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![picture from national chemistry Olympiad]

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I cant get the mechanism. Answer shows there will be formation of keto group. But how?

As I can understand the $\ce{H^+}$ is going to attack the $\ce{O}$ and can remove the $\ce{CH3}$ groups forming two adjacent alcohols which may join to form keton. I'm not sure.

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The mechanism to convert acetal to ketone in acid based hydrolysis is in following steps-

  • Protonation of (1) to make OCH3 a good leaving group forming (2)
  • O lone pair allows LG to leave forming oxonium ion and alcohol(3)
  • Water nucleophile attacks C of C=O (3) to give (4)
  • Base removes proton to give (5).
  • Reprotonates (5) to make OCH3 a better leaving group(6).
  • CH3OH leaves to give (7)
  • Base deproronates to give ketone (8)

Note :In this scheme, the base, B:, could be R-O-R, R-OH, H2O or the conjugate base of the acid catalyst

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References:

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  • $\begingroup$ Base is confusing. It is actually water. $\endgroup$ – Raoul Kessels Jan 28 '18 at 9:31
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    $\begingroup$ that was awesome! in the 5 step it will be h2o Not base. $\endgroup$ – Shariful Islam Jan 28 '18 at 9:34
  • $\begingroup$ @RoulKessels In this scheme, the base, B:, could be R-O-R, R-OH, H2O or the conjugate base of the acid catalyst $\endgroup$ – Chakravarthy Kalyan Jan 28 '18 at 9:35
  • $\begingroup$ @SharifulIslam In this scheme, the base, B:, could be R-O-R, R-OH, H2O or the conjugate base of the acid catalyst $\endgroup$ – Chakravarthy Kalyan Jan 28 '18 at 9:37
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Yes, the oxigen is protonated, water enters, methanol leaves and another $\ce{H^+}$ is released. Then the oxigen of the other $\ce{CH3O}$ group is protonated, the keto group is formed while the methanol leaves and $\ce{H^+}$ is released.

Edit:

The first part results in:

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and $\ce{CH3OH}$.

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  • $\begingroup$ what do you mean by water enters? $\endgroup$ – Shariful Islam Jan 28 '18 at 8:01
  • $\begingroup$ I have edited. I do not have a proper drawing program with me to draw the whole mechanism $\endgroup$ – Raoul Kessels Jan 28 '18 at 8:09
  • $\begingroup$ in the first step the lower O will be protonated,forming OH+(CH3).Then the CH3- Will be removed and OH+ WILl become alcohol group.From where does CH3-gets Oh-?And why the same will not occur for the upper och3 group? $\endgroup$ – Shariful Islam Jan 28 '18 at 9:08
  • $\begingroup$ No. Water enters and methanol is removed. Then the other -OMe is protonated and methanol is removed and the ketone is formed. $\endgroup$ – Raoul Kessels Jan 28 '18 at 9:25
  • $\begingroup$ I have found an image $\endgroup$ – Raoul Kessels Jan 28 '18 at 9:31

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