[rlabomba]

Had this on my test last week and i'm drawing a blank.  I said BR₂ with light but wouldn't that attach at the Pi bond carbon? 

[rolnor]

You would want to use a reagent that gives a low concentration of bromine, what could that be?

[rlabomba]

NBS? 

[rolnor]

Yes, that is the usual reagent for allylic bromination.

[Babcock_Hall]

@OP,

Do you know how the mechanism for this is different from the addition of bromine across a double bond?

[rlabomba]

Yes, that is the usual reagent for allylic bromination.

I was watching a video and the video showed Br₂ will accomplish the same reaction by adding the Br to the carbon adjacent to the double bonded carbon.  Is this true?

[rlabomba]

@OP,

Do you know how the mechanism for this is different from the addition of bromine across a double bond?

I know with NBS the Br will attach to the adjacent Carbon of the double bond, the Br radical will grab the Hydrogen to the adjacent double bond and will form a secondary radical on the substrate.  Then the Br will bond to this carbon.  Am I saying this right?

[Babcock_Hall]

You certainly have the key idea correct: allylic bromination is a radical process, unlike the addition across a double bond, which is a polar process.  Your description of the second portion of the radical process is unclear to me.  The carbon-centered radical reacts with a fresh molecule of Br₂, and creates a fresh bromine radical.