electrophilic addition of hydrogen halides.pptx)

7/31/2019 Electrophilic Addition of Hydrogen Halides.pptx)

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Electrophilic Addition of Hydrogen Halides

The

electrons of a double bond are more loosely held than those of the

bond.As a result, the electrons which extend above and below the molecular plane of the

alkene, can act as a nucleophile in a manner similar to that of more typical Lewis bases.

2,3-dimethylbutene

The electrophilic addition reactions of alkenes can be both regioselective and

stereospecific.


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Electrophilic attack by protons gives carbocations.

A strong acid may add a proton to a double bond to give a carbocation.

This reaction is simply the reverse of the last step in an E1 elimination reaction, andhas the same transition state.

At low temperatures, and with a good nucleophile, an electrophilic addition

product is formed.

Typically, the gaseous HX (HCl, HBr, or HI) is bubbled through the pure or dissolved

alkene. The reaction can also be carried out in a solvent such as acetic acid.


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The Markovnikov rule predicts regioselectivity in electrophilic

additions.

The only product formed during the reaction of propene with HCl is 2-

chloropropane:

Other addition reactions show similar results:


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If the carbon atoms participating in the double bond are not equally substituted,

the proton from the hydrogen halide attaches itself to the less substituted carbon.

The halogen, as a result attaches to the more substituted carbon.

This result is known as Markovnikovs rule and is based on the stability of the

carbocation formed by the addition of the proton.


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Markovnikovs rule can also be stated:

HX adds to unsymmetric alkenes in a way that the initial protonation gives the

more stable carbocation.

Product mixures will be formed from alkenes that are similarly substituted at both

sp2 carbon atoms.

If addition to an achiral alkene generates a chiral product, a racemic mixure will be

obtained.


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Carbocation rearrangements may follow electrophilic addition.

In the absence of a good nucleophile, a rearrangement of the carbocation may

occur prior to the addition of the nucleophile.

An example of such a rearrangement is the addition of trifluoroacetic acid to 3-

methyl-1-butene, where a hydride shift converts a secondary carbocation into a

more stable tertiary carbocation:


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The extent of carbocation rearrangement depends upon:

Alkene structure

Solvent

Strength and concentration of nucleophile

Temperature

Rearrangements are generally favored under strongly acidic, nucleophile-deficient

conditions.


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Electrophilic Addition of Halogens to Alkenes

Halogen molecules also act as electrophiles with alkenes giving vicinal dihalides.

The reaction with bromine results in a color change from red to colorless, which is

sometimes used as a test for unsaturation.

Halogenations are best carried out at or below room temperature and in inert

halogenated solvents (i.e. - halomethanes)


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Electrophilic Addition of Halogens to Alkenes

Bromination takes place through antiaddition.

Consider the bromination of cyclohexene. No cis-1,2-dibromocyclohexane is

formed.

Only anti addition is observed. The product is racemic since the initial attack of

bromine can occur with equal probability at either face of the cyclohexene.


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With acyclic alkenes the reaction is cleanly stereospecific:


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The Generality of Electrophilic Addition

The bromonium ion can be trapped by other nucleophiles.

Bromonation of cyclopentene using water as the solvent gives the vicinal

bromoalcohol (bromohydrin).

The water molecule is added antito the bromine atom and the other product is

HBr.


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Vicinal haloalcohols are useful synthetic intermediates.


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Oxymercuration-Demercuration: A special Electrophilic

Addition

The electrophilic addition of a mercuric salt to an alkene is called mercuration.

The product formed is known as an alkylmercury derivative.

A reaction sequence known as oxymercuration-demercuration is a useful

alternative to acid-catalyzed hydration:


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Oxymercuration is antistereospecific and regioselective.

The alcohol obtained from oxymercuration-demercuration is the same as that

obtained from Markovnikov hydration, however since no carbocation is involved in

the reaction mechanism, rearrangements of the transition state do not occur.


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Oxymercuration-demercuration in an alcohol solvent yields an ether:

electrophilic addition of hydrogen halides.pptx)

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