December 07, 2021, 08:38:38 PM
Forum Rules: Read This Before Posting


Topic: what reagent  (Read 2226 times)

0 Members and 1 Guest are viewing this topic.

Offline rlabomba

  • Regular Member
  • ***
  • Posts: 18
  • Mole Snacks: +0/-0
what reagent
« on: December 07, 2017, 10:28:37 AM »
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? 
« Last Edit: December 07, 2017, 11:39:14 AM by rlabomba »

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1668
  • Mole Snacks: +116/-8
Re: what reagent
« Reply #1 on: December 07, 2017, 10:32:54 AM »
You would want to use a reagent that gives a low concentration of bromine, what could that be?

Offline rlabomba

  • Regular Member
  • ***
  • Posts: 18
  • Mole Snacks: +0/-0
Re: what reagent
« Reply #2 on: December 07, 2017, 11:39:44 AM »
NBS? 

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1668
  • Mole Snacks: +116/-8
Re: what reagent
« Reply #3 on: December 07, 2017, 11:45:02 AM »
Yes, that is the usual reagent for allylic bromination.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4993
  • Mole Snacks: +292/-22
Re: what reagent
« Reply #4 on: December 07, 2017, 02:07:06 PM »
@OP,

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

Offline rlabomba

  • Regular Member
  • ***
  • Posts: 18
  • Mole Snacks: +0/-0
Re: what reagent
« Reply #5 on: December 07, 2017, 03:21:46 PM »
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?

Offline rlabomba

  • Regular Member
  • ***
  • Posts: 18
  • Mole Snacks: +0/-0
Re: what reagent
« Reply #6 on: December 07, 2017, 03:24:39 PM »
@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?

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4993
  • Mole Snacks: +292/-22
Re: what reagent
« Reply #7 on: December 07, 2017, 06:45:54 PM »
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.

Sponsored Links