[Lo.Lee.Ta.]

This is the answer for a past chem test...

When trying to draw a possible mechanism myself, I thought that things could start off by H3O+ protonating the gamma carbon, breaking the double bond between C3 and C4.

Then there would be a positive charge on C3, and I thought that the alpha carbon would then be able to lose an H to the water molecule, and create a double bond between C2 and C3.

Alpha carbons are acidic, so I thought that losing a H would not be a problem for it...


Is this a viable mechanism as well...? If not, why MUST we start by protonating the carbonyl Oxygen?
I would like to know why it happens the way the answer key is drawn and not the way I had thought...

Thank you so much!

[spirochete]

Your mechanism is a higher energy pathway because it forms a non resonance stabilized carbocation. I did not really read past that step because I know that is a major problem with it.

[spirochete]

I give you encouragement though, because these types of questions are the first step to becoming proficient in understanding mechanism.

[orgopete]

Is this a viable mechanism as well...? If not, why MUST we start by protonating the carbonyl Oxygen?
I would like to know why it happens the way the answer key is drawn and not the way I had thought...

The question is how do we know the carbonyl group is more basic than an alkene? This is a difficult question to answer by trying to find the comparable values in a table. You could think the absence of protonated alkenes is a hint that they are less basic than carbonyl groups. However, you could examine the mechanism of the Prins reaction and ask whether alkene protonation is preceding carbonyl protonation. You might also think alkene would be readily polymerized if treated with an acid.

The correct answer is the carbonyl group is more basic, it is protonated first. The rest follows from that step.