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Alkenes: Addition reactions

Catalytic addition of $H_2$

$H_2$ adds to an alkene in the presence of platinum or palladium catalysts or nickel/aluminum alloy.

The adsorption (1) of a molecule $H_2$ on an active site (2) of one of these catalysts activates molecule (3), allowing the addition (4):

Electrophilic additions of $HCl,HBr,HI,H_2O$

Principle:

$HCl,HBr,HI$ add easily on an alkene, $HF$ destroys the alkene

$H_2O$ adds at $150^oC$ in acidic medium

Orientation:

These additions are stereospecific:

Markovnikov's rule $H$ is always placed on the less substituted C atom of the C=C bond

Examples:

In red, the less substituted carbon (which carries least of substituents other than hydrogen):

1)

2)

Mecanism:

a) Electrophilic attack on the π doublet b) Capture of the chloride ion

Stability of carbocations:

(1) is more stable than (2), because the inductive effect of the two methyl groups "fills" the positive hole better than the effect of ethyl. (1) therefore has a longer life, leaving the chloride ion time to get fixed.

Addition of the halogens $Cl_2, Br_2$

Principle:

$Cl_2$ and $Br_2$ add easily on an alkene, $F_2$ destroys the alkene, $I_2$ is not added

Example: Bromine and hex-1-ene

Mechanism: Bromine and propene

a) Electrophilic attack of bromine (electronegative) on the double π bond! b) Nucleophilic attack from "behind" of the bromide ion

There is formation of a bromonium ion intermediate. Note the multicenter bond that does not stick with the traditional Lewis structures.

Proof:

The reaction of ethene on bromine performed in a solution of sodium chloride leads both to 1,2-dibromoethane and 1-bromo-2-chloroethane.