Chemistry

Aldehydes

Aldehydes


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Area of ​​Expertise - Organic chemistry

An aldehyde is a compound with aldehyde group (carbonyl group) -CH = O in the molecule.

Aldehydes are formed by the dehydrogenation of primary alcohols, hence the name.

The simplest aldehyde is formaldehyde (methanal, H-CH = O).

Learning units in which the term is dealt with

Wittig reaction25 min.

ChemistryOrganic chemistryAlkenes

This learning unit is an introduction to the Wittig reaction (Wittig olefination) and related types of reactions.

Interpretation of C, H, O compounds60 min.

ChemistryAnalytical chemistryIR / Raman spectroscopy

This chapter includes the interpretation of IR and Raman spectra of the compounds consisting of carbon, hydrogen and oxygen. On the basis of important group frequencies, after working through this section, you should be able to differentiate between the individual substance classes.

Reduction of aldehydes and ketones to alcohols20 min.

ChemistryOrganic chemistryReduction reactions

This learning unit shows the possibilities of reducing aldehydes and ketones to alcohols. Use in the laboratory and in industry is also discussed.

Reduction of esters via aldehydes to alcohols20 min.

ChemistryOrganic chemistryReduction reactions

This learning unit shows the possibilities of reducing esters to alcohols. The mechanism is explained using an example and the various possible applications both in the laboratory and in industry are presented.

Oxidation of aldehydes to carboxylic acids20 min.

ChemistryOrganic chemistryOxidation reactions

The learning unit shows the possibilities of oxidizing aldehydes to carboxylic acids. In addition to the reaction mechanism with the various oxidation possibilities, the biological significance is also discussed.

Meerwein-Ponndorf-Verley reduction20 min.

ChemistryOrganic chemistryReduction reactions

This learning unit explains the reaction principle of the Meerwein-Ponndorf-Verley reduction. Aldehydes and ketones can be reduced to the corresponding alcohol.

Cannizzaro reaction20 min.

ChemistryOrganic chemistryReduction reactions

This learning unit introduces the principle of the Cannizzaro reaction. The crossed Cannizzaro reaction is also discussed. The industrial importance is emphasized.

Reduction of carboxamides to amines20 min.

ChemistryOrganic chemistryReduction reactions

This learning unit describes the reduction of carboxamides to amines. The mechanism is discussed and the possible applications are presented.

The Grignard reaction20 min.

ChemistryOrganic chemistryOrganometallic compounds

It is one of the classic organometallic reactions for building CC bonds in synthetic organic chemistry and, thanks to its wealth of variants, opens up the possibility of converting a large number of different functional groups into others - the Grignard reaction, which has not been used for over 100 years is to be imagined without the laboratories.

Aldehydes and ketones30 min.

ChemistryOrganic chemistryCarbonyl compounds

In this learning unit, the substance class of aldehydes and ketones is presented. The enormous importance of these compounds both in technology and in the form of natural substances is explained using examples. In addition, the presentation and the properties are discussed.


Aldehyde is basically the generic term for a class of substances which has the aldehyde group (-HC = O) as a functional group as a distinguishing feature.

Alkanals are now a subgroup of the aldehydes because they have the aldehyde group on the one hand, but otherwise have a (possibly branched) chain-shaped hydrocarbon radical that only has single bonds and can therefore be derived from the alkanes.

So alkanals are a specific type of aldehyde.
Alkenals are another subgroup of aldehydes, cycloalkanals are another, and aromatic aldehydes are another. All aldehydes because they have the same functional group, but the rest of the molecule is different.

Thank you for the more than understandable and extensive answer :)


The properties of aldehydes

The carbon in the aldehyde group CHO has a double bond to the oxygen atom O. This forms the functional carbonyl group CO. Since the C = O double bond is very polar, intermolecular dipole-dipole forces occur between the aldehydes.

