Free Radical Addition of HBr To Alkenes
Description: Treatment of an alkene with HBr in the presence of catalytic amounts of a radical initiator (such as peroxides) in addition to heat or light leads to addition of HBr to the alkene in anti-Markovnikov fashion (note that the Br adds to the less substituted side of the alkene and the H adds to the more substituted side).
Notes: A common initiator for this purpose is t-butyl peroxide (t-BuO-Ot-Bu) but in general the peroxide will be written, “RO-OR”. Heat or light is required to break the weak O–O bond. Again, note that this reaction is in contrast to normal addition of HBr which proceeds in “Markovnikov” fashion.
Again, the initiator is generally catalytic (less than 1 equiv required)
Notes: Note that either light (hγ) or heat (Δ ) can be used to initiate the reaction, and the initiator can be written variably as “peroxides”, “RO-OR” or even given specifically, as in the case of t-butyl peroxide in the bottom example. The reaction is not stereospecific, so if new stereocenter(s) are formed, such as in the bottom example, a mixture of stereoisomers may be obtained.
Note also that Br adds to the less substituted carbon of the alkene.
Mechanism: The purpose of heat/light is to cause homolytic fragmentation of the weak O–O bond of the peroxide catalyst (Step 1 arrow A) . This leads to reversible formation of alkoxy radicals, which can then remove a hydrogen (homolytically) from H–Br to give the alcohol (R-OH) and bromine radical Br• (Step 2, arrows B and C) . The purpose of the peroxide is just to get the free radical chain reaction going (that is why only a catalytic amount is necessary). However, at least 1 equiv of H-Br is necessary to convert all the alkene to the addition product.
Once the bromine radical is formed, it can then add to the alkene. Note that since radical stability increases with increasing substitution (i.e. primary < secondary << tertiary) addition will occur such as to form the most substituted (i.e. most stable) radical which means that Br will add to the least substituted carbon of the alkene (step 3, arrows D and E). In this case a secondary radical is formed, which then reacts with H–Br to form the alkyl halide product and to regenerate bromine radical (Step 4, arrows F and G).
Although often not shown, termination occurs when the concentration of alkene or H-Br becomes low. Termination is the combination of two radicals to form a new bond. One possible termination step might be the combination of the alkyl radical with bromine radical, shown below:
Notes: This radical addition process only works for H-Br, not H-Cl or H-I.
Since free-radicals can participate in reactions from either face, if a stereocenter is formed, a mixture of stereoisomers will be obtained. For example, in the alkene below, the first addition (of Br•) can occur from either face, as can the attack of the radical on H-Br. Four stereoisomers are obtained.