Flow Charting Leaving Group Reactions James P. Hagen University of Nebraska at Omaha, Omaha, NE 68182 A popular trend in current organic chemisrry texts is early introducrion toalkvl halide chemistw (e.a., Streitwieser and ~eathcock-chapter 9, ~ a d e - ~ h a ~ t 5, e iMorrison and Boyd-Chapter 5, VollhardtChapters 6 and 7). Thus the substitutionand elmination reactions of the halides in many texts precede the chemistry of alkenes, which tmditi~~nnlly was used to introduce the student to solution reactions. While this trend has many pedagogical advantages, there is at least one drawback: the reactions of the halides are more complex mechanistically than ionic ar'ditions of alkenes. t substitution or The student must learn to ~ r e d i c whexher elimination wi 1 occur in k a l k y l halide reaction. This is a bewildering concept since so many factors (halide structure, solvent, nucleophilicity and basicity of the Lewis hases present) influence expected outcome. As a study aid for the students, I summarize the reactions of the halides in the form of a flow chart. This is presented as a hierarchical series of questions in which strong hases are ves
first located, then solvent and nucleophilicity effects are considered, and finally attention is directed to the structure of the halide. In this final stage stereochemical effects are considered also. The flow chart is equally applicable to reactions that may involve other leaving groups such as tosylates or henzoates. The flow chart I currently use is shown in the figure. I also include on this handout additional information that allows very reactive substrates (allylic and benzylic) as well as unreactive substrates to be considered when these structural types are covered later in the text. I find that the flow chart provides a good capsule review for the students both a t the beginning of the second semester of the year course and before the final exam. Presented March 28, 1987, at the University of Nebraska-Lincoln, symposium on organic chemistby teaching. 1 1 0 SN2 some E2 possible SN2 and E2 E2
(SN2
Strong bare?
3O
E2 E2 E2
- bimolecular. second order, inversion)
C.g. of poor nucleophlles: KOHlEtOH or lithium dllsopropylamide)
SN2, no E2 SN2, little E2 slow E2; S N and ~ El possible If ionizing solvent (SNl no
Good nucieophile? r strongly polarizable species e.g. RS-. R3P. I-, Br- In a polar solvent (weakly polarizable species like CI- are p o d nuclsophil~s in polar aprotlc solvents. )
I
\
- unlmolecular, first order, racemizalion) lo 2' 3'
I
no reaction Slow SNl, slow El SNl, E l
no no reaction 'Polar aprotic Solvents contain no OH br NH groups; e.g., acetone, dimethyllonnamide, dimethyisultoxide, hexamethylphosphoramide. and acetonitrile. Ionizing solvents are hydroxylic; e.g., methanol, ethanol, acetic acid, and water. Ailylic and benzylic halides are very reactive substrates and will react it either a l o or a 3' substrate reacts. Vinyl and phenyl substrates are quite unreactive. Very strong bases will give elimination (E2) however.
820
Journal of Chemical Education