This means that the aldehydes are more polar than alkanes, but less polar than alcohols, which is why the boiling points are between those of the alkanes and alcohols. The boiling points of the aldehydes are therefore higher than those of the alkanes and lower than those of the alcohols.

The boiling points rise due to the van der Waals forces with increasing chain length. Aldehydes or alkanes with a short chain are therefore relatively easily flammable.

Aldehydes with a short chain length are very soluble in water. This is due to the fact that aldehydes can form hydrogen bonds with water. These bonds are possible because the oxygen atom has two free electron pairs and is negatively polarizing.

Long-chain aldehydes, on the other hand, are not soluble in water, since the effect of non-polar alkyl groups is more pronounced.

A special property of aldehydes is that many have a special, characteristic odor, which is why they are often used in perfumes. However, you can smell aldehydes in food as well. In cinnamon, for example, you smell cinnamaldehyde (3-phenyl-2-propenal) or in almonds, benzaldehyde.


Aldehydes

AldehydesOrganic compounds belonging to the carbonyl compounds that contain one or more aldehyde groups -CHO in the molecule. Depending on the organic residue to which the aldehyde group is bound, a distinction is made between aliphatic, aromatic and heterocyclic A.

Nomenclature. The name of the aliphatic A. is formed according to the IUPAC nomenclature from the name of the corresponding hydrocarbon, from which the aldehyde can be formally derived by oxidation, and the suffix -al:



In practice, an older designation is also used for A., ​​which is derived from the Latin name of the carbonic acid formed by oxidation of the corresponding A.



The designation of aromatic and heterocyclic aldehydes, in which the aldehyde group is bonded directly to a carbon atom of the ring in question, is mainly given by common names, e.g. B. benzaldehyde, or is formed from the name of the parent compound and the beginning of -carbaldehyde, z. B. pyrrole-2-carbaldehyde.

Aldehydes. Tab .: Aliphatic and aromatic aldehydes.

When water is added to A., aldehyde hydrates (1,1-diols) are formed, but these are usually unstable because of the equilibrium



usually on the side of the reactants (Erlenmeyer rule). A stable connection of this type is e.g. B. chloral hydrate.

In an analogous manner, alcohols or thioalcohols can be added to A. in the presence of acidic catalysts, such as hydrogen halides or zinc chloride. The unstable hemiacetals formed in the process react under the specified reaction conditions mostly with another alcohol or thioalcohol molecule with the escape of water to form the acetal or thioacetal:



Due to the great resistance of the acetals to bases, they are often used instead of the free A. in syntheses. Acetals can easily be broken down into their original compounds by using dilute acids.

Reducing agents such as hydrogen, lithium aluminum hydride and sodium borohydride can convert A. into primary alcohols. The Meerwein-Ponndorf-Verley reduction succeeds in reversing the Oppenauer oxidation with isopropyl alcohol in the presence of aluminum isopropoxide. Under the conditions of the Wolff-Kishner reduction, A. are reductively converted into the corresponding hydrocarbons. The action of oxidizing agents on A. leads to the corresponding carboxylic acids. This reaction is extremely easy, since A. have a reducing effect. It is therefore often used to characterize this class of compounds and at the same time to differentiate it from the ketones. Tollens reagent, Nylander's reagent or Fehling's solution are used as oxidizing agents.

Another well-known addition reaction of A. is the addition of sodium hydrogen sulfite with the formation of poorly soluble, crystalline hydrogen sulfite compounds:



Since the bisulfite adducts can easily be broken down into the starting components again by dilute acids or sodium carbonate solution, this reaction is often used to purify the A.

An important reaction of A. for numerous syntheses is the formation of cyanohydrins by the addition of prussic acid to the carbonyl group:



A. initially react with ammonia to form the mostly unstable aldehyde ammonia adducts, which easily convert into aldimines with elimination of water:



A. condense with primary amines to azomethines:



When reacting with secondary amines, A. reacts with protons in a position to form enamines. In the absence of the corresponding hydrogen atom, aminals are formed. A. can be converted into secondary alcohols by Grignard reactions.

