Chemical processes of dehydration and dehydrogenation

What are the main characteristics of these processes?

n-butane 3D structure [Wikimedia]

In or­gan­ic chem­istry, re­ac­tions of de­hy­dro­gena­tion and de­hy­dra­tion are of­ten used for ob­tain­ing un­sat­u­rat­ed com­pounds – for ex­am­ple alkenes. To con­duct these pro­cess­es, sub­stances of dif­fer­ent struc­ture and com­po­si­tion, con­di­tions and cat­a­lysts, but in both re­ac­tions the same un­sat­u­rat­ed prod­ucts may be ob­tained.

De­hy­dra­tion of or­gan­ic com­pounds

De­hy­dra­tion is the re­ac­tion of a wa­ter mol­e­cule sep­a­rat­ing from an or­gan­ic mol­e­cule. Al­co­hols are of­ten sub­ject­ed to de­hy­dra­tion, as they have one or sev­er­al OH groups. Reagents may as­sist in sep­a­rat­ing wa­ter from an al­co­hol mol­e­cule, thus form­ing an un­sat­u­rat­ed com­pound. Un­der dif­fer­ent con­di­tions, de­hy­dra­tion may af­fect both bonds with­in a mol­e­cule and bonds be­tween them. This is the ba­sis for the clas­si­fi­ca­tion of de­hy­dra­tion – it can be in­ter­molec­u­lar and in­tramolec­u­lar ac­cord­ing­ly. Prod­ucts of these re­ac­tions are dif­fer­ent.

In­tramolec­u­lar de­hy­dra­tion

Phosphoric acid 3D structure [Wikimedia]

So that de­hy­dra­tion does not af­fect the ad­ja­cent atoms of the al­co­hol, the tem­per­a­ture of the re­ac­tion must ex­ceed 140 ᵒC (284 ᵒF). Con­cen­trat­ed sul­fu­ric (phos­phor­ic) acid is used as a de­hy­drat­ing sub­stance. The op­ti­mum tem­per­a­ture is 180 ᵒC (356 ᵒF).

Click here for oth­er ex­per­i­ments with or­gan­ic acids.

  • CH₃-CH₂-CH₂-OH = CH₃-CH=CH₂ + H₂O (a dou­ble bond forms be­tween car­bon atoms, from which a hy­drox­yl group and hy­dro­gen sep­a­rate; propy­lene forms);

If the al­co­hol is sec­ondary (the hy­drox­yl group is not lo­cat­ed at the fi­nal car­bon atom), the re­ac­tion may go in two di­rec­tions:

  • CH₃-CH₂-CH(OH)-CH₃ = CH₃-CH=CH-CH₃ + H₂O (butene-2);

  • CH₃-CH₂-CH(OH)-CH₃ = CH₃-CH₂-CH=CH₂ + H₂O (butene-1).

Trans-butene-2 [Wikimedia]

The re­ac­tion takes place ac­cord­ing to Za­it­sev’s rule (in the de­hy­dra­tion of al­co­hols, hy­dro­gen sep­a­rates from the less hy­drat­ed car­bon atom). In the cen­ter of the mol­e­cule, is car­bon with two hy­dro­gen atoms, and at the end with three; this means that sep­a­rate to a large ex­tent will take place from the group where there is less hy­dro­gen – in the mid­dle.

In­tramolec­u­lar de­hy­dra­tion

With low heat­ing (be­low 284 ᵒF or 140 ᵒC) al­co­hols de­hy­drate with the for­ma­tion of an es­ter:

СН₃-ОН + НО-СН₃ = СН₃-О-СН₃ + Н₂О.

De­hy­dra­tion takes place as fol­lows:

R-[ОН + Н]О-R = R-О-R + Н₂О (one hy­drox­yl group sep­a­rates en­tire­ly, and only hy­dro­gen from the sec­ond).

Non­sym­met­ri­cal es­ters can also be ob­tained:

С₃Н₇-ОН + НО-СН₃ = С₃Н₇-О-СН₃ + Н₂О (the reagents con­tain a mix­ture of al­co­hol).

via GIPHY

Si­mul­ta­ne­ous­ly, sec­ondary sym­met­ric es­ter prod­ucts form:

  • С₃Н₇-ОН + НО-С₃Н₇ = С₃Н₇-О-С₃Н₇ + Н₂О;

  • СН₃-ОН + НО-СН₃ = СН₃-О-СН₃ + Н₂О.

Pri­ma­ry al­co­hols en­ter into these re­ac­tions more of­ten. Ter­tiary and sec­ondary al­co­hols do not form es­ters, but turn into alkenes by the mech­a­nism of in­tramolec­u­lar de­hy­dra­tion.

С₃Н₇ОН [Wikimedia]

De­hy­dro­gena­tion re­ac­tion

The de­hy­dro­gena­tion re­ac­tion is the process of an even num­ber of hy­dro­gen atoms sep­a­rat­ing from a mol­e­cule of an or­gan­ic com­pound. This re­ac­tion does not usu­al­ly take place with­out a cat­a­lyst. Prod­ucts of de­hy­dro­gena­tion may be alkenes, alkines, di­enes – var­i­ous un­sat­u­rat­ed com­pounds. In the de­hy­dro­gena­tion of low­er alka­nes (their car­bon chain is not longer than C₂-C₄), the mol­e­cule re­mains acyclic. Clos­ing into the cy­cle takes place for alka­nes with a car­bon chain longer than 4 car­bon atoms.

In the de­hy­dro­gena­tion process, a break of the C-H bond takes place, hy­dro­gen atoms in ad­ja­cent car­bon atoms move away and form molec­u­lar hy­dro­gen, and in their place a dou­ble bond clos­es in the mol­e­cule. In de­hy­dro­gena­tion of hy­dro­car­bons (for ex­am­ple bu­tane of nor­mal struc­ture), a mix­ture of iso­mers may form (sub­stance with dif­fer­ent struc­ture or po­si­tion of atoms in space, but with an iden­ti­cal atom­ic com­po­si­tion and weight). 2 iso­mers of the prod­uct are ob­tained – butene-1 and butene-2.

2CH₃-CH₂-CH₂-CH₃ = CH₃-CH=CH-CH₃ + CH₂=CH-CH₂-CH₃ + 2Н₂ (in the pres­ence of a nick­el cat­a­lyst at a tem­per­a­ture of 500 ᵒC (932 ᵒF)).

At a tem­per­a­ture of 450-650 ᵒC (842-1202 ᵒF) on an alu­mo-chrome cat­a­lyst 2 mol­e­cules of wa­ter sep­a­rate and the alka­di­ene bu­ta­di­ene-1,3 forms:

CH₃-CH₂-CH₂-CH₃ = CH₂=CH-CH=CH₂ + 2H₂.

trans-butadiene [Wikimedia]

De­hy­dra­tion and de­hy­dro­gena­tion are re­ac­tions that are of­ten used in prac­tice in or­gan­ic syn­the­sis. It is im­por­tant when car­ry­ing them out to take into ac­count that of­ten it is not a pure sub­stance that forms as a prod­uct, but a mix­ture of iso­mer­ic sub­stances – if a cer­tain sub­stance must be ob­tained in pure form, it must be sep­a­rat­ed from the iso­mers and side prod­ucts of re­ac­tion.