Question

Using molecular deuterium in the presence of the metal catalyst palladium hydrogenate this alkene: 2,3-dimethylbut-1-ene

Answer 1

Hydrogenation has three components, the unsaturated substrate, the hydrogen (or hydrogen source) and, invariably, a catalyst. The reduction reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst.

Substrate

The addition of H₂ to an alkene affords an alkane in the prototypical reaction:

RCH=CH₂ + H₂ ? RCH₂CH₃ (R = alkyl, aryl)

Hydrogenation is sensitive to steric hindrance explaining the selectivity for reaction with the exocyclic double bond but not the internal double bond.

An illustrative example of a hydrogenation reaction is the addition of hydrogen to maleic acid to form succinic acid. Numerous important applications of this petrochemical are found in pharmaceutical and food industries.

An important characteristic of alkene and alkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with "syn addition", with hydrogen entering from the least hindered side. Typical substrates are listed in the table

Substrates for and products of hydrogenation

substrate	product	comments
alkene, R2C=CR'2	alkane, R2CHCHR'2	many catalysts, one application is margarine
alkyne, RCCR	alkene, cis-RHC=CHR'	over-hydrogenation to alkane can be problematic
aldehyde, RCHO	primary alcohol, RCH2OH	easy substrate
ketone, R2CO	secondary alcohol, R2CHOH	more challenging than RCHO, prochiral for unsymmetrical ketones
ester, RCO2R'	two alcohols, RCH2OH + R'OH	challenging substrate
imine, RR'CNR"	amine, RR'CHNHR"	easy substrate, often use transfer hydrogenation, actual precursor is N-protonated
amide, RC(O)NR'2	amine, RCH2NR'2	challenging substrate
nitrile, RCN	primary amine, RCH2NH2	product amine reactive toward precursor nitrile in some cases
nitro, RNO2	amine, RNH2	commercial applications use heterogeneous Ni and Ru catalysts; major application is aniline

Catalysts

With rare exceptions, no reaction below 480