Protonation Watch

username2176541
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#1
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If you protonate a weak acid e.g. benzoic acid.
+ +
So c6h5cooh + H+ ==> c6h5c(oh)2 or c6h5cooh2


which product do you get? both products have 1+ charge
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charco
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(Original post by Xenon17)
If you protonate a weak acid e.g. benzoic acid.
+ +
So c6h5cooh + H+ ==> c6h5c(oh)2 or c6h5cooh2


which product do you get? both products have 1+ charge
The oxygen of the carbonyl group will accept a proton.
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username2176541
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(Original post by charco)
The oxygen of the carbonyl group will accept a proton.
The markscheme gives the product as c6h5cooh2
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charco
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(Original post by Xenon17)
The markscheme gives the product as c6h5cooh2
It's an interesting one ...

I would go with the carbonyl, as this allows resonance delocalisation of the positive charge using the pi electrons.

This can't happen with protonation of the hydroxyl oxygen.

It is possible that the mark scheme is not actually showing the location of the proton, rather a general idea.

The esterification mechanism shows protonation of the carbonyl. The following image is taken from chemguide:

Name:  estermech2.gif
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MexicanKeith
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(Original post by Xenon17)
The markscheme gives the product as c6h5cooh2
The most basic site of the two is definitely the carbonyl Oxygen's lone pairs (the lone pairs on the other oxygen are delocalised and hence less available to accept a proton) so I would agree that the mark scheme isn't really trying to be representative of the product. In reality the protonation would be an equilibirum based process and so both oxygen atoms would be rapidly protonated and deprotonated.
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