One method for preparing alkenes is the removal of adjacent hydrogen and halide ions in alkyl halides. This is one example of an elimination reaction (hydrogen and the halide are eliminated from the molecule).
Alkynes can be prepared from either vicinal (attached at adjacent atoms) or geminal (attached at the same atom) dihalides with appropriate hydrogen atoms. Stronger bases are generally used in dehydrohalogenation to form alkynes than in forming alkenes.
When elimination can occur to form multiple alkene isomers, those with more substituted double bonds are formed in greater amounts. This observation is called Zaitsev's rule after the Russian chemist who first identified this behavior. The fourth example above shows the regioselectivity of the dehydrohalogenation reaction.
Elimination reactions can occur by two mechanisms. The mechanism which occurs is dependent on the strength of the base used to carry out the reaction. Stronger bases promote the concerted (one step) mechanism and weaker bases promote the unconcerted (two step) mechanism.
The concerted elimination mechanism is called E2 which stands for Elimination Bimolecular. Bimolecular refers to the fact that two molecules (the base and the alkyl halide) are involved in the rate-determining step and the rate of the reaction is dependent on the concentrations of both reactants.
The E2 mechanism involves removal of a proton by the strong base, the electrons from the carbon-hydrogen bond become the new double bond, and the halogen falls off as a halide ion. All of this happens in a single step. In order for this mechanism to occur, the hydrogen and bromine must be oriented in opposite directions and they must be in the same plane (anti-periplanar). This allows the orbitals involved in the movement of electrons to overlap to form the double bond. Rotate the molecule below by clicking on it and moving your mouse until you can see the anti-periplanar (or trans coplanar) orientation of the bromine (red) on carbon 2 and the hydrogen (white) on carbon 3. If the molecule does not appear, you need to download and install a copy of the Chime plug-in from the MDL Information Systems website.
The unconcerted mechanism is called E1 which stands for Elimination Unimolecular. Unimolecular refers to the fact that only one molecule (the alkyl halide) is involved in the rate-determining step (formation of the alkyl cation). The reaction depends only on the concentration of the alkyl halide.
The first step of the E1 mechanism involves loss of the halogen as a halide ion, leaving behind a positively charged carbocation. In the second step the relatively weak base removes an adjacent proton, whose electrons remain behind to form the double bond.
The E2 mechanism requires an anti-periplanar orientation between the hydrogen and the halide. This requirement results in the formation of one stereoisomer in greater amounts in the product for starting materials which can form multiple stereoisomers. See the mechanism above for an example.
The E1 mechanism passes through an intermediate (carbocation) with free rotation around the double-bond-to-be. This results in no preference for the formation of the possible alkene isomers and reactions occuring by the E1 mechanism are not stereoselective. See the mechanism above for an example.
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