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It goes without saying that every reduction agent has different reducing properties and strengths in terms of what sort of bond it can reduce/hydrogenate. But, why does it generally follow that hydrogen takes the place of an oxygen molecule during a hydrogenation/reduction reaction (in the presence of the necessary hydrogen or $\ce{H}$-donor of course)?

What is it about the properties of oxygen that makes it so ready to disassociate from the molecule when it gains an electron, allowing hydrogen to take its place? I'm sure there is something simple that I'm missing, but any explanation is appreciated.

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  • $\begingroup$ Welcome to Chemistry.SE. Could you add an example of what you are referring to as "hydrogen taking the place of oxygen"? Most organic reductions (involving oxygen-containing functional groups) are of the type where a carbon-oxygen double bond is converted into a carbon-oxygen single bond: $\ce{R2C=O -> R2CH-OH}$. $\endgroup$
    – Ben Norris
    Jan 17 '14 at 12:40
  • $\begingroup$ Yes that is a relatively good example. There is also the instance of catalytic transfer hydrogenolysis where R2CH-OH is converted into R2CH3 by cleavage of the oxygen. Or other instances with a strong enough hydrogenating agent were R-COOH can be converted to RCH3. My question is what is it about the intrincis properties of oxygen and hydrogen that make hydrogen so readily want to take its place in a highly reductive environment? $\endgroup$
    – JohanS
    Jan 17 '14 at 13:13
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But, why does it generally follow that hydrogen takes the place of an oxygen molecule during a hydrogenation/reduction reaction [...]

Ok, if at all, it's an oxygen atom ;)

I don't see that general trend towards complete deoxygenation, neither in catalytic hydrogenations, nor in hydride transfer reactions.

It usually takes

  1. rather drastical conditions
  2. quite some special reagents
  3. a particular type of derivatisation

or a combination of any of these to convert oxygen-containing compounds to alkanes.

Let's have a look at some examples:

$\ce{R1-CO-R2 -> R1-CH2-R2}$

  • Ketones and aldehydes may be directly reduced to alkanes in the Clemmensen reduction. The "mildest" variants use anhydrous $\ce{HCl}$ and amalgamated $\ce{Zn}$ in organic solvents.

  • The clasical Wolff-Kishner reduction, using hydrazine and sodium in diglyme, involves the in situ functionalization of the carbonyl and isn't a prototype for a mild reaction either. Admittedly, milder variants exist, using N-tert-butyldimethylsilylhydrazone and catalytical amounts of a Lewis acid, e.g. $\ce{Sc(OTf)3}$

  • Tosylhydrazones may be converted to alkanes using $\ce{NaCNBH3}$, $\ce{Na(CH3COO)3BH}$, or boranes.

  • Catalytic hydrogenation over Raney nickel is possible after converting the carbonyl to a dithiane; so it's actually a desulfurization.

$\ce{R1-CH2-OH -> R1-CH3}$

I'm not aware of any transformation that does not involve functionalization.

  • Alkyl tosylates can be deoxygenated using $\ce{LiEt3BH}$.
  • Acylation of alkohols with thiocarbonyl chlorides furnish thiocarbonates, which may be deoxygenated under using either tin hydrides ($\ce{R3SnH}$, Barton deoxygenation) or $\ce{B(CH3)3}$ in water/benzene.
  • Alkanes may also be obtained from alkohols under Mitsunobu conditions ($\ce{Ph3P}$, $\ce{DEAD}$, o-nitrobenzenesulfonylhydrazine).

$\ce{R1-CH2-COOH -> R1-CH3}$

Direct transformations of carboxylic acids to alkanes typically involve decarboxylation, i.e. the loss of $\ce{CO2}$. This may be achieved either by the Barton decarboxylation, or via photoinduced electron transfer reactions with electronically excited (electron) acceptors. In the course of the latter reactions, a carboxylate is oxidized to an acyloxy radical, which then undergoes decarboxylation.

To sum it up: It's not that easy ;)

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In the special case of reduction to an alcohol in acid (or other moiety that becomes a leaving group under reaction conditions), the resulting (stabilized) carbocation can be further reduced to the hydrocarbon (e.g., catalytic reforming using noble metal catalyst on acid zeolite under hydrogen). Benzylic positions often reduce to (unwanted) hydrocarbon. You can push this by using Lewis acid $\ce{AlH3}$ rather than $\ce{LiAlH4}$, $$\ce{3 LiAlH4 + AlCl3 → 4 AlH3 + 3 LiCl}.$$

Another entry into deoxygenation reduction is to use a potent oxophile like titanium, e.g., McMurray reaction, that is astounding effective for coupling hindered carbonyls into olefins by plucking out the oxygens. Its cousin the Tebbe methylenation does $\ce{R2C=O}$ to $\ce{R2C=CH2}$, convert a diaryl benzil's carbonyls into methylenes. Follow with olefin metathesis and ethylene extrusion (ADMET polymerization) ton give fully functionalized polyacetlyenes as living polymers, allowing block co-polymer builds, before quench. What fun! Here is an example with a p-phenylene spacer,

enter image description here

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