FireLion3
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Pd/H2 reduces aldehydes to alkanes or alcohols?
I haven't really been able to find a clear answer on this. Rather, many sources claim Pd takes aldehydes to alcohols while other sources claim Pd
(Pd/H2) takes aldehydes to alkanes. There seems to be an equally large number of sources making both claims.
This also has lead me to investigate the clemmensen reaction of reducing Aldehydes to Alkanes. Do aldehydes interact with zinc differently than with
palladium, thus resulting in alkanes versus alcohols, or is there simply a huge misnomer in so much literature claiming different to what Pd/H2
reduces aldehydes to?
Along with the many different literature contradictions, we have wikipedia over here claiming without reference:
http://en.wikipedia.org/wiki/Carbonyl_reduction
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Aldehydes can also be reduced to the alkane. An example is the reduction of an aromatic aldehyde to the methyl group using H2/Pd/C. Ketones and
aldehydes can also be reduced to alkanes using a zinc amalgam and hydrochloric acid via the Clemmensen reduction. |
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Nicodem
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Thread Moved
27-1-2014 at 08:04 |
Dr.Bob
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In some cases, such as for aromatic aldehydes, the reduction of the aldehyde provides a benzyl alcohol, which is easier to reduce than a simple
aliphatic alcohol.
So there are cases where is it doable, but mostly where the alcohol intermediate is activated somehow, like by an aromatic ring. But the ease of the
reaction will depend on the aromatic substituents and sterics of the molecule. You would have to provide more information to get a better answer, as
generic questions usually get generic answers.
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FireLion3
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We can use the example above. In the case of a benzyl alcohol, how easily does it go to the alkane? I know the alcohol group is a poor leaving group,
though at campus today I was looking through a textbook that claimed that in acidic solutions hydrogenolysis of a benzyl alcohol is more likely to
occur during a pd/h2 reduction of an aldehyde. | Quote: | | Further hydrogenolysis of benzyclic alcohols to alkane products can be a major problem with Pd/c catalyst, but can be controlled. In acidic solutions
hydrogenolysis is more likely to occur." |
I'm not sure how to interpret that. Is it saying subsequent hydrogenolysis of the alcohol from the aldehyde can be difficult to
prevent when using pd/c/h2? The wording "can be controlled" seems to imply that the hydrogenolysis happens by default.
Edit:
Did some more reading. So aromatic aldehydes can easily go to alcohols, which can then easily go to alkanes. Is an aromatic aldehyde only a molecule
which has a carbonyl group directly next to the aromatic ring? Or can it be a molecule that has a carbonyl group somewhere down a change branching off
the aromatic ring? Such as P-CH3-CH3-CH3-CHO
[Edited on 27-1-2014 by FireLion3]
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Random
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How does one explain hydrogenation of unsaturated ketone to rheosmin which doesn't ger further hydrogenated
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Nicodem
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Quote: Originally posted by FireLion3  | | We can use the example above. In the case of a benzyl alcohol, how easily does it go to the alkane? I know the alcohol group is a poor leaving group,
though at campus today I was looking through a textbook that claimed that in acidic solutions hydrogenolysis of a benzyl alcohol is more likely to
occur during a pd/h2 reduction of an aldehyde. |
Hydrogenolysis has nothing to do with nucleophilic substitution. For this reason, the "leaving group" property of the hydrogenolysed group is not
directly connected to the hydrogenolysis reactivity. It is not reasonable to use concepts from one mechanism and apply them to a different mechanism
without prior consideration on how exactly these properties relate to the reactivity. For example, some benzylamines are more readily debenzylated
than benzyl ethers when using Pd-C. Neither are benzyl chlorides necessarily much more readily hydrogenolysed than benzyl alcohols. Obviously, this is
not what one would expect by applying the leaving group tendency. We have theories in science that explain experimental results, by providing models.
The same goes for organic synthesis. It has theories with its own models. The more important theories in synthesis are the reaction mechanism
theories. I suggest you to get familiar with them.
| Quote: | | I'm not sure how to interpret that. Is it saying subsequent hydrogenolysis of the alcohol from the aldehyde can be difficult to
prevent when using pd/c/h2? The wording "can be controlled" seems to imply that the hydrogenolysis happens by default.
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Yes, complete hydrogenolysis all the way to the alkylaromatic can occur, but on some substrates the selectivity can be controlled. Acidic conditions
increase the rate of the benzyl alcohols hydrogenolysis (at least over Pd-C). The selectivity is highly substrate dependent. For example, acetophenone
is completely and rapidly hydrogenolysed all the way to ethylbenzene in mater of minutes over Pd-C, even at atmospheric hydrogen pressure. Some other
substrates will give a mixture of the benzyl alcohol and the alkylaromatic, sometimes even exclusively the alcohol.
| Quote: | | Is an aromatic aldehyde only a molecule which has a carbonyl group directly next to the aromatic ring? Or can it be a molecule that has a carbonyl
group somewhere down a change branching off the aromatic ring? Such as P-CH3-CH3-CH3-CHO |
The chemical terminology is most commonly functionally determined, hence an "aromatic aldehyde" stands an Aryl-CHO compound (where Aryl is an aromatic
or a heteroaromatic group). Same goes for aromatic ketones, which are aryl alkyl ketones or diaryl ketones.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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