Important condensation reactions, which are also of great importance in the separation and characterization of A., are reactions with hydrazine or substituted hydrazines, e.g. B. phenylhydrazine, 4-nitrophenylhydrazine and 2,4-dinitrophenylhydrazine, which proceed with the formation of the corresponding hydrazones:



The condensation reaction of A. with hydroxylamine, in which oximes are formed, plays a similarly important role:



Semicarbazones and thiosemicarbazones arise from A. and semicarbazide and thiosemicarbazide, respectively. They can also be used to purify and characterize the A.

A. with 945 hydrogen atoms react in the presence of bases or acids in the sense of the aldol reaction.

Aromatic A. disproportionate under these conditions according to the Cannizzaro reaction to the corresponding carboxylic acids and alcohols. In the presence of aluminum alcoholates, this disproportionation also succeeds with aliphatic A.

Similar to the aldol reaction, A. can also work with other CH-acidic reactants, such as malonic acid and malonic acid derivatives, according to the Knoevenagel condensation, with acetic anhydride / sodium acetate after the Perkin reaction and with & # 945 halocarbons & # 228 acid esters react in the sense of the Darzens-Erlenmeyer-Claisen condensation.

Numerous aromatic A. react under the influence of potassium cyanide, but also some thiazolium and imidazolium salts according to the benzoin condensation with the formation of benzoins (acyloin condensation). The structurally analogous aliphatic compounds, the acyloins, can be obtained directly from aliphatic A. only through the action of enzymes from certain types of yeast:



Analytical. In addition to chemical characterization in the form of derivatives, A. and ketones can be characterized by IR and NMR spectroscopic methods as well as by mass spectrometric fragmentation. In the IR spectra they have intense bands for the C = O stretching vibration in the range from 1680 to 1740 cm −1. The C-H stretching vibrations of A. appeared at 2665 to 2880 cm -1. The signals of the aldehyde protons are to be expected in the 1 H-NMR spectra in the range from 948 = 9 to 10 ppm. In the 13 C-NMR spectra, signals in the range from φ 948 = 180 to 210 ppm indicate with a high degree of probability the C = O grouping of A. and ketones. In the mass spectra of aromatic A. and ketones, the benzoyl cation appears as a characteristic key breaker.

Occurrence and extraction. Various A. occur naturally as plant constituents in low concentrations, especially in numerous & # 228therischen & # 214len, z. B. citral, citronellal. For the synthesis of A., there are numerous methods of preparation that are also generally applicable for ketones and special processes. The best known and most important for the synthesis of aliphatic A. is the partial oxidation or dehydrogenation of primary alcohols:



Chromium (VI) oxide or potassium dichromate in sulfuric acid, oxygen in the presence of heated copper or silver, manganese dioxide and selenium dioxide are suitable as oxidizing agents. Another important technical process is the pyrolysis of mixtures of a carboxylic acid and formic acid in the presence of manganese (II) oxide:



Aliphatic A. can also be produced by glycol cleavage and by hydroformylation of alkenes with carbon monoxide and hydrogen in the presence of dicobalt octacarbonyl at about 150 ° C and about 3 · 10 4 kPa. Further aldehyde syntheses are the Rosenmund reduction of carboxylic acid chlorides, the Grignard reaction of orthoformic acid esters with the Grignard reagent, the Sommelet reaction of alkyl halides with urotropine, the Stephen reduction of nitriles, the Nef reaction of primaries Nitroalkanes with dilute mineral acids, the Kr & # 246hnke reaction from nitrones, the Vilsmeier-Haack reaction of arenes, the Gattermann reaction of arenes with cyanide and hydrogen chloride in the presence of aluminum chloride, and the Gattermann-Koch reaction.

