Orgo chem reaction question

[Neil45]

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Can HCl be used to convert a carboxylic acid into an acyl chloride as shown below:

The answer says PCl3, PCl5, and SOCl can be used for this reaction which I understand. However, it says HCl cannot be used to do this. I thought that HCl would protonate the -OH group; this would create a good LG with the positive charge on the oxygen. Then the chloride ion can attack the carbonyl carbon and "kick out" the positively charge oxygen group. Does anyone know why HCl cannot be used?

 

[facecrime]

Cl ion is not a good nucleophile?

 

[ttpharm]

You need to understand the reactivities of carboxylic derivatives.

From least stable to stable:

Acyl chlorides < Anhydrides < Esters < Amides < Carboxylic Acids.

Conversions usually occur when going from something thats least stable to something more stable unless you use the exceptions such as SOCl which only converts Carboxylic acids -> Acyl Chlorides.

So lets say you have an acyl chloride, adding a methoxide would convert it to an ester. or H2O would convert it to a carboxylic acid. However, an Anhydride would not convert to an acyl chloride because its less stable. To do this, however, you would have to convert it to a carboxylic acid and then use SOCl to convert it into a acyl chloride.

 

[jwest88]

The carbonyl O is more basic than the OH in the carboxylic acid.(consider the resonance form which puts a negative charge on the carbonyl oxygen. When the carbonyl oxygen protonates, Cl- can attack but it will also be the best leaving group so it will be reversible.

 

[Dotoday]

Remember, base reacts with acid, not acid with acid. That would be easier to understand. Hope this helps.

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