ABSTRACT
Electrocatalytic hydrogenations (ECHs) of unsaturated organic compounds have been carried out sincethebeginningofthelastcentury[1-3].Ithasbeensuggestedquiteearlythatelectrochemical reductionsofunsaturatedorganiccompoundsinaqueousmedia,oncathodesofspongytransitionmetals,mightinvolvereactionoftheunsaturatedsubstratewithM(H)ads [4]. ECH involves thegenerationofchemisorbedhydrogen,M(H)ads (M is an adsorption site), by reduction of water (Equation 44.1)anditsreactionwithanadsorbedorganicsubstrate,M(Y=Z)ads (Equation 44.3) followedbydesorptionofthehydrogenatedproduct,M(YH-ZH)ads (Equation 44.4). If the YH-ZH sigmabondissufšcientlyweak,itshydrogenolysismayalsotakeplace(Equation44.5).Mostofthe organicfunctionalgroupsthathavebeenhydrogenatedbyheterogeneouscatalytichydrogenation (CHhet) have also been hydrogenated by ECH [5-12]. A large variety of catalytically active materialshavebeenusedascathode[5-18].AnumberofreviewsontheECHoforganiccompoundshave beenpublished[5-12].Thischapterdoesnotpresentanextensivereviewonthevariousorganic functionalgroupsthathavebeenhydrogenatedbyECH.Itratherconsistsofadiscussionwith examples:onthemechanismofECH,ontheadvantagesanddisadvantagesofECHwhencompared
I. Introduction ......................................................................................................................... 1657 II.Mechanistic Aspects of ECH .............................................................................................. 1658
A.General Aspects ........................................................................................................... 1658 B.Factors InŽuencing the Current Efšciency (the Efšciency) of ECH ........................... 1659
III.ECH versus Heterogeneous Catalytic Hydrogenation (CHhet) ............................................ 1660 A.Advantages of ECH over CHhet.................................................................................... 1661
1.Hydrogenation of Phenanthrene at Raney Ni ....................................................... 1661 2.Hydrogenation of Nitrobenzene to Aniline at Raney Cu ...................................... 1661
B.Disadvantages of ECH over CHhet ............................................................................... 1661 1.Hydrogenation of Phenanthrene to Perhydrophenanthrenes................................ 1662 2.ECH of Lignin Models and of Lignins ................................................................. 1662
C.Conclusions on the Comparison between ECH and CHhet .......................................... 1663 IV.ECH versus Electrohydrogenation by Electroreduction ...................................................... 1664 V.Electrohydrogenation of Nitro Compounds ........................................................................ 1664
A.Mechanistic Considerations ......................................................................................... 1664 B.Selectivity of Electrohydrogenation of Nitro Compounds on Raney Cu Cathodes .... 1666
1. Electrohydrogenation of Nitroalkyl Groups ......................................................... 1666 2.Electrohydrogenation of Nitroaryl Groups ........................................................... 1667
VI.ECH of Conjugated Enones................................................................................................. 1668 VII.Studies on Well-Dešned Surfaces ....................................................................................... 1669 References .................................................................................................................................... 1671
to CHhet and when compared to the hydrogenation of organic compounds via an electroreduction process(electronation-protonation[EP]mechanism),ontheselectivityofECHofnitrocompounds andofconjugatedenones,andonstudiesonwell-dešnedsurfaces(singlecrystals).Reviewshave also been published on studies on well-dešned surfaces [19-21].
