How are aldehydes formed


Substances from the group of aldehydes with natural origin are widespread in food and in fragrances as aroma or odor substances. In apples, pears and cherries, for example, you can find hexanal, an aliphatic aldehyde that is naturally formed from fatty acids. Benzaldehyde is found in bitter almonds and wine. Cinnamaldehyde is another aromatic aldehyde, it is the main component of cinnamon oil. This creates the aroma of the cinnamon spice.


The term aldehydes is of the term Alcohol dehydrogenatus derived. It was introduced by Justus von Liebig in 1835. The aldehydes can be produced in the laboratory by removing two hydrogen atoms from a primary alcohol. When methyl alcohol is oxidized with hot copper (II) oxide, the copper (II) oxide is reduced and, in addition to copper, methanal and water are formed:
Oxidation of methanol


If you immerse a heated and oxidized copper wire mesh in methanol,
Methanal is formed as an oxidation product. >Movie
Functional group

The resulting C = O double bond is called Carbonyl group. In addition to the aldehydes, it also occurs in the ketones. If at least one hydrogen atom is bonded to the carbonyl group, it is an aldehyde. The Aldehyde group refers to the functional group of the aldehydes. The name is formed from the basic structure of the comparable alkanes and receives the ending -al.  
Properties and evidence 
The aldehyde group is polar like the hydroxyl group (OH group) of alcohols, but not quite as strong. The short-chain aldehydes are therefore also water-soluble. Due to the polarity, their boiling points are higher than those of the alkanes, but somewhat lower than those of the corresponding alcohols. With increasing chain length, the water solubility decreases due to the decreasing polarity. The boiling temperatures rise as the chain length increases due to the increasing van der Waals forces.

Polarity of the carbonyl group

The short-chain aldehydes are, like the alcohols, highly flammable. In the laboratory, to differentiate it from a ketone, the sample is first carried out with Schiff's reagent, which results in a red-violet color. The aldehydes react positively in the Tollens test, aliphatic aldehydes also in the Fehling reaction.

Reaction of an aldehyde with Schiff's reagent
The C = O double bond is not very stable, so aldehydes such as formaldehyde show a wide variety of chemical reactions. The aldehydes can be oxidized to carboxylic acids; acetic acid can be produced from an aldehyde, for example. Aldehydes have a reducing effect, they reduce an ammoniacal silver nitrate solution, whereby a mirror of elemental silver forms in the test tube. The carbonyl group is oxidized, the silver ions are reduced:
Aldehyde detection with the Tollens sample

R-CHO + 2 Ag+ + 2 OH R-COOH + 2 Ag + H2O

Aldehydes reduce an ammoniacal silver nitrate solution to silver.
The short-chain aldehydes are important intermediate products in organic chemistry for the production of plastics and serve as disinfectants and preservatives. The longer-chain aldehydes are used as fragrances in the perfume industry. Aromatic aldehydes such as benzaldehyde are raw materials for the manufacture of medicines, perfumes and dyes.

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