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Alkynes are more reactive towards catalytic hydrogenation than that of alkenes, because they are more unsaturated. [kindly, tell me if these is not the correct reason.].

But why are carbonyl compunds more reactive towards catalytic hydrogenation than alkyenes? The $\ce{C=O}$ in carbonyl group is polar, but how does that affect?


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Option (D): $c>a>b$

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I doubt that it is carbonil group that is hydrogenated. It is more likely, that $C-Cl$ bond will be cleaved here. $COCl$ group is known to be very ready to react. – permeakra Jan 15 '13 at 7:56
up vote 3 down vote accepted

That is not true in general: the stronger nature of the C=O bonds (larger bond energy) means that they are not as easily hydrogenated as C=C bonds. However, your compound (c) is not a regular carbonyl group, but an acyl chloride, which is a quite reactive species and thus very easily hydrogenated.

For a bit of reading, see here the section on “Catalytic Hydrogenation”:

As a rule, the carbonyl group does not add hydrogen as readily as do the carbon-carbon double and triple bonds. Thus, it is fairly easy to reduce an alkene or alkyne function without affecting any carbonyl functions in the same molecule. By using a platinum catalyst and increased temperature and pressure, it is possible to reduce aldehydes and ketones to alcohols, but carboxylic acids, esters and amides are comparatively unreactive. The exceptional reactivity of acyl halides, on the other hand, facilitates their reduction under mild conditions, by using a poisoned palladium catalyst similar to that used for the partial reduction of alkynes to alkenes. This reduction stops at the aldehyde stage, providing us with a useful two-step procedure for converting carboxylic acids to aldehydes, as reaction #1 below demonstrates. Equivalent reductions of anhydrides have not been reported, but we might speculate that they would be reduced more easily than esters. The only other reduction of a carboxylic acid derivative that is widely used is that of nitriles to 1º-amines.

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