Use. A. are mainly used as starting components for numerous, also technically important syntheses and as odor and flavor substances in the food and cosmetics industries. The field of application of A. as intermediate products for the technical synthesis of plastics, styryl and azomethine dyes as well as many organic substance classes, e.g. B. carboxylic acids, alcohols, nitriles, amines.


Aldehydes

The following table shows some properties of selected alkanals:

nAlkanalCommon nameformulamolar massMelting pointboiling pointDensity [20 ° C]CAS No.
0MethanalformaldehydeCH2O30.03 g mol -1 - 117 ° C- 19 ° Cgaseous50-00-0
1EthanalacetaldehydeC.2H4O44.05 g mol -1 - 123.37 ° C20.2 ° C0.785 g cm -3 75-07-0
2PropanalPropionaldehyde
Propyl aldehyde
C.3H6O50.08 g mol -1 - 81 ° C49 ° C0.81 g cm -3 123-38-6
3Butanaln-butyraldehydeC.4H8O72.11 g mol -1 - 96.86 ° C74.8 ° C0.80 g cm -3 123-72-8
4Pentanaln-pentaldehyde
Valeraldehyde
Amylaldehyde
C.5H10O86.13 g mol -1 - 60 ° C103 ° C0.81 g cm -3 110-62-3
5Hexanaln-hexaldehyde
Capronaldehyde
C.6H12O100.16 g mol -1 - 56 ° C129 ° C0.81 g cm -3 66-25-1
6Heptanaln-heptaldehyde
Enanaldehyde
Heptylaldehyde
C.7H14O114.19 g mol -1 - 43.3 ° C152.8 ° C0.82 g cm -3 111-71-7
7Octanaln-octylaldehyde
Caprylaldehyde
C.8H16O128.21 g mol -1 - 23 ° C171 ° C0.82 g cm -3 124-13-0
8Nonanaln-nonyl aldehyde
Pelargon aldehyde
C.9H18O142.24 g mol -1 - 18 ° C191 ° C0.83 g cm -3 124-19-6
9 Decanaln-decylaldehyde
Capric aldehyde
C.10H20O156.26 g mol -1 7 ° C208.5 ° C0.83 g cm -3 112-31-2
10Undecanaln-undecylaldehydeC.11H22O170.30 g mol -1 -2 ° C120 ° C0.825 g cm -3 112-44-7
11Dodecanaln-dodecylaldehyde
Lauric aldehyde
C.12H24O184.32 g mol -1 12 ° C237-239 ° C0.83 g cm -3 112-54-9
12Tridecanaln-tridecylaldehydeC.13H26O198.35 g mol -1 14 ° C280 ° C0.83 g cm -3 10486-19-8
13Tetradecanaln-tetradecylaldehyde
Myristyl aldehyde
C.14H28O212.38 g mol -1 23.5 ° C260 ° C0.832 g cm -3 124-25-4
14Pentadecanaln-pentadecylaldehydeC.15H30O226.40 g mol -1 24.5 ° C286 ° C0.83 g cm -3 2765-11-9
15Hexadecanaln-hexadecylaldehyde
Palmityl aldehyde
C.16H32O240.42 g mol -1 35 ° C 629-80-1
16Heptadecanaln-heptadecylaldehydeC.17H34O254.45 g mol -1 36 ° C 629-90-3
17Octadecanaln-octadecylaldehyde
Stearyl aldehyde
Stearaldehyde
Stearaldehyde
C.18H36O268.49 g mol -1 38 ° C320 ° C0.831 g cm -3 638-66-4

Updated on December 05, 2017.

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Table of contents

According to the IUPAC nomenclature, aldehydes receive the name of the alkane with the same number of carbon atoms with the suffix -al or -carbaldehyde. Accordingly, the aldehyde derived from methane is called methanal, the ethanal derived from ethane. If another functional group has a higher priority, the prefix “Formyl-” is used. If, on the other hand, the compound is a natural substance or a carboxylic acid, the prefix “Oxo-” is chosen.

