Acetaldehyde

• Ethanal (IUPAC)
• Äthanal (deprecated)

A colorless liquid or colorless gas with a pungent odor

Liquid or gaseous

• 0.784 g · cm -3 ( 20 ° C)
• From 0.7904 to 0.7928 gcm -3 ( 10 ° C)

-123 ° C

20.4 ° ​​C

1007 hPa ( 20 ° C)

13.57 (25 ° C)

Miscible with water

2.750 (6 ) D ( 9.2 · 10-30 C · m)

1.3316 (20 ° C)

Risk

• 91 mg · m-3
• 50 ml · m-3

• -192.2 KJ / mol (liquid)
• -166.2 KJ / mol ( gas)

Template: Infobox chemical / molecular formula search available

Acetaldehyde [ ATSE ː t | t aldehydes ː ] Acetylaldehyd also named according to IUPAC ethanal, is an aldehyde, specifically, a alkanal with two carbon atoms; thus derived from ethane. Acetaldehyde has the half structural formula CH3 -CHO.

In the human body, acetaldehyde is formed as an intermediate in the degradation of ethanol by alcohol dehydrogenase. Acetaldehyde is responsible for the hangover the next morning, among other substances. Acetaldehyde is metabolised to acetate usually fast. After taking disulfiram or Coprin (active ingredient in Faltentintling ) the metabolism of acetaldehyde is inhibited and there is an accumulation in the body, which is associated with (mostly mild ) symptoms of poisoning ( Antabuse or Coprinus syndrome). Smouldering or burning of tobacco there arises as Neben-/Pyrolyseprodukt and thus reaches from the tobacco smoke through the alveoli into the blood. In addition, acetaldehyde was detected in plant extracts, essential oils and roasted coffee. Some deciduous trees, such as maples and poplars, give acetaldehyde from the transition from light to dark.

Biochemically acetaldehyde is also a common intermediate. So convert the resulting yeast pyruvate in glycolysis to ethanol in two stages in order by the first pyruvate with pyruvate decarboxylase, an enzyme from the class of lyases are converted into acetaldehyde and then by means of alcohol dehydrogenase, an enzyme from the class of oxidoreductases is converted to ethanol.

History

Acetaldehyde was probably first synthesized in the presence of sulfuric acid in 1781 by Carl Wilhelm Scheele in the attempt to oxidation of ethanol with manganese dioxide. However, the characterization was not until Justus Liebig in collaboration with Johann Wolfgang Dobereiner in 1835. He named the compound aldehyde (from Latin Alcoholus dehydrogenatus ). Acetaldehyde can be regarded as the first representative of the class of aldehydes.

Production

Historically acetaldehyde by oxidation of ethanol with a number of oxidizing agents, such as manganese dioxide / sulfuric acid ( Scheele ) or chromic acid ( Liebig ), given by the equation:

For large- scale synthesis of ethylene is prepared by catalytic addition of water with simultaneous air oxidation over fixed-bed catalysts in the context of the Wacker Process. Formally, the equations become:

In the 1970s, the worldwide production capacity for acetaldehyde increased to more than 2 million tons per year. Due to the development of new synthetic routes, which need not acetaldehyde as a reactant, currently the requirement is reduced, however.

Properties

Acetaldehyde is a colorless, highly volatile and highly flammable liquid that is miscible with water in all proportions; it is formed in an equilibrium reaction, the aldehyde hydrate. Unlike formaldehyde, however, the balance is only at about 50 % on the side of the hydrate.

Acetaldehyde must be stored in a cool, because it already boils at 20 ° C and forms explosive vapor - air mixtures. These vapors can ignite on hot heating surfaces due to the extremely low ignition point of 140 ° C. The flash point of acetaldehyde is -39 ° C and its UN number is 1089th

Acetaldehyde oligomerized slightly acid-catalyzed aldol addition to.

• The " dimer" ( CH3CHO ) 2 is the so-called aldol.
• Trimer ( CH3CHO ) 3 having a cyclic acetal structure (2,4,6 -trimethyl-1 ,3,5- trioxane). There is a liquid (bp 124 ° C) with the trivial name paraldehyde.
• Tetramer ( CH3CHO ) 4 is also cyclic acetal ( 2,4,6,8- tetramethyl-1 ,3,5,7 - tetroxocane ). It is a solid ( sublimes at 112 ° C ), also called metaldehyde. It is used as a solid fuel and because of its toxicity as a snail venom ( slug pellets ).
• Usually containing solid fuel and higher oligomers of acetaldehyde, such as pentamers ( CH3CHO ) and 5 hexamers ( CH3CHO ) 6

The oligomers with acetal can be easily cleaved again by acids.

Tautomerism

Acetaldehyde has an unstable tautomer, the vinyl alcohol or ethenol. This simplest enol isomerization in a free state under normal conditions immediately to acetaldehyde, unless - is stabilized - about as carbonyl iron or platinum complex Pt ( acac ) ( η2 - C2H3OH ) Cl.

Effects in the liver

The deleterious effects of acetaldehyde in liver are varied. Protein adducts it forms that activate the so called Kupffer cells (macrophages of the liver). They secrete enhanced substances, which alter the other cells of the liver, Ito cells in such a way that this then amplified forming collagen. This favors the formation of liver cirrhosis. In addition, acetaldehyde leads to increased formation of oxygen radicals, which damage the membranes of the cells so that they perish.

Use

Acetaldehyde is an important raw material in the chemical industry. Acetaldehyde is used for example for the production of acetic acid, acetic anhydride, butadiene, acrolein, and pentaerythritol.