"The mechanism of heterogenous hydrogenation involves (1) dissociative chemisorption of H2 on the catalyst, (2) coordination of the alkene to the surface of the catalyst, and (3) addition of the two hydrogen atoms to the activated π-bond in a syn- manner.… For low-pressure hydrogenations (1-30 atm), Pt, Pd, Rh, and Ru are used. The reactivity of a given catalyst decreases in the following order: Pt > Pd > Rh ~ Ru > Ni. For high-pressure hydrogenations (100-300 atm), Ni is usually the metal of choice. … The activity of a given catalyst generally is increased by changing from a neutral to a polar to an acid solvent. EtOAc, EtOH, and HOAc are the most frequently used solvents for low-pressure hydrogenations. … The reactivity of unsaturated substrates decreases in the following order: RCOCl > RNO, > RC-CR > RCH≡CHR > RCHO > RC≡N > RCOR > benzene. Both Pt and Pd catalysts fail to reduce RCOOR', RCOOH, and RCONH, groups. Pd is usually more selective than Pt. The ease of reduction of an olefin decreases with increasing substitution of the double bond. Conjugation of a double bond with a carbonyl group can markedly increase the rate of hydrogenation of the double bond. … With Pt and Pd catalysts, hydrogenation of allylic and benzylic alcohols, ethers, esters, amines, and halides is often accompanied by hydrogenolysis of the C-X bond where X = OH, OR, OAc, NR2, or halide, respectively."