How the oxidation of primary alcohols takes place

Al­co­hols are called pri­ma­ry if their hy­drox­yl group is lo­cat­ed at the first car­bon atom. Com­pounds of this type ox­i­dize the most eas­i­ly: if you com­pare the ease with which pri­ma­ry, sec­ondary and ter­tiary al­co­hols ox­i­dize, the first ox­i­dize the most eas­i­ly, and the third with the most dif­fi­cul­ty. With the in­com­plete ox­i­da­tion of al­co­hols of dif­fer­ent struc­tures, dif­fer­ent sub­stances form:

• pri­ma­ry al­co­hols ox­i­dize to alde­hy­des;

• sec­ondary al­co­hols ox­i­dize to ke­tones.

Ter­tiary al­co­hols ox­i­dize to car­bon­ic acids (usu­al­ly a mix­ture of acids forms, as the de­struc­tion of the car­bon skele­ton takes place).

Com­plete ox­i­da­tion of pri­ma­ry al­co­hols takes place in 3 stages:

• for­ma­tion from an alde­hyde from the al­co­hol;

• for­ma­tion of car­bon­ic acid from the alde­hyde;

• break­down of car­bon­ic acid into car­bon diox­ide and wa­ter.

Slow ox­i­da­tion of pri­ma­ry al­co­hols into car­bon­ic com­pounds

In the ox­i­da­tion of pri­ma­ry al­co­hols, alde­hy­des and car­bon­ic acids can be ob­tained. The spe­cif­ic prod­uct of re­ac­tion de­pends on the con­di­tions of con­duct­ing the re­ac­tion. It is im­por­tant that the alde­hy­des are of­ten the prod­ucts of the in­com­plete ox­i­da­tion of pri­ma­ry al­co­hols. These prod­ucts may also be ox­i­dized, and car­bon­ic acids ob­tained.

Ob­tain­ing alde­hy­des and car­bon­ic acids from pri­ma­ry al­co­hols is slow ox­i­da­tion, in which the at­tach­ment of atom­ic oxy­gen takes place. As ox­i­diz­ers (sources of [O]), oxy­gen potas­si­um per­man­ganate KM­nO₄, cop­per (II) ox­ide CuO and oth­ers may be used (for sim­plic­i­ty, we do not write out the full re­ac­tions with ox­i­diz­ers, but re­ac­tions with atom­ic oxy­gen):

• СН₃-OH + [O] = HCOH + H₂O (in slow ox­i­da­tion of methanol, formalde­hyde is formed – the cor­re­spond­ing alde­hyde; be­fore the for­ma­tion of formalde­hyde an in­ter­me­di­ary un­sta­ble com­pound forms – methane­di­ol CH₂(OH)₂, which wa­ter breaks off from);

• HCOH + [O] = HCOOH (in the sub­se­quent ox­i­da­tion of formalde­hyde, methanoic or formic acid forms).

This is the two-step method for ob­tain­ing car­bon­ic acid from al­co­hol. If only an alde­hyde is to be ob­tained, af­ter its for­ma­tion the ox­i­diz­ers must be re­moved from the re­ac­tive mix­ture, as the trans­for­ma­tion into car­bon­ic acid takes place quite quick­ly (alde­hy­des ox­i­dize more swift­ly than al­co­hols).

In fur­ther ox­i­da­tion, the break­down of car­bon­ic acid into car­bon diox­ide and wa­ter is pos­si­ble:

HCOOH + [O] = CO₂ + H₂O.

Al­co­hol burn­ing

Here the un­sta­ble in­ter­me­di­ary com­pound of the type [HO-COOH] forms, which breaks down into car­bon diox­ide and wa­ter.

Rapid ox­i­da­tion of pri­ma­ry al­co­hols to car­bon diox­ide

Rapid ox­i­da­tion (com­bus­tion) usu­al­ly takes place with the re­lease of a large quan­ti­ty of heat and light. Pri­ma­ry al­co­hols, like any oth­er or­gan­ic com­pounds, burn well in air, with the for­ma­tion of car­bon diox­ide and wa­ter.

C₂H₅OH + 3О₂ = 2CO₂ + 3H₂O (com­bus­tion takes place at a high tem­per­a­ture and is ac­com­pa­nied by the re­lease of a large quan­ti­ty of heat – up to 1370 kJ).

Ox­i­da­tion of sec­ondary and ter­tiary al­co­hols

In the slow ox­i­da­tion of a sec­ondary al­co­hol, a ke­tone can be ob­tained:

CH₃-CH(OH)-CH₃ + [O] = (CH₃)₂C=O + H₂O (from iso­propyl al­co­hol, ace­tone is formed).

Ke­tones are rather re­sis­tant to ox­i­da­tion, and only with con­sid­er­able heat­ing and the ap­pli­ca­tion of strong ox­i­diz­ers is their tran­si­tion to car­bon­ic acids pos­si­ble:

2CH₃-СН₂-C(O)-СН₂-СН₂-CH₂-CH₃ + 6[O] = CH₃­COOH + CH₃-CH₂-COOH + CH₃-CH₂-CH₂-COOH + CH₃-CH₂-CH₂-CH₂-COOH.

Ter­tiary al­co­hols are the most re­sis­tant to ox­i­da­tion: they can only be ox­i­dized un­der harsh con­di­tions (heat­ing, acidic medi­um) by strong ox­i­diz­ers. The car­bon skele­ton of the ini­tial mol­e­cule breaks down. This is ev­i­dent from the ex­am­ple of the ox­i­da­tion of 2-methylbu­tanol-2.

• CH₃-C(CH₃)(OH)-CH₂-CH₃ = CH₃-C(CH₃)=CH-CH₃ + H₂O (at the first stage, the de­hy­dra­tion of al­co­hol takes place with the for­ma­tion of an alkene (2-methyl­butene-2); the process takes place with heat­ing in an acidic medi­um);

• CH₃-C(CH₃)=CH-CH₃ + 3[O] = (СН₃)₂С=О + СH₃­COOH (with harsh ox­i­da­tion of the alkene, the de­struc­tion of its car­bon skele­ton takes place on its dou­ble bond with the for­ma­tion of a ke­tone (in the case an ace­tone) and car­bon­ic acid (acetic).

On com­bus­tion, any sec­ondary and ter­tiary al­co­hols, re­gard­less of struc­ture, form equiv­a­lent com­pounds to pri­ma­ry al­co­hols – car­bon diox­ide and wa­ter:

2СH₃-CH(OH)-CH₃ + 9O₂ = 6CO₂ + 8H₂O.

There are sev­er­al ex­per­i­ments with oxy­gen in MEL Chem­istry sub­scrip­tion.

Ox­i­da­tion of pri­ma­ry al­co­hols is one of the meth­ods for ob­tain­ing alde­hy­des and car­bon­ic acids. Com­bus­tion of al­co­hols is ac­com­pa­nied by the re­lease of a large quan­ti­ty of heat. For ex­am­ple, be­cause of this prop­er­ty ethanol is con­sid­ered to be the most promis­ing and eco­log­i­cal­ly sound fuel (a re­place­ment for petrol in in­ter­nal com­bus­tion en­gines). In the lab­o­ra­to­ry, ethanol is used in spir­it burn­ers.