The common name is derived from the Latin name for the carboxylic acid that is created when an oxygen atom is added. For methanal (H – CHO) this is methanoic acid (lat. acidum shapeicum, H – COOH), hence shapealdehyde, for ethanal the ethanoic acid (lat. acidum aceticum, CH3–COOH), hence Acetaldehyde. The other common names are derived accordingly. Dicarboxylic acids in which a carboxylic acid group has been reduced to an aldehyde group are occasionally used Semialdehydes called.

Homologous series of the alkanals edit

number
(Carbon atoms)
IUPAC
description
Common names Molecular formula Structural formula boiling point
in ° C [2]
1 Methanal formaldehyde CH2O 0 −19,1
2 Ethanal acetaldehyde C.2H4O −0 20,1
3 Propanal Propionaldehyde
Propyl aldehyde
C.3H6O −0 48
4 Butanal n-Butyraldehyde C.4H8O −0 74,8
5 Pentanal Valeraldehyde
Amylaldehyde
n-Pentaldehyde
C.5H10O − 103
6 Hexanal Capronaldehyde
n-Hexaldehyde
C.6H12O − 131
7 Heptanal Enanthaldehyde
Heptylaldehyde
n-Heptaldehyde
C.7H14O − 152,8
8 Octanal Caprylaldehyde
n-Octylaldehyde
C.8H16O − 171
9 Nonanal Pelargon aldehyde
n-Nonylaldehyde
C.9H18O − 191
10 Decanal Capric aldehyde
n-Decylaldehyde
C.10H20O − 208,5
12 Dodecanal Lauric aldehyde
Dodecyl aldehyde
C.12H24O − 238
14 Tetradecanal Myristyl aldehyde
Tetradecylaldehyde
C.14H28O − 260

The general empirical formula of the alkanals is C.nH2nO (n = 0, 1, 2, 3, 4, …).

There are also many other groups of aldehydes for which historical names are mostly used:

    is derived from propene - an alkene. is derived from benzene and is therefore an aryl aldehyde. (Furfurol, furan-2-carbaldehyde) is derived from furan, so it is a heteroarylaldehyde. [3]

Dipole-dipole forces arise between the aldehyde groups of alkanals, since the C = O double bond is very polar. Hydrogen bonds do not form because there is no oxygen-bonded hydrogen atom. That is why the boiling points of aldehydes are between those of alcohols and alkanes. Aldehydes can form hydrogen bonds with water because the oxygen atom has two free electron pairs and is negatively polarized. This is why short-chain aldehydes are readily soluble in water. In the case of longer-chain aldehydes, the effect of the non-polar alkyl radicals predominates, which makes the compounds insoluble in water. Many aldehydes have a characteristic odor.

Aldehydes are widely used as flavorings in foods such as wine. Often these arise in fruit and vegetables from substances containing oleic, linoleic or linolenic acid during harvest, chopping or preparation. Hexanal can be found e.g. B. in apples, pears, peaches and cherries. (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 cucumbers next to (E.,E.) -2,4-nonadienal, (E.,Z) -2,6-nonadienal and (Z,Z) -3,6-nonadienal as an odorous flavoring substance. [4]

Above a certain concentration, however, such carbonyl compounds are often rated as rancid, fishy, ​​metallic or as cardboard-like aromas and generally cause an old taste. [4]

The mild oxidation of primary alcohols in a non-aqueous medium produces aldehydes. They can be further oxidized to carboxylic acids.

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

The hydroformylation of an alkene produces a mixture of the n-Aldehyde (middle) and des i-Aldehyde (right).

Formaldehyde (methanal) is produced in large quantities (21 million tons per year worldwide), more than any other aldehyde. It is used as a disinfectant, as a preservative for perishable goods such as cosmetics (formalin solution) and as a raw material in the chemical industry. The largest quantities were processed into aminoplasts and phenoplasts in the plastics industry until 1990. In medicine, methanal in a 4–8% solution (formalin) is used as a fixative in histotechnology.

