Aldehyde

Aldehydes ( from neulateinisch Alcoholus dehydrogenatus " dehydrated alcohol " or " alcohol, the hydrogen was removed from " ) are chemical compounds with the functional group -CHO, the aldehyde or formyl group is mentioned. The carbonyl group (> C = O) of the aldehydes bearing a hydrogen and a carbon substituent in contrast to the ketones. An exception is the simplest aldehyde, methanal (formaldehyde ), which carries two hydrogen substituents. Aldehydes with an alkyl group (ie, alkane derivatives) are referred to as alkanals; the homologous series is derived according to the nomenclature of the homologous series of alkanes from. Next Mehrfachaldehyde exist - such as glyoxal, the simplest dialdehyde.

• 9.1 Nucleophilic Addition
• 9.2 Addition of Water
• 9.3 Addition of Alcohols
• 9.4 addition of nitrogen nucleophiles
• 9.5 aldol reaction
• 9.6 Mixed aldol reaction
• 9.7 pinacol coupling

Nomenclature

Aldehydes obtained by the IUPAC nomenclature the name of the alkane with the same number of carbon atoms with the suffix - al or -carbaldehyde. Accordingly, the derived from methane aldehyde methanal, ethanal is called the derived from ethane. If any other functional group has a higher priority, the prefix " formyl " is used. The compound is, however, a natural material, or a carboxylic acid, then the prefix "oxo " is selected.

The trivial name is derived from the Latin name for each generated when adding an oxygen atom carboxylic acid. For methanal (H -CHO) which is the methane acid (Latin acidum formicum, H -COOH), therefore, formaldehyde, ethanal for the acetic acid (Latin acidum aceticum, CH3 -COOH), therefore acetaldehyde. Accordingly, the other common names derive. Dicarboxylic acids in which a carboxylic acid group has been reduced to an aldehyde, are sometimes called semialdehydes.

Homologous series of alkanals

Empirical formula of alkanals: CnH2n 1 CHO (n = 0, 1, 2, 3, 4, ...)

There are also many other groups of aldehydes, are used for the most historical names:

• Acrolein is derived from propene - an alkene - from.
• Benzaldehyde is derived from benzene, is thus an aryl aldehyde.
• Furfural ( furfurol, furan -2-carbaldehyde ) is derived from of furan, is thus a Heteroarylaldehyd.

Properties

Between the aldehyde groups of alkanals it comes to dipole -dipole forces, as the C = O double bond is very polar. Hydrogen bonds do not form because there is no oxygen- bonded hydrogen atom is present. Therefore, the boiling points of aldehydes between those of alcohols and alkanes. Aldehydes can form hydrogen bonds with water because the oxygen atom has two lone pairs of electrons and is negatively polarized. Therefore, short-chain aldehydes are readily soluble in water. With long-chain aldehydes outweighs the effect of the non-polar alkyl groups, making the compounds insoluble in water. Many aldehydes have a characteristic odor.

Occurrence

Aldehydes are widely used as flavorings in food. Often they occur in fruits and vegetables from oleic, linoleic or linolenic acid - containing substances during harvesting, crushing or preparation. Hexanal can be found eg in apples, pears, peaches and cherry. (E )-2- hexenal is found in apples, peaches, cherries and plums, the isomeric (Z )-2- hexenal in apples, pears, oranges and strawberries. (Z )-3 -nonenal comes in addition to cucumbers (E, E ) -2,4- nonadienal, (E, Z ) -2,6- nonadienal and (Z, Z) -3,6- nonadienal as odor imaging flavoring ago.

Above a certain concentration such carbonyl compounds are however often rated as rancid, fishy, metallic or cardboard -like flavors and cause an overall Altgeschmack.

Production

By mild oxidation of primary alcohols in the non-aqueous medium result aldehydes. They can be further oxidized to carboxylic acids.

The technically most important processes for the production of aldehydes is the oxo - synthesis, also known as hydroformylation. In this case, a mixture of an alkene with carbon monoxide and hydrogen in the presence of a suitable catalyst is reacted:

Use

Formaldehyde ( methanal ) is produced in large quantities (worldwide 21 million tonnes per year), more than any other aldehyde. It is used as a disinfectant used as a preservative for perishable goods, such as cosmetics ( formalin ), and as a raw material in the chemical industry. The largest volumes are further processed in the plastics industry to amino resins and phenolic resins.

