Lactaldehyde

Structural formula
Structural formulas of ( S ) -lactaldehyde (left) and ( R ) -lactaldehyde
General
Surname	Lactaldehyde
other names	2-hydroxypropionaldehyde Lactic acid aldehyde
Molecular formula	C 3 H 6 O 2
Brief description	odorless solid with a slightly bitter taste
External identifiers / databases
CAS number 598-35-6 (racemate) 3946-09-6 [( R ) -lactaldehyde] 3913-64-2 [( S ) -lactaldehyde] EC number 809-415-9 ECHA InfoCard 100.237.284 PubChem 855 ChemSpider 832 DrugBank DB03776 Wikidata Q4381828
properties
Molar mass	74.08 g mol −1
Physical state	liquid
Melting point	105 ° C
safety instructions
GHS labeling of hazardous substances Caution H and P phrases H: 315-319 P: ?
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Lactaldehyde (or 2-hydroxypropanal ) is a chemical compound from the series of hydroxyaldehydes . The synthesis and properties of the compound were first described by Alfred Wohl in 1908 .

Isomers

Lactaldehyde is chiral ; due to the stereocenter at the α-C atom, it forms two enantiomers , which are called ( R ) - and ( S ) -lactaldehyde [synonyms: D - (-) - and L - (+) - lactaldehyde] become.

Isomers of lactaldehyde
Surname	( S ) -lactaldehyde	( R ) -lactaldehyde
other names	L- lactaldehyde (+) - lactaldehyde	D- lactaldehyde (-) - lactaldehyde
Structural formula
CAS number	3913-64-2	3946-09-6
	598-35-6 (racemate)
EC number	-	-
	809-415-9 (racemate)
ECHA info card	-	-
	100.237.284 (racemate)
PubChem	439231	439350
	855 (racemate)
DrugBank	DB03776	-
	- (racemate)
Wikidata	Q28529704	Q27102246
	Q4381828 (racemate)

synthesis

During the initial synthesis, the calcium salt of glyoxylic acid 1 is esterified with ethanol and hydrochloric acid to give ethyl diethoxyacetate 2 and acetalized . This intermediate stage is then reacted with piperidine to give the carboxamide 3 . The Grignard reaction of piperidide with methyl magnesium iodide gives ketone 4 - the aldehyde acetal of methyl glyoxal . This ketone can be reduced to the acetal of lactaldehyde 5 with sodium in ethanol . The acetal is then saponified with dilute sulfuric acid to give lactaldehyde 6 .

In an alternative synthesis, described by Hermann Otto Laurenz Fischer and Erich Baer in 1935 , diethoxyacetaldehyde 7 is converted in a Grignard reaction with methylmagnesium iodide to lactaldehyde diethyl acetal 8 and then saponified to lactaldehyde 6 .

In 1952, John Kenyon Netherton Jones published another synthetic approach to lactaldehyde. The aldehyde group of methylglyoxal 9 is selectively converted into the dibutyl acetal 10 using butanol and p -toluenesulfonic acid ( p TsOH) as a catalyst . The reduction with lithium aluminum hydride in ether leads to lactalaldehyde dibutyl acetal 11 , which is hydrolyzed to lactaldehyde 6 in a mixture of glacial acetic acid , water and formic acid in the final step .

properties

Wohl was already able to show that lactaldehyde is present as a dimer in a freshly prepared aqueous solution and slowly changes into the monomolecular form . This behavior was confirmed in 1982 by NMR , Raman and infrared spectroscopy and examined in detail. Three crystal forms can be isolated by recrystallization from various solvents, which are configurational isomers of 3,6-dimethyl-1,4-dioxane-2,5-diol - a cyclic hemiacetal with a 1,4-dioxane basic structure . These arise from the dimerization of the open-chain DL -lactaldehyde:

In a DMSO- d ₆ solution is obtained from the three different crystal forms after some time, an identical ¹ H-NMR spectrum different from the superposition of the spectra of configurational isomers 2 , 3 and 4 yields:

These configurational isomers are thus in equilibrium with one another via the lactaldehyde 1 . The proportions of the three species 2 , 3 and 4 in the equilibrium mixture are 53.6%, 38.6% and 7.8%. The speed at which equilibrium is reached varies widely. Starting with isomer 4 , equilibrium is reached in solution after about one month, while starting with isomer 3, equilibrium is reached in two to three days.

The three dimeric forms of lactaldehyde are each formed by the reaction of a D and an L monomer. The dimers from two D or two L monomers are not observed.

In the NMR spectrum of the equilibrium mixture, there are also weak signals that can be assigned to a five-membered dimer, as well as a very weak signal for the aldehyde proton of the monomer.

Individual evidence

1. ↑ ^{a b c d} A. Wohl: On the knowledge of the three-carbon series . In: Reports of the German Chemical Society . tape 41 , no. 3 , October 1908, p. 3599 , doi : 10.1002 / cber.19080410340 .
2. ↑ There is not yet a harmonized classification for this substance . A label of [No public or meaningful name is available] in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on July 6, 2020, is derived from a self-classification by the distributor .
3. ↑ A. Wohl, M. Lange: Structure of lactic acid aldehyde . In: Reports of the German Chemical Society . tape 41 , no. 3 , October 1908, p. 3612 , doi : 10.1002 / cber.19080410341 .
4. ↑ External identifiers or database links for diethoxyacetaldehyde : CAS number: 5344-23-0, EC number: 807-284-2, ECHA InfoCard: 100.234.849 , PubChem : 219609 , ChemSpider : 190379 , Wikidata : Q82950805 .
5. ^ Hermann OL Fischer, Erich Baer: About Glyoxal-semiacetal. (II. Communication on Glyoxal) . In: Helvetica Chimica Acta . tape 18 , no. 1 , 1935, p. 514 , doi : 10.1002 / hlca.19350180169 .
6. ↑ L. Hough, JKN Jones: 773. The synthesis of sugars from simpler substances. Part IV. Enzymic synthesis of 6-deoxy-D-fructose and 6-deoxy-L-sorbose . In: Journal of the Chemical Society . 1952, p. 4052 , doi : 10.1039 / jr9520004052 .
7. ↑ Hiroaki Takahashi, Yoko Kobayashi, Norio Kaneko: Conformational studies of DL-lactaldehyde by 1H-NMR, Raman and ir spectroscopy . In: Spectrochimica Acta Part A: Molecular Spectroscopy . tape 39 , no. January 6 , 1983, pp. 569 , doi : 10.1016 / 0584-8539 (83) 80108-1 .