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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on August 02, 2019, 02:42:05 PM

Title: Catalytic Hydrogenation - reactivity order
Post by: xshadow on August 02, 2019, 02:42:05 PM

Hi

If I have a ""theoric"" substrate with:

-Nitro group
- Aldehydic group
-double bond
-triple bond

What is the reactivity order toward the catalytic hydrogenatoon (for example Pd/H2)


 Usually I associate Hydrogenation / H₂   with alkenes and alkynes (double and triple bond C-C )

But now my slides say that  nitro group is the most reactive group  towards hydrogenation
.is it correct?


So can be a reactivity order like this:

1) NO group
2)triple bond C≡C
3)double  bond C=C
4) C=O derivates (usually ketones and aldehydes)
???

Thanks!!

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: hollytara on August 03, 2019, 12:12:52 AM

This depends on how you define reactivity.  The order you list  nitro>alkyne>alkene>carbonyl tracks the reaction enthalpies: nitro is the most exothermic, carbonyl is the least. 

But reactivity could also refer to the rate of the reaction, and this might be different depending on the catalyst that is used.

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: xshadow on August 04, 2019, 05:20:51 AM

If I use:

1)  Pd/C
2)Pt

With palladium is the correct order?

My doubt is about nitro group vs C=C hydrogenation
Thx

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: rolnor on August 04, 2019, 08:36:36 AM

I think both reactions will be very fast, maybe not possible to reduce one without the other.

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: hollytara on August 04, 2019, 03:43:28 PM

If memory serves, reaction enthalpies are about 550 kJ/mol for nitro to amine, about 300 kJ/mol for alkyne (double reduction to alkane), about 150 kJ/mol for alkene, and about 70 kJ/mol for carbonyls. 

You can hydrogenate alkenes selectively over ketones with Pt or Pd and H2.  Some examples show alkenes and nitro groups both hydrogenated under these conditions. 

Wilkinsons Catalyst in THF/t-BuOH will reduce the alkene but not the nitro.  J. Org. Chem.  2002, 67 (9) 3163-3164

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: OrganicDan96 on August 04, 2019, 05:01:57 PM

I know the op didn't actually ask this but what about hydrogenolysis of benzyl ethers vs hydrogenation of nitriles? can a nitrile be hydrogenated selectively?

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: hollytara on August 04, 2019, 06:15:51 PM

A lot depends on the catalyst.  Your best bet is to look in the literature to find out if there is one with the selectivity you need. 

Title: Re: Catalytic Hydrogenation - reactivity order
Post by: rolnor on August 05, 2019, 03:57:25 AM

You can use nickel boride to reduce nitriles in presence of benzyl ethers I think.