COOH and halogen

[Electrons]

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R-CH2COOH + Br2/PBr3 => ?

Answer: R-CBrHCOOH (The Br binds to alpha Carbon).

Why? I thought answer is RCH2COBr, but it's not. Isn't OH more acidic than the alpha H on the alpha Carbon?

 

[doc3232]

I agree with your answer.
Never seen the other one.

 

[Electrons]

The above is from Marye Anne Fox & James Whitesell flash card.

I just found that in Destroyer Roadmap #4 top middle also address this, it would RCH2COBr instead of the above answer. Hrm...interesting.

 

[artanis]

R-CH2COOH + Br2/PBr3 => ?

Answer: R-CBrHCOOH (The Br binds to alpha Carbon).

Why? I thought answer is RCH2COBr, but it's not. Isn't OH more acidic than the alpha H on the alpha Carbon?

That is the correct answer. If i am not mistaken, it sho0uld be written as PBr3/Br2, anyways, within the mechanism you get the terminal carbons OH turning into BR, you then get a Br2 attacking the Alha carbon (the one adjacent to the terminal) at which point it knocks of an H, and by the end the Br at the terminal carbon leaves once again and becomes an -OH group once again. If any of you have McMurray's 6th Ed this is the best book to look at this stuff. If you do review chapter 22 at the back of the chapter in the summary.

 

[Electrons]

Wow. You are right. I just checked chapter 22 summary. They even got a name for it Hell-Volhard-Zelinski bromination of acids. Ok, so maybe Destroyer needs some correction. Thanks.

 

[doc3232]

This reaction is meaningless. It will not come up on the section referred to as the most straightforward section.
I have seen carboxylic acids react with PBr3/Br2 and form acid halides.

 

[Electrons]

This reaction is meaningless. It will not come up on the section referred to as the most straightforward section.
I have seen carboxylic acids react with PBr3/Br2 and form acid halides.

lol. They got a name for it. It has be correct and must be already reviewed by many before getting such research published.