Characteristics of alkynes and hydrogenation of acetylene

Alkynes (or acety­lene hy­dro­car­bons) are un­sat­u­rat­ed hy­dro­car­bons, with the com­mon for­mu­la of Cn­H₂ₙ₋₂ (n is the num­ber of car­bon atoms). Com­pounds of this type have a triple co­va­lent bond in their struc­ture. The two car­bon atoms are in a trou­ble bond, each of which is in the state of sp-hy­bridiza­tion (this is a spe­cial type of over­lap of atom­ic or­bitals, when one s and one p-or­bital are mixed, and the oth­er two are lo­cat­ed per­pen­dic­u­lar­ly and form a p-bond or hold a lone elec­tron pair) – this is why the ge­om­e­try of this com­pound of atoms is lin­ear. For ex­am­ple, the sim­plest alkyne – acety­lene C₂H₂ is lo­cat­ed on a plane lin­eal­ly: the an­gle be­tween car­bon atoms in the com­pound is 180 de­grees.

Phys­i­cal prop­er­ties

The first three alkynes in their ho­mol­o­gous row (in this row sub­stances dif­fer by length – each new hy­dro­car­bon re­sem­bles the pre­vi­ous one by its struc­ture, but dif­fers by one struc­tural unit of CH₂) are gas­es:

• HC≡CH (C₂H₂ - ethyne, or acety­lene);

• HC≡C-CH₃ (C₃H₄ - propy­ne);

• HC≡C-CH₂-CH₃ (C₄H₆ - bu­tyne).

The prin­ci­ple of ad­di­tion in the ho­mol­o­gous row of ho­mol­o­gous dif­fer­ence of CH₂ is clear:

С₂Н₂ + СН₂ = С₃Н₄;

С₃Н₄ + СН₂ = С₄Н₆ (with the ad­di­tion of CH₂ to the pre­vi­ous alkyne, the next one is ob­tained).

All alkynes, start­ing with petyne and end­ing with alkyne with 16 car­bon atoms, ex­ist in nor­mal con­di­tions in the form of liq­uids. The high­er alkynes, start­ing with C₁₇H₃₂, take the form of solids. All com­pounds with a triple bond dis­solve poor­ly in wa­ter, and well in non­po­lar sol­vents (or­gan­ic – for ex­am­ple ben­zol).

Iso­merism of alkynes

Iso­merism is the phe­nom­e­non of com­pounds which are iden­ti­cal in com­po­si­tion but dif­fer­ent in struc­ture. For alkynes, there are three types of iso­merism:

• in­ter­class (alkynes iso­mer­ized by alka­di­enes, hy­dro­car­bons with two dou­ble bonds – for ex­am­ple propy­ne and propa­di­ene – in­ter­class iso­mers with the for­mu­la C₂H₄ (propy­ne has one triple bond be­tween car­bon atoms and one or­di­nary, while propa­di­ene has two dou­ble bonds);

• iso­merism of the po­si­tion of the triple bond (in bu­tyne the triple bond may be lo­cat­ed at the first and sec­ond car­bon atom – ac­cord­ing­ly, bu­tyne-1 and bu­tyne-2 dif­fer re­spec­tive­ly: CH≡C-CH₂-CH₃ и CH₃-C≡C-CH₃);

• iso­merism of chain (the struc­ture of the chain in alkynes may dif­fer – for ex­am­ple hexyne-2 CH₃-CH≡C-CH₂-CH₂-CH₃ is iso­mer­ized by 4-methyl pen­tyne-2 CH₃-CH≡C-CH₂(CH₃)-CH₃).

Spa­tial (geo­met­ric) iso­merism, in which a com­pound has iden­ti­cal struc­ture and com­po­si­tion but dif­fer­ent lo­ca­tion of atoms in space, is not char­ac­ter­is­tics for alkynes.

Ob­tain­ing alkynes

The com­mon method for ob­tain­ing all ho­mologs of acety­lene is the re­ac­tion of di­haloalka­nes with an al­co­hol so­lu­tion of an al­ka­li (elim­i­na­tion re­ac­tion). Halo­gens may be lo­cat­ed on ad­ja­cent atoms of car­bon, and on the same one (sum­ma­ry equa­tions are giv­en):

• CH₃-CH₂-CHBr₂ + 2NaOH = CH₃-C≡CH + 2NaBr + 2H₂O;

• CH₃-CHCl-CHCl-CH₃ + 2KOH = CH₃-C≡C-CH₃ + 2KCl + 2H₂O.