Aldehydes and ketones are also used for the production of plastics, solvents, dyes, perfumes and medicines. Starting from acrolein is DL -methionine, a feed additive, produced in quantities of more than 100,000 tons per year.

In medicine, formaldehyde and glutaraldehyde are used as surface and instrument disinfectants. Both aldehydes have good activity against many microorganisms. In particular, non-enveloped viruses and spore-forming bacteria (eg, anthrax ), which are only a few disinfectants available, can be achieved. Since aldehydes are irritating to skin and mucous membranes and occasionally cause allergies, must be handled carefully with these agents.

In perfumery aldehydes have been used since 1921 ( Chanel No.. 5).

Physiological significance

In the metabolism of the cells there are a number of aldehydes. Plays a special role ethanal ( acetaldehyde trivial name ), which is formed during the degradation of ethanol and is responsible for the so-called alcohol hangover.

Evidence

• Tollensprobe
• Fehling test
• Schiff test

Spectroscopy of aldehydes

In IR spectra of aldehydes and ketones to find the intense characteristic band of the C = O stretching vibration in the range of 1690-1750 cm -1.

13C - NMR spectra are found, the signal of the carbonyl carbon of aldehydes and ketones in a range of 195 and 210 ppm. The corresponding proton of the aldehyde can be found in the 1H - NMR spectra as a sharp peak at about 10 ppm. This feature makes the identification by NMR spectroscopy particularly simple, since in this high range only a few protons have a resonance.

Reactions

Aldehydes are reactive compounds and can be very easily oxidized to the carboxylic acid.

• The C = O bond of the carbonyl group is strongly polar with the partial positive charge ( δ ) on the carbon atom, can be attacked at the nucleophile.
• Aldehydes with a hydrogen atom bonded to the α - carbon atom adjacent to the carbonyl group may be in the keto and the enol form - see keto -enol tautomerism.
• With aldehydes, it is observed that the hydrogen atoms at the adjacent carbon atom to the carbonyl group are significantly more acidic than hydrogen atoms in a "normal" carbon atoms. This is due to the fact that the carbonyl carbon is electron deficient and a - I effect exerts on neighboring bonds, on the other hand, after deprotonation of the negative charge are delocalized on the oxygen of the carbonyl group ( - M effect ).

Nucleophilic addition

After attack of the nucleophile, the π - electron pair is derived entirely to the now negatively charged oxygen. In protic solvents, this is compensated by proton uptake, whereby an OH group is formed in place of the carbonyl group.

Addition of water

Water ⇒ aldehyde aldehyde hydrate ( gem- diol )

Aldehyde are in an aqueous solution with the corresponding gem- diol, that is, a hydrocarbon with two hydroxyl groups on a carbon atom at equilibrium. In general, the equilibrium is on the side of the aldehyde. However, in the case of Trichloracetaldehyds the equilibrium is on the side of the gem- diol.

Addition of alcohols

Alcohol aldehyde hemiacetal ⇒

Hemiacetal alcohol water ⇒ acetal

Example: ring closure of dextrose (glucose )

See also: acetal

Addition of nitrogen nucleophiles

Prim amine aldehyde ⇒ imine ( Schiff base ) water

Sec. Amine aldehyde ⇒ enamine water

Oxidation to the carboxylic acid (important for proof )

Aldol reaction

The CH- acidic hydrogen atom in the α - position can be split off by bases. The resulting enolate anion is added to the carbonyl carbon of another aldehyde molecule. The result is an aldol, an addition product of an alcohol group (OH group ), and aldehyde. In this way, carbon-carbon bonds can be made. If the aldol formed then dehydrated, it is called aldol condensation, thereby arise α, β -unsaturated aldehydes.

Mixed aldol reaction

Mixed aldol reactions are usually not in a one-pot reaction feasible since there are four possible products can form and also form. An exception is when one of the aldehydes is not enolisable, which means does not have a CH- acidic H- atom. In this case, only a mixed aldol is possible. An example of non- enolizable aldehydes, aromatic aldehydes (see: benzaldehyde ). In this way, in a Knoevenagel condensation cinnamaldehyde, an important fragrance obtained.

Pinacol coupling

Substituting aldehydes with an alkali metal (for example sodium) to such a radical anion, which is quickly dimerizes. The hydrolysis provides a pinacol ( traditional name for a 1,2- diol, which is a diol having vicinal hydroxyl groups ). Starting from an α, ω -dialdehyde is obtained analogously by reaction of an intramolecular cyclic 1,2- diols.