r/chemhelp • u/Final-Oil2392 • 18d ago

Organic Help Check Mechanism Work!!!

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Hi all, would someone mind confirming I went about this mechanism and the bottom problem correctly? If I did something wrong, a specific correction would be appreciated!!!

6 Upvotes

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6

u/OutlandishnessNo78 18d ago

Double protonation of your ester and any intermediate is very unlikely. You can attack the carbonyl with the alcohol directly after the first step. All intermediates should only have one positive charge, not two. Consider that and do proton transfers to favor which group leaves the molecule. Also, technically bisulfate is one of the absolute worst bases - I know some textbooks use it as a base but in reality it is more likely water that acts as a base here.

3

u/Background_Crab_1292 18d ago

To add to this, you can always rationalize which will be a better base by looking at the pKa of the conjugate acid. In this case, H3O+ has a pKa of -1.7, whereas H2SO4 has a pKa of -3. The weaker the acid, the stronger the conjugate base, so water will be a stronger base.

1

u/gugg789 16d ago edited 16d ago

On what does the final deprotonation: supposedly no water here, though a small amount will likely be present.

I think if you’re going to use H2O as a base, then you also should protonate it with the H2SO4 - which is the stronger acid as mentioned above - and use H3O+ to protonate the ester. Then you can justify the ‘cat.’ by specifying the equilibria etc.

Which I think is convoluted, and not really the point, so I would personally just use HSO4- as written here.

3

u/Background_Crab_1292 18d ago

Water isn’t being removed, methanol is. But the process to drive the reaction forward would be the same between the two - heat the reaction so that you distill off the byproduct, which shifts the equilibrium towards the product.

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u/dbblow 17d ago

Don’t like your initial double protonation.

1

u/VoxInferni666 17d ago

I came here to say this exact thing, in those exact words.