Main in­dus­tri­al method of ob­tain­ing acety­lene CH≡CH (C₂H₂):

6CH₄ + O₂ = 2C₂H₂ + 2CO + 10H₂ (re­ac­tion is car­ried out at a tem­per­a­ture of around 1500 ᵒC or 2732 ᵒF).

A sim­i­lar prod­uct of re­ac­tion can be ob­tained by the ther­mic break­down of meth­ane:

2CH₄ = C₂H₂ + 3H₂ (tem­per­a­ture of break­down – around 1200 ᵒC or 2192 ᵒF).

The sim­plest alkyne (acety­lene) can be ob­tained from cal­ci­um car­bide:

CaC₂ + 2H₂O = C₂H₂ + Ca(OH)₂.

In the pres­ence of a strong al­ka­li, acety­lene can be ob­tained from di-haloalka­ne in 2 stages:

1. CH₂­Cl-CH₂­Cl + KOH(al­co­hol) = CH₂=CH-Cl + KCl + H₂O;

2. CH₂=CH-Cl + KNH₂ = C₂H₂ + KCl + NH₃.

Chem­i­cal prop­er­ties of alkynes

By their re­ac­tive abil­i­ty, alkynes are in many ways sim­i­lar to alkenes, but there are also oth­er spe­cial re­ac­tions for them that are only char­ac­ter­is­tics for com­pounds with a triple bond of re­ac­tion.

Alkynes en­ter into re­ac­tions of sub­sti­tu­tion, at­tach­ment, ox­i­da­tion and poly­mer­iza­tion. The most im­por­tant and wide­spread re­ac­tions of alkynes are giv­en be­low.

• Re­ac­tions of ox­i­da­tion of alkynes

• Ox­i­da­tion of alkynes can be com­plete and in­com­plete. In the first case, the com­bus­tion of un­sat­u­rat­ed car­bon with a triple bond takes place:

2CH≡CH + 5O₂ = 4CO₂ + 2H₂O (sooty flame).

• In­com­plete com­bus­tion is car­ried out in the pres­ence of potas­si­um per­man­ganate in a neu­tral medi­um, or of potas­si­um dichro­mate in an acidic medi­um (for sim­plic­i­ty, atom­ic oxy­gen [O] is giv­en in the re­ac­tion, as this par­tic­u­lar is re­leased in the con­tact of per­man­ganates or dichro­mates with or­gan­ic mat­ter):

СH₃-C≡CH + 3[O] + H₂O = CH₃-COOH + HCOOH (a mix­ture of car­bon­ic acids forms – acetic and formic);

CH≡CH + 4[O] = HOOC-COOH (ox­al­ic acid).

• Re­ac­tions of poly­mer­iza­tion

• Dimer­iza­tion (for­ma­tion of a com­pound from two monomers):

CH≡CH + CH≡CH = CH₂=CH-C≡CH (vinyl acety­lene forms in an acidic medi­um in the pres­ence of salts of mono­va­lent cop­per and am­mo­ni­um chlo­ride NH₄­Cl).

• Lin­ear trimer­iza­tion:

3CH≡CH = CH₂=CH-C≡C-CH=CH₂ (a lin­ear trimer forms in an acidic medi­um in the pres­ence of salts of uni­va­lent cop­per and am­mo­ni­um chlo­ride NH₄­Cl).

• Zelin­sky’s re­ac­tion – cy­clotrimer­iza­tion (for­ma­tion of ben­zol and its aro­mat­ic de­riv­a­tives):

3СH≡CH = C₆H₆ (at 500 ᵒC or 932 ᵒF in the pres­ence of ac­ti­vat­ed char­coal, ben­zol forms).

If propy­ne CH₃-С≡CH is heat­ed with crys­tal sul­fur acid, we can ob­tain 1,3,5-trimethyl­ben­zol (methyl groups break down across one car­bon atom in a ben­zol ring).

• Re­ac­tions of sub­sti­tu­tion (char­ac­ter­is­tic only for alkynes with the group -С≡СH)

On re­act­ing with strong bases, the hy­dro­gen atom in the alkyne is sub­sti­tut­ed to the met­al (in­sol­u­ble acetylenides form):

CH₃-С≡CH + [Ag(NH₃)₂]OH = CH₃-СH≡CAg + 2NH₃ + 2H₂O.

