carboxylic acid halogenation

[yoyohomieg5432]

I'm looking at pg.15 of the biological sciences content outline. The reaction is shown on wiki here:

http://en.wikipedia.org/wiki/Hell–Volhard–Zelinsky_halogenation

I don't understand the first step. I.e. how is the alcohol getting replaced with Br?

Also, I read somewhere that this converts the carboxylic acid so that it can be converted into enolizable form. Why can't a carboxylic acid enolate?

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[Gauss44]

I'm looking at pg.15 of the biological sciences content outline. The reaction is shown on wiki here:

http://en.wikipedia.org/wiki/Hell–Volhard–Zelinsky_halogenation

I don't understand the first step. I.e. how is the alcohol getting replaced with Br?

Also, I read somewhere that this converts the carboxylic acid so that it can be converted into enolizable form. Why can't a carboxylic acid enolate?

I suspect the following: The bromine product only enolates because Br likes to make one bond only. Oxygen is almost the opposite, it likes to have double bonds. So on a carboxylic acid the electrons follow the path of least resistance to the other oxygen instead of the path of greater resistance toward the carbons. The bromine product doesn't have that option, so if the electrons go anywhere on the molecule, they are stuck with the direction of the carbons rather than the bromine direction.

 

[loltopsy]

I don't see any alcohol being replaced with bromine in that page. If you mean the hydroxyl group of the carboxylic acid, in that case, the first step is addition to the carbonyl's oxygen. Trivalent phosphorous love oxygen. It forms a 4 centered complex with an oxygen of the carboxylic acid (draw it bound to the carbonyl oxygen). This allows the proton bound to the carboxylic acid to eliminate forming a sort of ester bond with the phosphorous. The negative charge on the phosphorous facilitates a bromine leaving with its electrons to form a bromide anion. This then adds into the ester at the carbon, breaking the carbon-oxygen double bond. Now the negatively charge oxygen forms a double bond with the central carbon and eliminates the oxygen-phosphorous complex.

I realize that's difficult to read, so I drew the mechanism.

http://i.imgur.com/4OBG3qz.jpg?1

 

[AureatEagle]

I'm looking at pg.15 of the biological sciences content outline. The reaction is shown on wiki here:

http://en.wikipedia.org/wiki/Hell–Volhard–Zelinsky_halogenation

I don't understand the first step. I.e. how is the alcohol getting replaced with Br?

Also, I read somewhere that this converts the carboxylic acid so that it can be converted into enolizable form. Why can't a carboxylic acid enolate?

loltopsy is correct with respect to the PBr3 mechanism. For your second question, a carboxylic acid cannot easily be enolized because of it's OH group. Enolizing any carbonyl functional group results in an anion which would be destabilized by the OH, as it is an electron donating group.

 

[Gauss44]

loltopsy is correct with respect to the PBr3 mechanism. For your second question, a carboxylic acid cannot easily be enolized because of it's OH group. Enolizing any carbonyl functional group results in an anion which would be destabilized by the OH, as it is an electron donating group.

An enol requires an OH in it's -ene form. http://en.wikipedia.org/wiki/Enol Remember that an enol fluctuates between having a double bond between two carbons, and having a double bond between an oxygen and a carbon.

 

[yoyohomieg5432]

so am i understand this correctly: the alpha hydrogen is less acidic with the OH there which makes the enolization challenging because the OH is donating into the pi*. Replacing with Br solves this problem.

Also, thanks to loltopsy for posting the mechanism that helped a lot

 

[loltopsy]

The bromine structure, the bromine cannot make a double bond to the carbon (it would be way too weak due to the positive charge created on bromine), so the bromine's electronegativity takes over. It withdraws electron density from the pi electron system, pulling the electrons closer to the carbon closest to the bromine. In order for the double bond to break, the electron density must be on the alpha carbon. Moving the electron density to the carbon attached to the bromine stabilizes the structure.