General characteristics of aldehydes, oxidation
General properties of aldehydes and interesting reactions with aldehydes
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).
Often popular names are also used instead of systematic names:
Methanal – formaldehyde, formic aldehyde;
Ethanal – acetic aldehyde;
Pentanal – valeric aldehyde.
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.
- 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.
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.
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:
2CH₃CHO + 5O₂ = 4CO₂ + 4H₂O;
- Hydration (reduction by hydrogen):
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;
- 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):
R-CH=O + 2Cu(OH)₂ = RCOOR + Cu₂O + 2H₂O;
CH₂=O + Cu(OH)₂ = HOOH + Cu + H₂O.
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):
R-CH=O + 2Cu(NH₃)₄₂ = RCOOH + Cu₂O + 4NH₃ + 2H₂O;
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.