Sub­sti­tu­tion also takes place with uni­va­lent cop­per chlo­ride, sil­ver ox­ide and al­ka­line met­als:

CH≡CH + 2Cu­Cl = Cu-C≡C-Cu + 2HCl (di-sub­sti­tut­ed uni­va­lent cop­per acetylenide);

CH₃-C≡CH + CuCl = CH₃-C≡C-Cu + HCl (mono­sub­sti­tut­ed uni­va­lent cop­per acetylenide);

2CH≡CH + 2Na (in liq­uid NH₃) = 2CH≡C-Na + H₂;

CH≡CH + Ag₂O (in NH₃·H₂O) = Ag-C≡C-Ag + H₂O.

The most im­por­tant prop­er­ties of alkynes for study are their ad­di­tion re­ac­tions – in­clud­ing the hy­dro­gena­tion of acety­lene.

Ad­di­tion re­ac­tions of alkynes, hy­dro­gena­tion of acety­lene

For alkynes, all ad­di­tion re­ac­tions which af­fect the dou­ble bond are char­ac­ter­is­tic. For ex­am­ple, a qual­i­ta­tive re­ac­tion can be car­ried out for the dou­ble bond – the dis­col­oration of bromine wa­ter (as two p-bonds are present, the re­ac­tion of com­plete bromi­na­tion takes place in 2 stages):

The first one:

CH≡CH + Br₂ = CHBr=CHBr;

The sec­ond one:

CHBr=CHBr + Br₂ = CHBr₂-CHBr₂ (1,1,2,2-tetra­bro­moethane forms).

With hy­dro­halo­gens, the re­ac­tions take place ac­cord­ing to Markovnikov’s rule (hy­dro­gen moves from acid to the most hy­dro­genat­ed car­bon atom, and the halo­gen to the least). The re­ac­tion also has two stages:

The first one:

CH₃-С≡CH + HCl = CH₃-C(Cl)=CH₂;

The sec­ond one:

CH₃-C(Cl)=CH₂ + HCl = CH₃-C(Cl)₂-CH₃ (prod­uct - 2,2-dichlor­propane).

In the pres­ence of acids and salts of bi­va­lent mer­cury, Kucherov’s re­ac­tion is pos­si­ble – the re­ac­tion of alkynes with wa­ter (hy­dra­tion):

• CH≡CH + Н₂О = СН₃-СОН (ac­etalde­hyde forms – acetic alde­hyde; only in the re­ac­tion of acety­lene with wa­ter);

• CH₃-С≡CH + Н₂О = СН₃-С(О)-СН₃ ((with any oth­er alkynes be­sides acety­lene, ke­tones are formed ac­cord­ing to Kucherov’s re­ac­tion – for ex­am­ple, ace­tone).

The hy­dra­tion of any alkynes, in­clud­ing acety­lene, also takes place in stages – first one p-bond is hy­drat­ed, then an­oth­er. A re­ac­tion with molec­u­lar hy­dro­gen is pos­si­ble with heat­ing in the pres­ence of a plat­inum or nick­el cat­a­lyst (full hy­dra­tion can be car­ried out in this way):

1. CH≡CH + Н₂ = СН₂=СН₂ (the prod­uct of the in­com­plete hy­dro­gena­tion of acety­lene – eth­yl­ene);

2. СН₂=СН₂ + Н₂ = СН₃-СН₃ (prod­uct of re­ac­tion – eth­ane; on com­plete hy­dro­gena­tion of acety­lene alka­nes form – sat­u­rat­ed aliphat­ic (non­cycli­cal) hy­dro­car­bons).

For hy­dro­gena­tion to take place ful­ly, the ini­tial hy­dro­car­bon and molec­u­lar hy­dro­gen should be tak­en in the ra­tio of 1:2.

Acety­lene

In or­der to stop the re­ac­tion when eth­yl­ene is pro­duced – the prod­uct of in­com­plete hy­dro­gena­tion of acety­lene – less re­ac­tive cat­a­lysts should be used, for ex­am­ple Pd/Ca­CO₃/Pb(CH₃­COO)₂. Oth­er alkynes be­sides acety­lene also form alka­nes on com­plete hy­dro­gena­tion:

CH₃-С≡CH + 2Н₂ = СН₃-СН₂-СН₃ (propane).

Here you can find more ex­per­i­ments with hy­dro­gen.

Acety­lene is used much more of­ten in in­dus­try than oth­er alkynes. It has found wide ap­pli­ca­tion in or­gan­ic syn­the­sis, the man­u­fac­ture of syn­thet­ic ma­te­ri­als (for ex­am­ple PVC) and sol­vents. Its char­ac­ter­is­tic re­ac­tion is of­ten used for this – for ex­am­ple, the re­ac­tion with halo­gens or hy­dro­halo­gens.