Organic Chemistry - "Change", Part 9: Peroxide Initiated Radical Addition of HBr to an Alkene

 Greetings,

This post covers the radical addition of HBr to an Alkene.  Key points, along with the mechanism, will be discussed.

Key Points

These are the main things to know in order to better understand the radical addition of Hydrogen Bromide to an Alkene.

• Initiated by Heating an Alkyl Peroxide
• Cleavage (Breaking) of HBr Bond is Homolytic
• Bromine Adds First
• Addition is Anti-Markovnikov
• Only HBr Adds as Radicals

Initiated by Heating an Alkyl Peroxide

Alkyl Peroxides have the general formula, R1O-OR2. The O-O bond is relatively weak and can be broken by heating alone, without the aid of any additional reagents (such as a catalyst). Alkyl Peroxide cleavage happens first, before the formation of radicals from HBr. Alkyl Peroxide radicals are short-lived (like other radicals), but exist long enough to produce H. and .Br radicals.

Cleavage of HBr is Homolytic

Breaking of the HBr bond must be homolytic (even splitting of the HBr bond) to produce radicals. Homolytic cleavage must be initiated by another radical species (Alkoxide Radicals in our case, here).

Bromine Adds First

The Br radical is much more stable than the H radical. Br radical exists longer (than H radical) because of its larger size - greater number of electrons stabilizes the Br radical. The H radical must immediately bond to the O atom of an alkoxide radical, leaving the Br radical available to react with the alkene.

Addition is Anti-Markovnikov

Hydrogen adds to the carbon atom with the lower number of hydrogens bonded to it. This is anti-markovnikov and happens because the H atom must add after the Br atom. Br adds to the carbon atom with more bonded H atoms because this places the unpaired radical electron on the carbon atom bonded to a greater number of other carbon atoms (more stable distribution of charge concept). Generally, for a direct addition to an alkene, the first atom to add always bonds to the lesser substituted carbon (greater number of C-H Bonds), which results in a more stable intermediate state - Lower energy state translates to "reaction pathway of least resistance".

Only HBr Adds as Radicals

HF and HCl compounds have stronger covalent bonds than HBr: It takes much more energy to produce radicals from HCl and HF than from HBr (again, "path of least resistance"!). The I radical, formed from HI breaking apart, is relatively too unreactive to bond with an alkene carbon: I radical will bond with the alkoxide radical before bonding with an alkene because there is much less energy required.

The following image shows the mechanism of the reaction.

The H radical (Step 4 addition) comes from a second HBr molecule, which produces another Br radical, along with the main product. Production of another radical causes this reaction to be self-perpetuating (it is a chain reaction).

That concludes this post. As always, Thank you for reading!