An elimination reaction is a reaction in which two or more atoms, one of which is usually hydrogen, are removed from adjacent atoms in the reactant, resulting in the formation of a multiple bond.
The relationship between addition reactions and elimination reactions
Alkenes can be readily prepared from the alkylhalide (X = Cl, Br, I) or the alcohol.
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Electrophilic Addition
The C=C bonds of alkenes are electron-rich and nucleophilic. Commonly used plastics such as polyethylene, polypropylene and polystyrene are all formed through the reactions of alkenes. These materials continue to find use in our society because of their valuable properties, such as high strength, flexibility and low weight.
Alkenes undergo addition reactions. Most commonly they add a proton to one end of the double bond and another group to the other end.
Alkenes are reactive because they have a high-lying pair of π-bonding electrons. These electrons are loosely held, being high in energy compared to σ-bonds. The fact that they are not located between the carbon nuclei but are found above and below the plane of the double bond, also makes these electrons more accessible.
Alkenes can donate their electrons to strong electrophiles other than protons and their reactivity pattern is a little different than the simple addition across the double bond.
Elimination reactions
This type of elimination can be described by two model mechanisms:
- it can occur in a single concerted step (proton abstraction at Cαoccurring at the same time as Cβ-X bond cleavage)
- in two steps (Cβ-X bond cleavage occurring first to form a carbocation intermediate, which is then ‘quenched’ by proton abstraction at the alpha-carbon).
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The most common elimination reactions are dehydrohalogenation and dehydration. In the mechanism above, X could be Cl, Br, or I for the dehydrohalogenation where there is a loss of HX from an alkyl halide. For dehydration, X would be an OH group in the above mechanism where the overall loss is water from an alcohol. These mechanisms, termed E2 and E1, respectively.
Halohydrins from Alkenes
Addition of HOX
The addition of chlorine and bromine to alkenes, proceeds by an initial electrophilic attack on the pi-electrons of the double bond. Dihalo-compounds in which the halogens are bound to adjacent carbons:.
R2C=CR2 + X2 ——> R2CX-CR2X
Another electrophilic addition to an alkene is the reaction of an alkene with the other halogen-containing reagents, HOX, to form halohydrins. The alkene is reacted with Br2 or Cl2 in the presence of water. These reagents are unsymmetrical (unlike Br2 or Cl2), so their addition to unsymmetrical double bonds may in principle take place in two ways. In practice, these addition reactions two isomers with one being the most favorable. The electrophilic moiety in both of these reagents is the halogen.
(CH3)2C=CH2 + HOBr ——> (CH3)2COH-CH2Br
The Markovnikov rule.
The bonding of an electrophilic species to the double bond of an alkene should result in preferential formation of the more stable (more highly substituted) carbocation, and this intermediate should then combine rapidly with a nucleophilic species to produce the addition product.
Halogenation of alkenes
Mechamism
Step 1. is the same as halogenation of alkenes. The halogen, in this case bromine, reacts with the alkene to form a cyclic bromonium ion.
Step 2. water acts as the nucleophile. Water is available at a higher concentration and a better nucleophile than the bromine ion, so is more likely to be available in the correct orientation for the nucleophilic attack.
Step 3. There is a loss of a proton from the oxygen to the solvent (water) to form the neutral halohydrin. In this case, a bromohydrin is the product of the addition reaction.
Electrophilic halogen reagents to alkene groups.
