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


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.

  • $\begingroup$ For your followup - how many resonance structures can you count for 4? What about 1? $\endgroup$
    – kosyumote
    Jul 19, 2016 at 11:26
  • $\begingroup$ Also, see chemistry.stackexchange.com/questions/26610/…. $\endgroup$
    – kosyumote
    Jul 19, 2016 at 11:37
  • $\begingroup$ three resonating structures for 4 and two for 1 $\endgroup$
    – Prakhar
    Jul 19, 2016 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, 2016 at 12:47

1 Answer 1


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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