Aldehydes and ketones are also used to make plastics, solvents, dyes, tannins, perfumes and medicines. Starting from acrolein will 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 are effective against many different microorganisms. In particular, non-enveloped viruses and spore-forming bacteria (e.g. anthrax), which are only accessible to a few disinfectants, can be reached in this way. Since aldehydes have an irritating effect on the skin and mucous membranes and occasionally cause allergies, these agents must be used carefully. [5]

Aldehydes have been used in perfume production since 1921 (Chanel No. 5).

A number of aldehydes are found in the metabolism of cells. Acetaldehyde (ethanal) plays a special role.

In the IR spectra of aldehydes and ketones one finds the intense characteristic band of the C = O stretching vibration in the range of 1690–1750 cm −1.

In 13 C-NMR spectra, the signal of the carbonyl carbon atom of aldehydes and ketones is found in a range of 195 and 210 ppm. The corresponding proton of the aldehyde group can be found in 1 H-NMR spectra as a sharp signal at around 10 ppm. This property makes identification by means of NMR spectroscopy particularly easy, since only a few protons have a resonance in this high range. [6]

Aldehydes are reactive compounds and can be easily oxidized to carboxylic acids.

  • The C = O bond of the carbonyl group is strongly polar with the positive partial charge (δ +) on the carbon atom, which can be attacked by nucleophiles.
  • Aldehydes with a hydrogen atom bonded to the α-carbon atom directly next to the carbonyl group can be in the keto or enol form - see also keto-enol tautomerism.
  • In the case of aldehydes, it is observed that hydrogen atoms on the carbon atom adjacent to the carbonyl group are significantly more acidic than hydrogen atoms on "normal" carbon atoms. This is due on the one hand to the fact that the carbonyl carbon is very poor in electrons and has an −I effect on neighboring bonds, and on the other hand, after deprotonation, the negative charge can be delocalized on the oxygen of the carbonyl group (−M effect).

Nucleophilic addition edit

After attack by the nucleophile, the π-electron pair goes entirely to the oxygen, which is now negatively charged. In the protic solvent, this is compensated for by the uptake of protons, which creates an OH group instead of the carbonyl group.

Addition of water edit

Aldehydes are in aqueous solution with the corresponding according to-Diol, that is, a hydrocarbon with two hydroxyl groups one Carbon atom, in equilibrium. Usually the equilibrium is on the side of the aldehyde. In the case of trichloroacetaldehyde, however, the equilibrium is on the side of the geminal diol.

Addition of alcohols edit

Hemiacetal + alcohol ⇒ acetal + water

Example: ring closure of grape sugar (glucose)

Addition of Nitrogen Nucleophiles Edit

Prim. Amine + aldehyde ⇒ imine (Schiff base) + water

Sec. Amine + aldehyde ⇒ enamine + water

Oxidation to carboxylic acid (important for evidence)

Aldol reaction edit

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

Mixed Aldol Reaction Edit

Mixed aldol reactions cannot usually be carried out in a one-pot reaction, since four possible products can and do form. An exception is when one of the two aldehydes cannot be enolized, i.e. does not have a CH-acidic hydrogen atom. In this case only a mixed aldol is possible. An example of non-enolizable aldehydes are aromatic aldehydes (see: Benzaldehyde). In this way, cinnamaldehyde, an important fragrance, is obtained in a Knoevenagel condensation.

Pinacol coupling edit

If aldehydes are reacted with an alkali metal (example: sodium), a radical anion is formed which quickly dimerizes. The hydrolysis produces a pinacol (traditional name for a 1,2-diol, i.e. a diol with vicinal hydroxyl groups). Starting from an α, ω-dialdehyde, cyclic 1,2-diols are obtained analogously by an intramolecular reaction.


Video: An Overview of Aldehydes and Ketones: Crash Course Organic Chemistry #27 (May 2022).