice Ref. ( 3 ) ) A allows one to bring in many &les of biochemical importance (6). Because the students in this course had not had an inorganic chemistry course, a discussion of substitution a t transition metal complexes was preceded by an introduction to theories of bonding in these systems. There are a number of good sources for this material (see Refs. (5)and (7)). Throughout the discussions of substitutions a t various ~~
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118 1 Journal of Chemical Education
atoms, similarities and differences with previously studied atoms were stressed. Both this and the similarity of methods used to study all the systems provided cohesiveness to suhject matter not normally treated in a single course. Student response to the coursewas very positive; comments indicate that a lot was learned. My own feeling is that the strength of the course is attributable to heine able to deal with material a t a level more soljhisticated than usually achieved in undereraduate courses. This was nossible hv restrictine studv to a iingle reaction type, rather i h m surve$ng mech&ismsof all the types of reactions. Appendix: Nucleophlllc Substitution. Course Outline I. Reaction Parameters-introduch tomethods which yield mechanistic information 1. Products-ineluding isotope labeling end exchange studies 2. Kinetics 3. SUre"ehemi.try I . SuhrtrateChange8 a. SuhrfituentEffects h. Leaving Group Effeds c. 1wtope Effects 6. Medium Chsnlres
Nucleophiliesubstitution 11. Nucleophili I . Saturated Carbon a. Ingdd work-SN1~SN2 h. Winslein work-Ion pair involvement e. Rocen, develollmenra 2. Silicon :3. Other representative elements lP.S.0, etc.) 4. square Planar complexla 5. Oelahedrai Complexes
Literature Cited ( I ) A ~atisfsetorytext for this material is. Jackso". R. A:'Mochsnism: A" lntroduetion to thestudy o i o r p n i c Reactionr: Clarendon Press, Oxford, 1972.Thisbook hsaan
adequate number of pmblema with hints and annwem. 12, Thore are a number of good sources for reading msuria1 on this subject, including ebeptemon nucloophilienubstitution in thestandard physicdownicbmlu b~ingold. Hammetl, Gould, Liherlor, ete. Gould is partiollarly good ksa source for problems. 131 The classic in this area is Sommer, L. H., "Sfereochemistry, Meehsnism and Silicon: MeCraw~Hill,N.Y.. 1965. More concise diaeusrions of this material can he found in 14) I"k-"l< University Press. Ithaea, New 151 Basolo. F..and Pesrson. R. C..'
