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Which hydrogen of 3-methylcyclohex-2-en-1-one is the least acidic?

3-methylcyclohex-2-en-1-one

According to me it should be 2, as on its removal there won't be any resonance stabilization as would happen in the other cases. In addition to that a negative charge residing on a carbon attached to a double bond would be highly unstable.
But the answer key says that it is 3.

A follow up question: Which is the most acidic hydrogen?

According to me it should be 4 as it leads to a greater number of resonance structures.

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  • $\begingroup$ For your followup - how many resonance structures can you count for 4? What about 1? $\endgroup$ – kosyumote Jul 19 '16 at 11:26
  • $\begingroup$ Also, see chemistry.stackexchange.com/questions/26610/…. $\endgroup$ – kosyumote Jul 19 '16 at 11:37
  • $\begingroup$ three resonating structures for 4 and two for 1 $\endgroup$ – Prakhar Jul 19 '16 at 11:43
  • $\begingroup$ Did they ask something like "which of sp3 carbons have the least acidic hydrogen?" Or did they ask "which of these four hydrogens is the least acidic?" 2 indeed looks least acidic among all, because it is the only anion that doesn't have appropriate resonance structures. $\endgroup$ – sixtytrees Jul 19 '16 at 12:47
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You are correct. Position 2 is the least acidic.

Position 3 is not the least acidic hydrogen. It might be the most acidic hydrogen. Position 2 is the least acidic hydrogen of the four marked.

What does position 2 have going for it? Why might it not have been marked as the least acidic? The lone pair that would be left in the conjugate base is sitting in a sp² orbital.

3-methylcyclohex-2-en-1-one deprotonation at position 2

All else equal, sp² hybrid orbital containing $\ce{C-H}$ bonds are more acidic than sp³ hybrid orbital containing $\ce{C-H}$ bonds. The $\mathrm{p}K_\mathrm{a}$ of ethane $\ce{CH3CH3}$ is reported to be $\sim 50$, and the $\mathrm{p}K_\mathrm{a}$ of ethene $\ce{CH2=CH2}$ is reported to be $\sim 44$.

However, all else is not equal. Resonance, especially with a carbonyl group, stabilizes conjugate bases more. Acetone $\ce{CH2COCH3}$ has a $\mathrm{p}K_\mathrm{a}$ of $\sim 19$. Position 2 is not stabilized by resonance. The conjugate bases from positions 1, 3, and 4 are all resonance stabilized through the carbonyl group.

Position 1 has two resonance structures.

3-methylcyclohex-2-en-1-one deprotonation at position 1

Position 3 has three resonance structures.

3-methylcyclohex-2-en-1-one deprotonation at position 3

Position 4 has three resonance structures.

3-methylcyclohex-2-en-1-one deprotonation at position 4

It is between positions 3 and 4 for the most acidic. I chose position three because alkyl groups (the rest of the ring) are electron donating by induction, but alkyl group also stabilize pi bonds, so position 4 is a good candidate for most acidic as well.

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