Free Radical Bromination of Alkanes

by James

Description: When treated with bromine (Br2) and light (hν) alkanes are converted into alkyl bromides. In the absence of any double bonds, with Br2 this is selective for tertiary carbons.
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Notes: Light (hν) is the initiator for this reaction.

Examples: 

Notes: Note that each of these reactions produces HBr as a byproduct. If this reaction occurs at a stereocenter, a mixture of alkyl bromides will be obtained.

This table allows for the relative comparison of the selectivity of radical chlorination and bromination (reaction rates with typical primary, secondary, tertiary alkyl C-H bonds)

radical selectivity

Mechanism: When treated with light, Br2 fragments to bromine radicals (Step 1, arrow A). At any given time only a small amount of these radicals is present. There’s lots of leftover Br2! Remember this as it comes up in step 3. Bromine radical then abstracts a hydrogen from the tertiary carbon, leaving behind the tertiary radical (Step 2, arrows B and C). The tertiary radical then reacts with Br2, giving the tertiary bromide (Step 3, arrows D and E).

Notes: Avoid the common mistake of having the radical react with Br• in the third step – that’s not a propagation reaction, that’s termination!

Note that tertiary radicals are more stable than secondary or primary radicals and thus require the least energy to form (another way of saying this is that tertiary C-H bonds are weaker than secondary and primary C-H bonds). The higher selectivity of this reaction for tertiary C-H (when compared with chlorination) is a reflection of the fact that formation of the weaker H–Br bond provides less of a driving force for this reaction as compared to H–Cl (which is a stronger bond).

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{ 9 comments… read them below or add one }

vu quoc

Dear Sir,
I see that exciting topic. But you give me some example for “heat may also suffice”. What difference hv and heat are inside this reaction ?
“Notes: Although light (hν) is commonly employed as the initiator, heat may also suffice.”

thanks

Reply

james

Thanks for asking for clarification – what I should have said is that one can also use heat and peroxides. A common example of a peroxide for these types of reactions is tert-butyl peroxide, (CH3)3C–O–O–C(CH3)3 . When this is heated, it fragments to give the free radical (CH3)3CO• , which can fragment Br–Br homolytically to give Br• and start the chain reaction.

Reply

MzF

this may seem silly, but i just cant remember–are rearrangements possible in radical bromination?

Reply

James Ashenhurst

If you’re asking if H and alkyl shifts are possible, for our purposes the answer is no.

Reply

stefani johnson

what about chain termination?

Reply

James

Chain termination can be drawn by showing ANY of the radicals depicted here combining with the carbon radical. Any time you have two radicals combining to form a new bond, that is chain termination.

Reply

Petr Menzel

Hi, there is table with comparison of selectivity. For Br-: 0,007; 1; 220; 19,4. You write that Br- “likes” tertiary carbon. Why there is not the highest number? Thx, P.

Reply

James

Did you mean to say “19,400 ” ?

Reply

Petr Menzel

Sorry, I can see 19 and there is “comma”, not “dot” —> 19 400. It is clear for me now.

Reply

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