poor nucleophile because it will experience difficulty in approaching the carbon bearing the leaving group. Hence sterically bulky bases such as potassium t-butoxide (the potassium salt of t-butanol) favour b-elimination over substitution:
Where more than one elimination product can be formed the more highly substituted (i.e. more stable) alkene is normally favoured (Zaitsev's rule):
However using a bulkier base favours the less substituted alkene:
KOC2H5 in C2H5OH 71% 29%
KOC(CH3)3 in (CH3)3COH 28% 72%
KOC(C2H5)3 in (C2H5)3COH 11% 89%
Alkene formation may also proceed by an E1 (elimination, unimolecular) mechanism which bears the same relationship to E2 that SN1 bears to SN2:
E2: Rate = k x [RX] x [Base]
E1: Rate = k x [RX]
Unlike the case of the E2 mechanism - there is no steric requirement for the conformation of the substrate in an E1 reaction.
Where more than one possible alkene can result from deprotonation of the intermediate carbocation then Zaitzev's rule operates:
The same factors which promote SN1 substitution in alkyl halides also favour E1 elimination of HX - i.e. a high degree of alkyl substitution at the carbon atom bearing the leaving group, a good leaving group and a polar solvent. Hence the SN1 and E1 mechanisms usually compete in the case of the reactions of nucleophiles with tertiary halides or tosylates. Using a sterically hindered (and therefore poorly nucleophilic) base will tend to promote E1 over SN1.
Reactivity of Alkenes:
Electrophilic addition of polar reagents to carbon-carbon double bonds:
(1) Electrophilic addition of HX to alkenes - regiospecific Markovnikov addition to form alkyl halides:
Why does this reaction produce only one of the two possible structurally isomeric products?.
The reaction is a two step bimolecular process and proceeds via a carbocation intermediate:
Long before anything was known about the mechanism of this reaction it was recognised that 'Addition of HX to an alkene will proceed in such a way as to attach hydrogen to the least substituted carbon and X to the most substituted carbon'. This is known as Markovnikov's Rule after the Russian chemist who first put it forward.
Markovnikov's Rule can now be restated: Addition of HX (or any other polar species) to an alkene will take place in such a way as to produce the most stable - i.e. the most highly substituted - carbocation intermediate.
Remember that the order of stability of carbocations is:
Another feature of polar additions is structural rearrangement - a process in which a compound or intermediate changes its structure without changing its composition. The driving force is the formation of the more stable carbocation:
1,2-alkyl (i.e. CH3-, carbanion) shifts to generate a more stable carbocation are also possible:
Note that in these rearrangements the migrating atom (H) or group (R) carries a bonding electron-pair along with it when it moves, i.e. the migrating species is to be regarded as either a hydride anion (H-) or as a carbanion (R-).
(2) Electrophilic addition of H2O to alkenes - Markovnikov hydration to form alcohols:
This is not a useful laboratory preparation of alcohols but is used industrially for the preparation of t-butanol:
In the laboratory the Markovnikov hydration of alkenes is usually carried out indirectly via oxymercuration with mercury(II) acetate:
(3) Electrophilic addition of borane (BH3) to alkenes - Anti-Markovnikov hydration to form alcohols:
This synthesis of alcohols by addition of BH3 to alkenes (hydroboration) followed by oxidation will be dealt with as part of Module CM2005.
(4) Electrophilic addition of halogens to alkenes - Formation of 1,2-dihaloalkanes:
In the transition state for the SN2 attack of a nucleophile on a cyclic mercurinium or bromonium cation bond-breaking of the C-Hg or C-Br bond is more advanced than bond-formation with the incoming nucleophile. Hence the SN2 transition state for this reaction has partial carbocationic (i.e. SN1) character and therefore nucleophilic attack is at the most substituted carbon atom.