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112 Chapter 6 PROPERTIES AND REACTIONS OF HALOALKANES: BIMOLECULAR NUCLEOPHILIC SUBSTITUTION Consideration of the nucleophile follows. The section discusses two characteristics that can give rise to high nucleophilicity: high as in hydroxide ion. and large size of the nucleophilic atom. as in iodide ion. The reasons behind this are explored in detail and are relatively straightforward. In attempting to understand these, consider, again, how the characteristics in question influence the relative stability of the transition state of the reaction. Get into the habit of doing this whenever you are faced with totally new concepts relating to reactions and their kinetic favorability. A third factor can affect the competition between base strength and atom size in determining relative nucle- ophilicity: the solvent in which the reaction is performed. Some solvents do not affect nucleophiles. These are the polar, aprotic solvents, of which there are several including N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). When nucleophilic substitutions are carried out in polar, aprotic solvents, the com- petition between more basic nucleophiles and larger nucleophilic atoms is close but usually won by the stronger base. Therefore, fluoride, the strongest base among the halides. reacts fastest in SN2 displacements carried out in polar aprotic solvents such as DMF. The situation changes when an SN2 reaction is run in a protic solvent, such as water or an alcohol. Protic solvents interact the strongest with (especially) negatively charged small nucleophilic atoms. The nature of this interaction is hydrogen bonding, and it effectively gets in the way of the nucleophilic atom, inhibiting it from reacting. Fluoride, the smallest halide ion, is most affected by hydrogen bonding in a protic solvent, and is most severely inhibited from reacting. This hydrogen bonding has the effect of canceling out the base-strength advantage of fluoride as a nucleophile. Fluoride goes from being the best halide nucleophile in polar, aprotic solvent, to the worst in polar, protic solvent: Nucleophilicity order in polar, aprotic solvent (acetone. DMF, DMSO)-basicity dominates: > Cl⁻ > > Nucleophilicity order in polar, protic solvent (water, alcohols)-size dominates: > Br⁻ > F⁻ Consideration of substrate structure is much simpler, as the major consideration for an SN2 reaction is, simply, How crowded is the "back side" of the atom being attacked? The less crowded, then the less steric hindrance to approach of the nucleophile on route to the transition state, so the better will be the situation for an reaction. The extreme importance of steric hindrance is emphasized by the fact that tert-butyl and neopentyl halides are virtually incapable of reacting by the SN2 mechanism. Solutions to Problems 31. (a) Chloroethane (b) 1.2-Dibromoethane (c) (d) 1-lodo-2.2-dimethylpropane (e) (f) Tribromomethane CH₂CH₃ 32. (a) (b) H CI CH₂Br (c) (d) CCh (e) H