Elimination Reactions
An elimination reaction is promoted by a base and is always in competition with substitution. The ratio of elimination can be maximized by using strong, sterically hindered bases.
The bulkiest base can hardly attack any hydrogens other than the methyl ones, which are the least hindered ones. That leads to a higher ratio of the thermodynamically least stable olefin.
This is a clear-cut case of kinetic vs. thermodynamic control. Let's recall the amusing story...
Alexander Mikhaylovich Zaitsev (also spelled Zaytsev, Saytzeff and Saytzev; 2 July 1841 – 1 September 1910) was a Russian chemist. He worked on organic compounds and proposed Zaitsev's rule, which predicts the composition of the products of an elimination reaction.
August Wilhelm von Hofmann (8 April 1818 – 5 May 1892) was a German chemist.
Remember that elimination through an E2 mechanism requires that the leaving group and at least one hydrogen in the neighboring carbon be in anti arrangement.
This stringent stereoelectronic requirement makes some reactions stereospecific:
The 2R,3R and/or 2S,3S isomers (together, as a racemate, or individually) of 2-bromo-3-methylpentane by E2 elimination lead exclusively, stereospecifically, to the E isomer of 3-methyl-2-pentene.
The 2S,3R and/or 2R,3S isomers (together, as a racemate, or individually) of 2-bromo-3-methylpentane by E2 elimination lead exclusively, stereospecifically, to the Z isomer of 3-methyl-2-pentene.
The treatment of an alcohol with a mineral acid at high temperature leads to water elimination (dehydration) and olefin formation.
The reaction proceeds by an E1 mechanism and hence through carbocations.
As usual, carbocations may suffer rearrangement and yield unexpected olefins.
One important elimination reaction is the Hofmann degradation of amines. Click here to refresh your memory.