Aldehyde

Nomenclature

The aldehyde will have the name of the corresponding alkane, to which one adds the suffix “- Al”. For the remainder, the rules are the same ones as those stated for the Alcène S. When the aldehyde sets up the priority functional group of the molecule, it is not necessary to allot an index of position to him, because it is always in final position (1).

CH₃ - COH: éthanal

CH₃ - CH (CH₃) - CH₂ - COH: 3 - méthylbutanal

When the aldehyde is a substituent on a cyclic compound like the Benzène, it bears then the name of carbaldéhyde or carboxaldéhyde which is two equivalent terms. For example, the following compound can name benzènecarbaldéhyde or benzènecarboxaldéhyde (the name running benzaldéhyde is also accepted):

C₆H₅ - COH

When the aldehyde group is not priority, one allots to him a prefix of the form alcanoyle . For example, the following compound names acid ethanoylbenzenesulfonic :

HOC - CH₂ - C₆H₅ - SO₃H

Principal aldehydes

Properties, reactivity

They are general with aldehydes and ketones. See the general article carbonyl compound

Formation

By oxidation of alcohols

One obtains aldehydes by oxidation of primary education alcohols and the Cétone S by secondary alcohol oxidation (tertiary alcohol can oxidize only under extreme conditions, one often regards it as not being able to be oxidized).

This oxidation thus puts concerned the redox cell:

However, the aldehydes are still of good reducers. They thus oxidize very easily in carboxylic acid:

There is even a case for which the reaction continues: methanol. It oxidizes initially in méthanal, then in methanoic acid, and finally out of carbon dioxide.

Industrial oxidation

oxidation by O₂ of the air

One can use as Fe₂O₃ catalyst or MnO₃ with 400°C, or a catalyst containing money towards 600°C.

catalytic Dehydrogenation of primary education alcohols

Alcohol is gas. One uses as CuO catalyst towards 400-500°C, Ag, Cu towards 300°C, ZnO or ZrO towards 300-400°C.

Proceeded of Wacker-Hoechst

Chemical oxidation

An oxidant " should be used; doux" , under strict operating conditions, in order to avoid the second oxidation. One uses in general a derivative of chromium VI in stoechiometric proportions. There are 2 historical methods:

For the aldehydes having a short chain carbonaceous, and which are thus consequently more volatile, one uses dichromate K₂Cr₂O₇ the potassium, in acid medium, with higher temperature of the boiling point of aldehyde, which allows its extraction after formation.
For other aldehydes, one uses the reagent of Sarett, chromates CrO₃ in the Pyridine, at low temperature (0°C).

However the pyridine and the compounds containing chromium are very toxic, and cannot be used any more. Of another methods were developed:

PCC and PDC, always containing chromium, but in less large quantities;
methods with the activated DMSO (of which the Oxidation of Swern);
Periodates (reactive of Of Martin, IBX).

By ozonolyse of olefinic hydrocarbons

Aldehydes can be obtained by Ozonolyse of olefinic hydrocarbons. It is important to use a reducing additive during the reactions, like a sulfide (classically diméthylsulfure) or a phosphine (the triphénylphosphine, less odorous than the sulfur compounds), in order to trap under oxidizing products (hydrogen peroxide). In the contrary case, a peroxydization of aldehyde in Carboxylic acid is observed.

Use of Aldehydes

The aldehydes are very powerful products of synthesis which bring, independently of their own odor, an extraordinary capacity of diffusion to the compositions. In Perfumery this relates to especially aliphatic aldehydes. the discovery of aldehydes gave to the creators a new pallet. Their marked use is at the origin of the perfumes of the type known as " Aldéhydé". The aldehydes are used as means of disinfection and with the organic manufacture of preparation. Aldehydes and ketones are used with the manufacture of plastics, solvent, dye perfume and drug. For example the aldehyde C 17 has a cherry odor.

Acétalisation

The acetalisation is a reversible reaction making it possible to transform a carbonyl compound and two alcohols (or a diol) into acetal . This reaction allows in fact to protect the carbonyl group, or alcohol.

Reaction

Conditions

acid Heating
Catalysis (in general APTS)

Characteristics and remarks

When an aldehyde is used, one names the product obtained " acétal". When a ketone is used, one names the product obtained " cétal". However, more and more one uses the acetal term as generic term to indicate the 2 products. One on the other hand always calls the reaction (as well for aldehydes and ketones) " acétalisation "

This reaction is in general in favor of acetal (contrary to the cétals). If one wants to support it in this direction, one uses an alcohol excess (which is used consequently occasion as solvent).

One can also distill water (heteroazeotropic distillation) using the apparatus of Dean and Stark. To support the reaction reverses, it is the opposite: one puts a water excess to hydrolize acetal.

When one wants to protect the function carbonyl, one generally uses a diol, like the ethane-1,2-diol:

This reaction can very be also well used to protect a function alcohol, as local private individuals the diols.

In certain cases, the reaction can stop with hemi-acetal (case of cyclic hemi-acetals).

Action of a organomagnésien

A mixed Organomagnésien reacts on an aldehyde to form a secondary alcohol (- > reaction).

Reaction of Wittig

The Réaction of Wittig transforms an aldehyde into ethylene derivative. It transforms in fact connection C=O in connection C=C.

Reduction of aldehydes

Out of alcohols

Industrially

One uses the Dihydrogène H₂ in an inert solvent, in the presence of a catalyst (heterogeneous Catalyze). They are very often metals, like the Platine (Pt), the palladium (Pd), the Nickel (Ni) or the Rhodium (Rh). If ever the aldehyde has also a connection C=C, that Ci also is hydrogenated (the reaction is easier on C=C than on C=O). the reaction is Exothermique.

By the hydrides

Out of alkane

Lengthening of the carbonaceous chain

Aldolization

The carbonaceous chain of an aldehyde can increase: there is condensation. This condensation is carried out in basic medium.

R-CH ₂-CHO + HO^- => R-CH-C=O + H₂O Substitution of the hydrogen carried by carbon in alpha function C=O O | R-CH-CHO + R-CH₂-CHO => R-CH₂-CH | R-CH- C=O | H

O | R-CH ₂-CH + H₂O => R-CH₂-CH-CH-C=O | | | | R-CH- C=O OH R H | H

This reaction is called: Reaction of β aldolization

This product in general undergoes a reaction of crotonisation, supported by the conjugation of double connection C=C with C=O:

R-CH ₂-CH-CH-C=O → R-CH₂-CH=CH-C=O + H₂0 -->

Tests of recognition

To highlight the presence of aldehyde, one can make a test of recognition:

Addition of composed of type Z-NH₂ (amine): formation of a precipitate.
the test with the Fehling's solution, who takes a red color hot brick in the presence of aldehyde.
the test known as of the " mirror of money " , using the reagent of Tollens, leading to the creation of a solid money deposit.
the test with the reagent of Schiff, which cold, colors the solution pink.

But these methods using of the chemical reactions with aldehyde destroy it. One uses now more readily the methods of spectrometry (NMR and infra-red) allowing to detect this function without destruction of the sample.

Random links: