How does propane react with bromine under radical conditions?

1 Answer
Feb 28, 2017

It is conceived to be a radical process......

Explanation:

Halogenation of alkanes is a radical process, that tends to be indiscriminate (i.e. dihalogenated products are possible in that radical mechanisms can be quite unselective):

#"H"_3"CCH"_2"CH"_3 +"Br"_2 stackrel(hnu)rarr "H"_3"CCHBrCH"_3 +"HBr"#

The mechanism of halogenation is in your text, and the starting point is the homolysis of the bromine molecule by UV light:

#Br_2 +hnurarr2dotBr#

The #dotBr# radical, a 7 electron species, is very reactive inasmuch as when it reacts it generates another radical species to continue the chain of reaction:

#R-CH_3+dotBr rarrR-dotCH_2 +HBr#

#R-dotCH_2 +Br_2 rarr RCH_2Br + dotBr#

And the #dotBr# radical can continue the chain of reaction.......The reaction can terminate by the coupling of 2 radicals:

#R-dotCH_2 +dotCH_2R rarr RH_2C-CH_2R#

The presence of such #C-C# coupled products is good evidence for the intermediacy of these radicals. Most of the time, however, radical halogenation of alkanes will lead to a mess (whereas radical halogenation of benzyl groups is a bit more discriminating in that the stabilization of a benzyl radical allows selective formation of #PhCH_2Br#).