Presentation on theme: "Unimolecular Elimination: E1 7-6 An alternative reaction of nucleophiles towards haloalkanes is the abstraction of a proton and loss of halide, rather."— Presentation transcript:
Unimolecular Elimination: E1 7-6 An alternative reaction of nucleophiles towards haloalkanes is the abstraction of a proton and loss of halide, rather than the addition of the nucleophile. The abstraction of a proton leads to the formation of a double bond. This process is called elimination.
When 2-bromo-2-methyl propane is dissolved in methanol, a solvolysis reaction occurs (S N 1). However, a second minor reaction (20%) also occurs forming an alkene. Kinetic analysis shows that the rate of alkene formation depends only upon the starting haloalkane concentration and is thus 1 st order. This reaction is termed “E1” and has the same rate-determining step as the S N 1 reaction: the formation of a carbocation.
The orbital picture of the proton abstraction looks like: The complete mechanism is:
There are 9 possible protons which could be abstracted by the methanol molecule, however, each would lead to the same product. Other substrates may give more than one product:
The nature of the leaving group has no effect on the ratio of substitution to elimination. At low base concentration, addition of base only has a small effect on the product ratio, however at high concentrations of a strong base, the ratio of elimination to substitution rises greatly.