Acetal

Acetals are chemical compounds that are characterized by two alkoxy or aryloxy -OR, which are bonded to the same carbon atom, distinguished. They are geminally located diethers having the general structure R'2C (OR) 2, wherein R a may be a hydrogen atom. "

Originally, ( a hydrogen atom ) and the diethers formed from ketones, ketals as mentioned differences between the diethers formed from aldehydes. Nowadays, according to IUPAC ketals are usually regarded as a subclass of acetals.

In the formation of acetals are formed as intermediates, semi - or hemiacetals ( R'2C (OR ) (OH) ), which can only be isolated in some cases, but especially when a ring closure to stabilize in relatively relaxed ring systems leads. So many monosaccharides are present as stable, cyclic hemiacetals (as lactols ).

Synthesis

Acetals are formed in the reaction of aldehydes ( eg acetaldehyde ) and mono-or dihydric alcohols (eg glycol ) with elimination of water. Usually, this condensation reaction is conducted with acid catalysis:

Acetals as protecting groups

In synthetic chemistry, acetals are frequently used as protective groups for carbonyl groups. This can be a diol, a dialcohol with two hydroxy groups ( OH groups), mostly 1,2-ethanediol, to react with the carbonyl group to be protected in the presence of catalytic amounts of acid, so that a good yield, a cyclic acetal is formed, which bases, organometallic reagents and customary reducing agents is not attacked. Acetals are masked carbonyl groups in this sense. Conversely, diols protected as acetals. Here, acetone is often used as the carbonyl component, such as no stereogenic center is generated at Acetalkohlenstoff. The products of this reaction are called Acetonide.

Fehling's sample, which is used to detect an aldehyde group ( -CHO) in aldehydes and reducing sugars, therefore is positive even if those materials with a hemiacetal OH group.

Related compounds

O, N- acetals

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

Thioacetals

Acetals can also be formed with thiols these are called thioacetals (more precisely, dithioacetals, since both oxygen atoms of the acetal can be replaced by sulfur atoms). They differ in the reactivity of the acetals, which may have synthetic utility ( Corey -Seebach reaction). Substituting an aldehyde or ketone with acid catalysis, however, to come up with an alcohol of the type HO -CH2-CH2 -SH, the result is the elimination of water a Monothioacetal.

S, N- acetals

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