Acetals

Acetals

Acetals (in the narrower sense) - derived from aldehydes - which, together with the ketals (below) as a subgroup, form the group of acetals (in the broader sense). The following applies: R 1 to R 3 are organyl residues (alkyl residues, aryl residues, arylalkyl residues, etc.), but - with the exception of R 1 - no hydrogen atom. - For most acetals, R 2 = R 3 . Acetals with R 2 ≠ R 3 are called mixed acetals.

General structure of ketals, a subgroup of acetals in the broader sense. The following applies: R 1 to R 4 are organyl residues (alkyl residues, aryl residues, arylalkyl residues, etc.), but not a hydrogen atom.

Acetals are chemical compounds with two alkoxy or aryloxy groups (-OR) on the same carbon atom. This makes them one of the geminal dieters . The name for this group of substances goes back to acetaldehyde diethylacetal, which was initially simply called acetal .

Acetals are formally derived from aldehydes or ketones , although a distinction was originally made between diethers formed from aldehydes (with one hydrogen atom) and diethers formed from ketones ( ketals ). Nowadays, according to IUPAC, ketals are mostly regarded as a subclass of the acetals.

When acetals are formed, semi- acetals or hemi- acetals are formed as intermediate products , which react to form acetals in the presence of acids .

synthesis

Acetals are formed when aldehydes or ketones react with alcohols . The reaction is usually carried out with acid catalysis with elimination of water . Half-acetals are formed as an intermediate stage, as the following exemplary reaction of acetaldehyde and methanol shows:

When using dihydric alcohols ( diols ), the second step of this reaction takes place intramolecularly and cyclic acetals are formed . The addition of a dilute acid leads to the hydrolysis of the acetals, so that the equilibrium of the above reaction is shifted to the left.

Acetals as protecting groups

Use of a cyclic acetal as a protective group for an aldehyde function.

In synthetic chemistry, cyclic acetals in particular are often used as protective groups for carbonyl groups . The high resistance of the acetals to bases , reducing and oxidizing agents is used to protect the carbonyl group from oxidation , for example . After the desired reaction has been carried out in an alkaline medium , the splitting off of the diol and thus the reformation of the carbonyl compound can be achieved by acidification . Conversely, diols can also be protected as acetals. Acetone is often used here as the carbonyl component, since it does not create a stereogenic center on the acetal carbon. The products of this reaction are called acetonides.

Related links

O , N acetals

If the carbonyl group of an aldehyde or a ketone is reacted with a 1,2-amino alcohol with elimination of water, an N , O acetal is formed.

Thioacetals

Acetals can also be formed with thiols , these are called thioacetals (more precisely: di thioacetals, since both oxygen atoms of the acetal are replaced by sulfur atoms). They differ in their reactivity from acetals, which can be of synthetic use ( Corey-Seebach reaction ). Substituting an aldehyde or ketone under acid catalysis, however, with an alcohol of type HO-CH ₂ -CH ₂ -SH in a way, one obtains elimination of water, a mono thioacetal.

S , N -acetals

If the carbonyl group of an aldehyde or a ketone is reacted with a 1,2-aminothiol with elimination of water, an N , S -acetal is formed. Thiazolidines and 3- thiazolines are examples of heterocyclic N , S acetals.

See also

Compounds in which three alkoxy groups or aryloxy groups -OR are bonded to the same carbon atom are called orthoesters and do not count among the acetals.

Individual evidence

1. ↑ Olaf Kühl: Organic Chemistry . Wiley-VCH, Weinheim 2012, ISBN 978-3-527-33199-4 , p. 221.
2. ↑ Entry on acetals . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00062 .
3. ↑ Entry on ketals . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.K03376 .
4. ↑ ^{a b} Bernd Schmidt: Fundamentals of organic chemistry . 5th, revised and updated edition. De Gruyter, Berlin 2015, ISBN 978-3-11-033105-9 , pp. 479-481 .
5. ↑ Paula Y. Bruice: Organic Chemistry: Study compact . 5th updated edition. Pearson Studium, Munich 2011, ISBN 978-3-86894-102-9 , pp. 746-747 .
6. ↑ Entry on acetonides . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00064 .