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waleed_hamoda
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Elimination reactions are important as a method for the preparation of alkenes.
The term "elimination" describes the fact that a small molecule is lost during the process.
A 1,2-elimination indicates that the atoms that are lost come from adjacent C atoms.
The two most important methods are:
Dehydration (-H2O) of alcohols, and
Dehydrohalogenation (-HX) of alkyl halides.
There are three fundamental events in these elimination reactions:
removal of a proton
formation of the CC p bond
breaking of the bond to the leaving group
Depending on the relative timing of these events, different mechanisms are possible:
Loss of the LG to form a carbocation, removal of H+ and formation of C=C bond : E1 reaction
Simultaneous H+ removal, C=C bond formation and loss of the LG : E2 reaction
Removal of H+ to form a carbanion, loss of the LG and formation of C=C bond (E1cb reaction)
In many cases the elimination reaction may proceed to alkenes that are constitutional isomers with one formed in excess of the other. This is described as regioselectivity.
Zaitsev's rule, based on the dehydration of alcohols, describes the preference for eliminations to give the highly substituted (more stable) alkene, which may also be described as the Zaitsev product. The rule is not always obeyed, some reactions give the anti-Zaitsev product.
Similarly, eliminations often favour the more stable trans-product over the cis-product (stereoselectivity).
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The term "elimination" describes the fact that a small molecule is lost during the process.
A 1,2-elimination indicates that the atoms that are lost come from adjacent C atoms.
The two most important methods are:
Dehydration (-H2O) of alcohols, and
Dehydrohalogenation (-HX) of alkyl halides.
There are three fundamental events in these elimination reactions:
removal of a proton
formation of the CC p bond
breaking of the bond to the leaving group
Depending on the relative timing of these events, different mechanisms are possible:
Loss of the LG to form a carbocation, removal of H+ and formation of C=C bond : E1 reaction
Simultaneous H+ removal, C=C bond formation and loss of the LG : E2 reaction
Removal of H+ to form a carbanion, loss of the LG and formation of C=C bond (E1cb reaction)
In many cases the elimination reaction may proceed to alkenes that are constitutional isomers with one formed in excess of the other. This is described as regioselectivity.
Zaitsev's rule, based on the dehydration of alcohols, describes the preference for eliminations to give the highly substituted (more stable) alkene, which may also be described as the Zaitsev product. The rule is not always obeyed, some reactions give the anti-Zaitsev product.
Similarly, eliminations often favour the more stable trans-product over the cis-product (stereoselectivity).
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waleed_hamoda
Well-Known Member
waleed_hamoda
Well-Known Member
waleed_hamoda
Well-Known Member
waleed_hamoda
Well-Known Member
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