according to this rule, product I i.e., -bromopropane is expected. In actual practice, this is the principal product of the reaction. This generalisation of Markovnikov rule can be better understood in terms of mechanism of the reaction. Mechanism Hydrogen bromide provides an electrophile, H + , which attacks the double bond to form carbocation as shown below : (i) The secondary carbocation (b) is more stable than the primary carbocation (a), therefore, the former predominates because it is formed at a faster rate.
(ii) The carbocation (b) is attacked by Br – ion to form the product as follows : -Bromopropane (major product) (a) less stable primary carbocation (b) more stable secondary carbocation Anti Markovnikov addition or peroxide effect or Kharash effect In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with HBr but not with HCl and Hl. This addition reaction was observed by M.S. Kharash and F.R.
Mayo in at the University of Chicago. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule. CH – CH=CH +HBr (C H CO) O CH –CH CH Br –Bromopropane ( . ) Mechanism : Peroxide effect proceeds via free radical chain mechanism as given below: (ii) C .
H +H–Br Homolysis C H + B . r The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary. This explains the formation of -bromopropane as the major product. It may be noted that the peroxide effect is not observed in addition of HCl and HI.
This may be due to the fact that the H–Cl bond being stronger ( . kJ mol – ) than H–Br bond ( . kJ mol – ), is not cleaved by the free radical, whereas the H–I bond is weaker ( . kJ mol – ) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.
Problem . Write IUPAC names of