General characteristics of aldehydes, oxidation

General properties of aldehydes and interesting reactions with aldehydes

Silver mirror reaction with aldehydes [Wikimedia]

Alde­hy­des are a class of or­gan­ic com­pounds which con­tain the alde­hyde group COH (-CH=O) in their com­po­si­tion. In sys­tem­at­ic names, alde­hy­des can be de­ter­mined by the suf­fix -al (CH₃­COH – ethanal).

Aldehyde group structure [Wikimedia]

Of­ten pop­u­lar names are also used in­stead of sys­tem­at­ic names:

  • Methanalformalde­hyde, formic alde­hyde;

  • Ethanal – acetic alde­hyde;

  • Pen­tanal – va­ler­ic alde­hyde.

Ob­tain­ing alde­hy­des

There are many ways of ob­tain­ing alde­hy­des:

  • Ox­i­da­tion of pri­ma­ry al­co­hols (it is easy to ox­i­dize them fur­ther to car­bon­ic acids):

CH₃OH + [O] = HCOH + H₂O;

HCOH + [O] = HCOOH.

Formic acid forms when formaldehyde oxidizes [Wikimedia]
  • Ox­i­da­tion by bi­va­lent cop­per ox­ide:

C₂H₅OH + CuO = CH₃-CH=O + Cu + H₂O;

  • De­hy­dra­tion of al­co­hol va­pors on a heat­ed cop­per, chromi­um or zinc cat­a­lyst:

C₂H₅OH = CH₃-CH=O + H₂;

  • *Hy­dra­tion of acety­lene in the pres­ence of salts of bi­va­lent mer­cury (Hg²⁺) – Kucherov’s re­ac­tion:

H-C≡C-H + H₂O = CH₃-COH;*

  • Ox­i­da­tion of eth­yl­ene in the pres­ence of pal­la­di­um chlo­ride Pd­Cl₂:

2CH₂=CH₂ + O₂ = 2CH₃­COH.

Some ex­per­i­ments with oxy­gen are in­clud­ed in MEL Chem­istry sub­scrip­tion.

Phys­i­cal prop­er­ties

At room tem­per­a­ture, formalde­hyde is gaseous. All oth­er alde­hy­des with a car­bon skele­ton of a length up to C₁₃ are liq­uids, while alde­hy­des with a longer un­branched skele­ton are solids.

Formaldehyde 3D structure [Wikimedia]

Chem­i­cal prop­er­ties

The car­bonyl group present in alde­hy­des is strong­ly po­lar (the shift of elec­tron den­si­ty is ob­served to­wards oxy­gen, on atoms of car­bon and oxy­gen in the car­bonyl group, par­tial pos­i­tive and neg­a­tive charges ap­pear re­spec­tive­ly). The com­pound is more ac­tive the high­er the par­tial pos­i­tive charge on the car­bonyl car­bon is.

The main chem­i­cal prop­er­ties of alde­hy­des are the fol­low­ing:

  • Com­bus­tion:

2CH₃­CHO + 5O₂ = 4CO₂ + 4H₂O;

HCHO + H₂ = CH₃OH (heat­ing and the pres­ence of a Ni cat­a­lyst are re­quired);

  • Shift of hy­dro­gen atoms in the hy­dro­car­bon rad­i­cal to the al­pha po­si­tion: СH₃-CH₂-COH + Cl₂ = CH₃-CHCl-COH + HCl.

There are many re­ac­tions of at­tach­ment to alde­hy­des. Pos­si­ble at­tach­ment:

  • Sodi­um hy­dro­sul­fate with the for­ma­tion of sodi­um al­pha-hy­drox­y­sulfto­nates:

C₂H₅-CH=O + NaH­SO₃ = C₂H₅-CH(OH)-SO₃­Na;

  • Al­co­hol with for­ma­tion of semi-ac­etals:

CH₃-CH=O + C₂H₅OH =CH₃-CH(OH)-O-C₂H₅;

Acetaldehyde 3D structure [Wikimedia]
  • Hy­drazine with for­ma­tion of hy­dra­zones:

CH₃-CH=O + NН₂-NH₂ = CH₃-CH=N-NH₂ + H₂O;

  • Amines with for­ma­tion of N-sub­sti­tut­ed imines:

C₆H₅-CH=O + H₂NC₆H₅ = C₆H₅CH=NC₆H₅ + H₂O;

  • Wa­ter with for­ma­tion of gem-di­ols:

R-CH=O + H₂O = R-CH(OH)-OH;

  • Hy­dro­cyan­ic acid with for­ma­tion of ni­t­ryls:

CH₃-CH=O + HCN = CH₃-CH(CN)-OH;

  • Thi­ols with for­ma­tion of dithioac­etals (in an acidic medi­um):

CH₃-CH=O + C₂H₅SH = CH₃-CH(SC₂H₅)-SC₂H₅ + H₂O.

Ox­i­da­tion of alde­hy­des

Alde­hy­des ox­i­dize quite eas­i­ly – for the re­ac­tion to take place, even mild ox­i­diz­ers can be used – for ex­am­ple bi­va­lent cop­per hy­drox­ide or sil­ver ox­ide (the alde­hyde group turns into a car­boxyl group):

  1. R-CH=O + 2Cu(OH)₂ = RCOOR + Cu₂O + 2H₂O;

  2. CH₂=O + Cu(OH)₂ = HOOH + Cu + H₂O.

Copper hydroxide [Wikimedia]

It is im­por­tant that the cop­per hy­drox­ide is fresh­ly pre­pared. The re­ac­tion can also be car­ried out with the am­mo­ni­um com­plex of cop­per hy­drox­ide and a com­plex with salts of tar­tar­ic acid (Fehling’s so­lu­tion):

  1. R-CH=O + 2Cu(NH₃)₄₂ = RCOOH + Cu₂O + 4NH₃ + 2H₂O;

  2. R-CH=O + 2Cu(OH)₂/salt of tar­tar­ic acid = RCOOH + Cu₂O + 2H₂O.

Sil­ver mir­ror re­ac­tion:

HCOH + 2[Ag(NH₃)₂]OH = HCOOH + 2Ag + 4NH₃ + H₂O.

Alde­hy­des also ox­i­dize well with strong ox­i­diz­ers (to car­bon­ic acids):

HCOH + [O] = HCOOH.

Among all alde­hy­des, the most wide­ly used in in­dus­try is formalde­hyde, which is used for dip­ping grain, tan­ning leather and pro­duc­ing resins. Cin­nam­ic alde­hyde is used in per­fumes.