Reaction of hydrogenation of alkenes

Alkenes or olefins are hy­dro­car­bons which con­tain a short (dou­ble) bond be­tween car­bon atoms. The gen­er­al for­mu­la of alkenes is CₙH₂ₙ. As a con­se­quence of the dou­ble bon in alkene mol­e­cules, var­i­ous re­ac­tions of at­tach­ment with an open­ing of the dou­ble bond can be car­ried out. Dif­fer­ent mol­e­cules can be at­tached – halo­gens, hy­dro­halo­gens, wa­ter, hy­dro­gen etc. By the re­ac­tion of hy­dro­gena­tion, sim­i­lar sat­u­rat­ed hy­dro­car­bons can be ob­tained from alkenes – alka­nes.

Fea­tures of the hy­dro­gena­tion re­ac­tion

The hy­dro­gena­tion of olefins (alkenes) is a re­ac­tion of the at­tach­ment of hy­dro­gen by a dou­ble bond with the for­ma­tion of alka­nes – sat­u­rat­ed aliphat­ic (non­cycli­cal) hy­dro­car­bons.

CH₂=CH₂ + H₂ = CH₃-CH₃ (with heat­ing in the pres­ence of Ni, eth­ane is formed from eth­yl­ene).

The re­ac­tion can also be ac­cel­er­at­ed by pres­sure. The re­ac­tion takes place in a gaseous phase with a re­duc­tion in the vol­ume of the sys­tem (in ini­tial reagents, ₂ vol­umes of gas are present – eth­yl­ene and hy­dro­gen, and in the prod­ucts only one – eth­ane). With a re­duc­tion of the vol­ume of gas, pres­sure in the sys­tem drops, and so the re­ac­tion takes place less in­ten­sive­ly. This prop­er­ty of the sys­tem obeys Le Chate­lier’s prin­ci­ple (the equi­lib­ri­um in a sys­tem shifts to the side which weak­ens the ex­ter­nal im­pact on the sys­tem). So in these sys­tems, in­creased pres­sure is usu­al­ly cre­at­ed.

The re­ac­tion of the hy­dro­gena­tion of alkenes is an ox­i­da­tion-re­duc­tion re­ac­tion, as it is ac­com­pa­nied by a change in ox­i­da­tion states:

CH₃-CH=CH₂ + H₂ = CH₃-CH₂-CH₃ (in propy­lene, in the car­bon atoms from left to right the ox­i­da­tion states are equal to -3, -1 and -2, and in propane - -3, -2 and -3).

Molec­u­lar hy­dro­gen in the re­ac­tion ox­i­dizes, ac­quir­ing an ox­i­da­tion state of +1:

H₂ -2e = 2H⁺, car­bon atoms with a dou­ble bond are re­duced (schemat­i­cal­ly the re­duc­tion process takes place as fol­lows):

• C⁻¹ + e = C⁻²;

• C⁻² + e = C⁻³.

MEL Chem­istry sub­scrip­tion in­cludes an ex­per­i­ment with hy­dro­gen com­bus­tion.*

De­pend­ing on the cat­a­lyst used, hy­dro­gena­tion may be het­eroge­nous (if the cat­a­lyst is a met­al) and ho­moge­nous (if the cat­a­lyst is a com­plex of a tran­si­tion met­al, for ex­am­ple rhodi­um or ruthe­ni­um).

Con­di­tions of the re­ac­tion process

The re­ac­tion does not take place in nor­mal con­di­tions – both heat­ing and cat­a­lysts are re­quired. The op­ti­mum cat­a­lyst for the hy­dro­gena­tion of alkenes is plat­inum. Ow­ing to the high price of the met­al, it is of­ten re­placed by a cheap­er equiv­a­lent – nick­el (cat­a­lyst – Raney nick­el), in the pres­ence of which the hy­dro­gena­tion re­ac­tion also takes place. The most ac­tive cat­a­lysts of the process may be ruthe­ni­um and rhodi­um, but these cat­a­lysts have not re­ceived wide­spread use in in­dus­try.

When nick­el is used as a cat­a­lyst, in the sys­tem a pres­sure of 5-10 at­mos­pheres is usu­al­ly cre­at­ed, and the tem­per­a­ture is raised to 50-100 ᵒC or 122-212 ᵒF. These are rather harsh con­di­tions, as the nick­el cat­a­lyst is less re­ac­tive than its equiv­a­lents.

Plat­inum is used in the form of Adams’ cat­a­lyst – plat­inum diox­ide PtO₂, which when hy­dro­gen is passed into the re­ac­tive mix­ture is re­duced to plat­inum black – fine­ly dis­persed metal­lic plat­inum. When it is used, gen­tler con­di­tions of hy­dro­gena­tion are re­quired – the re­ac­tion is car­ried out with heat­ing up to 20-50 ᵒC or 68-122 ᵒF in al­co­hol or acetic acid un­der nor­mal pres­sure.

At the end of the re­ac­tion, the sol­vent is evap­o­rat­ed, and the cat­a­lyst fil­tered.

What cat­a­lyst is bet­ter, het­eroge­nous or ho­moge­nous?

Fre­quent­ly, met­als are used in in­dus­try as cat­a­lysts of hy­dro­gena­tion. But het­eroge­nous hy­dro­gena­tion has a num­ber of short­com­ings:

• or­di­nary bonds be­tween car­bon atoms can break off, as a re­sult of which hy­dro­car­bons form which are less branched or which have a short­er car­bon chain;

• the alkene can iso­mer­ize – form com­pounds of sim­i­lar atom­ic com­po­si­tion, but of a dif­fer­ent struc­ture.

These fac­tors cause the for­ma­tion of sec­ondary prod­ucts of re­ac­tion, and there­fore ef­fect the yield and pu­ri­ty of the fi­nal prod­uct. This does not hap­pen when ho­moge­nous cat­a­lysts are used.

Hy­dro­gena­tion in the pres­ence of com­plex­es may be car­ried out in or­di­nary con­di­tions:

CH₂=CH₂ + H₂ = CH₃-CH₃ (at a tem­per­a­ture of 273 Kelvin and a pres­sure of 1 at­mos­phere in the pres­ence of Wilkin­son’s cat­a­lyst, the com­plex of rhodi­um [C₆H₅P]₃RhCl – tris-(triph­enylphos­phane) rhodi­um chlo­ride.

Het­eroge­nous cat­a­lysts can also se­lec­tive­ly re­duce short bonds. If there are sev­er­al short bonds in a com­pound, in the present of these cat­a­lysts only one of them can be re­duced – for ex­am­ple, the dou­ble bond will re­duce much more quick­ly which has few­er sub­sti­tutes (mono- or di-sub­sti­tu­tion) than the one with more sub­sti­tutes (tri- or tetra-sub­sti­tu­tion):

CH₂=CH-CH₂-CH=C(CH₃)₂ + H₂ = CH₃-CH₂-CH₂-CH=C(CH₃)₂ (the less sub­sti­tut­ed dou­ble bond is re­duced more quick­ly in the pres­ence of [C₆H₅P]₃RhCl).

Poly­mer­iza­tion

the process can be used for mak­ing fuel with a high oc­tane num­ber.