《有机化学精品教学》（汕头大学）Chap 6 Halohydrocarbon（2012）.pptVIP

The only reactant that is undergoing change in the first (rate-determining) step is the alkyl halide, so we expect such reactions would be unimolecular and follow a first-order rate equation. Hence the name SN1 is applied to this mechanism. b. Stereochemistry of the SN1 Reaction Nu- The crucial intermediate in the reaction is the planar carbocation, an chiral molecule. Once the free, planar carbocation is formed, optical activity is inevitably lost. Addition of the nucleophile must take place at both faces of the cation forming equal amounts of (R) and (S) product. So racemization is the result. The rate of the SN1 reaction: The rate of the SN1 reaction is much greater for tertiary than for secondary substrates, and simple primary alkyl and methyl halides do not react at all. Because the carbocation forming is the key step, so the SN1 reaction is favored for more stable carbocation. We have known that the stability of carbocations increase with substitution. The reactive activities of the alkyl halides: Stability of carbocation: + + + 1. The rate of reaction is proportional to the concentrations of the substrate. 2. The reaction is completed in two steps. An initial ionization is followed by attack of the nucleophile. 3. Racemization. 4. Sometimes rearrangement is occurred. Summary of SN1 reaction: rearrangement α β α β α β β For many combinations of alkyl halides and nucleo-philes, elimination reactions may compete with substitution, or even be the predominant reaction path. If we hope to understand why one or the other mode of reaction is preferred in a given case, we must study elimination reactions with the same care as we studied substitution. Ⅱ. Elimination Reaction Elimination reaction: a double bond is formed between two adjoined carbon atoms by eliminating a small molecule such as H2O, HX. + HX Saytzeff elimination: Formation of the more substituted alkene in an elimination reaction. In other words, when halogen hydride is rem