In acidic catalyzed enolization of butan-2-one:

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We have 2 alpha hydrogens available. From the mechanism of acid catalyzed enolization there is a certain carbocation character at the carbonyl carbon because of resonance, and thus I thought that the elimination would be an E1 styled one where the alkene formed is the more substituted one.

But if we look at from an 'acidic hydrogen' perspective the hydrogen on the least substituted end is the most acidic whose extraction will lead to a less substituted alkene in the enol form.

So which enol would be major in the enolization of butan-2-one under acidic conditions? Would the same enol be major in basic medium?

  • $\begingroup$ In the acidic conditions, first the oxygen atom of $\ce{C=O}$ gets protonated by taking $\ce{H^+}$ from $\ce{H3O^+}$ and after that water molecule abstracts proton from the $\alpha$ - carbon to from the enol in which there is $\ce{C=C}$ double bond. So, definitely more substituted enol will be the major one, as the proton abstraction factor doesn't play a key role in acidic conditions, but in basic conditions it does. $\endgroup$ – Soumik Das Feb 5 '19 at 9:23
  • $\begingroup$ @SoumikDas But as per research.cm.utexas.edu/nbauld/teach/ch610bnotes/ch19.htm in both acidic and basic medium proton abstraction is RDS. $\endgroup$ – Akshat Joshi Feb 5 '19 at 10:32
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    $\begingroup$ I agree that the proton abstraction step is the rds, so the less substituted enol will be kinetically favoured, as it's easier, whereas the more substituted enol will be the thermodynamically controlled product. If you let your reaction run for much longer time, you will mostly get the more substituted one. $\endgroup$ – Soumik Das Feb 5 '19 at 12:32
  • $\begingroup$ @SoumikDas Why is the kinetic product favoured over the thermodynamic product in basic medium? $\endgroup$ – Akshat Joshi Feb 5 '19 at 12:50
  • $\begingroup$ @YUSUFHASAN It is not E2. You need a considerably strong base incase of E2. Even aqueous KOH fails to do E2. But some sources write the proton abstraction in a concerted fashion(but they do show carbocation resonance structure). $\endgroup$ – Akshat Joshi Feb 6 '19 at 10:17

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