What is the mechanism for the reduction of ketone to alkanes using $\ce{H2}$, $\ce{Pd/C}$ in ethanol? Searching on Google mostly yields the reduction of ketones to secondary alcohols using LAH.

  • $\begingroup$ Is there acid present? $\endgroup$
    – Beerhunter
    Jun 7 '17 at 15:14
  • $\begingroup$ For the reduction of ketone to alkane, no. $\endgroup$ Jun 7 '17 at 15:14
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    $\begingroup$ The ketone is reduced to an alcohol first. I suggest looking up benzyl alcohol reduction to toluene. $\endgroup$
    – Beerhunter
    Jun 7 '17 at 15:16
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    $\begingroup$ Actually, you can convert ketones to alkanes, though not realistically with just palladium on carbon (It would probably only give the alcohol product). You could convert it to a dithiane using a dithiol and then reducing it with Raney Nickel. $\endgroup$
    – Linus Choy
    Jun 7 '17 at 17:23

The reduction of a ketone all the way down to an alkane using catalytic hydrogenation over palladium on charcoal is not a general/widely used process.

Quoting from Comprehensive Organic Functional Group Transformations III:

...while the reduction of benzylic ketones by hydrogenation over palladium is often an efficient procedure , it ... sees limited use. An isolated report of the reduction of aliphatic ketones by hydrogenation over platinum on Montmorillonite K-10 merits ...attention here and further study

The majority of examples of this reaction occur using benzylic ketones, carrying out the reaction in the presence of aqueous acid in order to facilitate the loss of the hydroxyl caused upon initial reduction of the ketone. The adjacent aromatic ring helps stabilise the developing (+) charge as the C-O bond cleaves, which is what makes benzylic ketones good substrates for this reaction.

Reduction of benzylic ketone with catalytic hydrogenation

Ref: J. Med. Chem. 2001, 44, 3424

In the context of lab scale chemistry (i.e. not industrial processes), several good methods exist including the Wolff–Kishner (or variants as Jan pointed out in the comments), the Clemmensen, and the Mozingo reduction. Some success is also noted via initial reduction, mesyaltion, and treatment with a hydride source.

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    $\begingroup$ Plus Wolff-Kishner variants such as Huang-Minlon. $\endgroup$
    – Jan
    Jun 7 '17 at 22:31
  • $\begingroup$ Out of all the methods discussed, catalytic hydrogenation is the most efficient and practised at tonne scale. Acidic conditions, as mentioned, are best. $\endgroup$
    – Beerhunter
    Jun 8 '17 at 13:55
  • $\begingroup$ @Beerhunter, are they under conditions applicable to the lab? Or is it a case of high temperature, high high pressure, strong acid on simple feedstocks? $\endgroup$
    – NotEvans.
    Jun 8 '17 at 17:57
  • $\begingroup$ Pressure obviously but on tetracycline antibiotics with HCl or methanesulfonic acid, less than 100degC. Pressure depends on metal and substrate $\endgroup$
    – Beerhunter
    Jun 8 '17 at 18:04

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