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Halogenation of alkenes ... why?

When you add Br2 to RCH=CH2 why is it that the first Br always attatches to to the C that is not connected to the R chain ??
franks
When you add Br2 to RCH=CH2 why is it that the first Br always attatches to to the C that is not connected to the R chain ??



the first Br will attach to the CH2 so as to form an intermediate that is more stable as the positive charge is in the middle of the molecule (2º carbonium ion)

RCH=CH2 + Br-Br ---> R-CH(+)-CH2Br + Br-

This secondary carbonium ion is formed faster as it is energetically more stable that the corresponding 1º carbonium ion R-CHBr-CH2(+)

Ihn most organic reactions there are competing side reactions and the most successful is the fastest (which has the lowest activation energy due to the stability of the intermediate) In this case the first stage of the reaction is faster for the 2º C+ ion formation.
Reply 2
Avatar for dinesh
dinesh
Just to add:
the stability of the secondary carbo cation , is attributed to the fact that CH3( and all alkyl groups for that fact) have a positive inductive effect. This means that it "pushed" electrons. The electrons will reduce that positive charge around the C and thus increase the stability of the intermediate.
To be MORE accurate, and this is preferred by examiners to my knowledge,
draw it is
R-CH---CH2
\ BR+/
(if it doesn't show clearly the bond is from the C in the CH to the BR and then from the C in the CH2 to the Br, not the R if it shows up like that)

This shows the formation of the bromonium .THe BR(-) then attacks it from behind.
forming the dibromo compound.


Note though that in the case of Hydrogen halides they follow mackovnikovs rule.
(can't spell it oh well.)
of course one also has steric considerations. It is easier (i.e. the reaction cross section is larger) for a bromine to attack the CH2 group rather than the CHR group because you haven't got the girt big R in the way. Dinesh is right that a transition state is (to the best of our knowledge) stabilised in such a fashion as described. This is more intuitive as the Br- also attacks the less sterically hindered end of the molecule.
Reply 4
Avatar for Twiglet
Twiglet
OP
2
thanks! Makes everything make much more sense now :biggrin:
xxx

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