Alkene HBr mechanism

This is the more detailed mechanism of what happens when an addition reaction takes place between an alkene and HBr. 

Starter of free radicals called the INITIATION stage CI2 in bright sunlight — 2C1  

The star means it has an odd number of electrons. CI2 breaks and gives one electron to each atom. 

HBr is polarised as the molecule approaches the high election density in the C=C bond the electrons of Br in the H —Br bond are repelled 

As discussed earlier, if propene gas and HBr gas are mixed then the H of the H-Br attacks the carbon atom with the most number of hydrogens on it and the Br goes to the other carbon atom. 

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