paraldehyde

Structural formula
Structural formula of the trimer of acetaldehyde without stereochemistry
General
Non-proprietary name	paraldehyde
other names	2,4,6-trimethyl-1,3,5-trioxane Paracetaldehyde
Molecular formula	C 6 H 12 O 3
Brief description	colorless liquid with a sweet odor
External identifiers / databases
CAS number 123-63-7 EC number 204-639-8 ECHA InfoCard 100.004.219 PubChem 31264 ChemSpider 21106173 DrugBank DB09117 Wikidata Q424342
Drug information
ATC code	N05 CC05
Drug class	Sedative, hypnotic
properties
Molar mass	132.16 g mol −1
Physical state	liquid
density	0.99 g cm −3 (20 ° C)
Melting point	12 ° C
boiling point	124 ° C
Vapor pressure	10.3 h Pa (20 ° C) 18.8 hPa (30 ° C) 32.8 hPa (40 ° C) 55 hPa (50 ° C)
solubility	easily in water (120 g l −1 at 20 ° C) miscible with ethanol, chloroform, ether
Refractive index	1.405 (20 ° C)
safety instructions
Please note the exemption from the labeling requirement for drugs, medical devices, cosmetics, food and animal feed GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary Caution H and P phrases H: 226 P: 210
Toxicological data	2711 mg kg −1 ( LD 50 ,  rat ,  oral )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Paraldehyde is a chemical compound . It is the trimer of acetaldehyde . It is a cyclic full acetal . Accordingly, the molecule is unstable, even with dilute acids . Paraldehyde is an active ingredient from the group of sedatives with calming, sleep-promoting and antispasmodic properties. It is rarely used these days.

Extraction and presentation

Paraldehyde is formed by cyclizing three molecules of acetaldehyde in the presence of sulfuric acid . The product formation of the cyclization reaction is temperature dependent. Formation of the trimer is preferred at room temperature. At lower temperatures around −10 ° C the tetramer metaldehyde is more likely to form .

The trimerization reaction proceeds exothermically, where a heat of reaction of −113 kJ mol ⁻¹ can be estimated.

properties

Physical Properties

Paraldehyde is a colorless liquid with a pungent ethereal odor. It is miscible with typical organic solvents and readily soluble in water. The solubility in water decreases with increasing temperature.

Compilation of the most important thermodynamic properties

property	Type	Value [unit]	Remarks
Standard enthalpy of formation	Δ f H 0 liquid Δ f H 0 gas	−681.8 kJ mol −1 −636 kJ mol −1	as a liquid as a gas
Enthalpy of combustion	Δ c H 0 liquid	−3394.2 kJ mol −1	as a liquid
Heat capacity	c p	257.3 J mol −1 K −1 (25 ° C) 1.947 J g −1 K −1 (25 ° C)	as a liquid
Critical temperature	T c	563 K
Enthalpy of fusion	Δ f H	13.52 kJ mol −1
Enthalpy of evaporation	Δ V H	41.5 kJ mol −1	at normal pressure boiling point
viscosity		1.31 mPas −1	at 20 ° C
Refractive index		1.4049	at 20 ° C

Stereochemistry

Two stereoisomers can be formulated for the molecular structure , known as cis- (1) and trans-paraldehyde (2). The previous two are in equilibrium with their respective conformational isomers (4) and (3), which are, however , very unstable for steric reasons, since the methyl groups would interfere with each other.

Safety-related parameters

Paraldehyde forms highly flammable vapor-air mixtures. The compound has a flash point of 27 ° C. The explosion range is between 1.3% by volume (70 g / m³) as the lower explosion limit (LEL) and 17% by volume as the upper explosion limit (UEL). The limit gap width was determined to be 1.01 mm. This results in an assignment to explosion group IIA. The ignition temperature is 201 ° C. The substance therefore falls into temperature class T3.

safety instructions

Storage is difficult due to instability. You add an antioxidant and store away from light and heat or in brown glass bottles. The opened quantities should be kept as small as possible and used quickly.

proof

• The aldehyde reacts in the Tollensprobe with a silver diammine complex and forms a silver mirror from elemental silver .
• The CH ₃ -CO group can be detected in the iodoform reaction .

Acid hydrolysis of the paraldehyde to acetaldehyde is required for both reactions .

Paraldehyde is monographed in pharmacopoeias .

See also

• Metaldehyde
• Paraformaldehyde

Individual evidence

1. ↑ ^{a b c d e f g h i j k l m n o p} Entry on 2,4,6-trimethyl-1,3,5-trioxane in the GESTIS substance database of the IFA , accessed on June 9, 2020(JavaScript required) .
2. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-410.
3. ↑ Paraldehyde data sheet from Sigma-Aldrich , accessed on April 18, 2011 ( PDF ).
4. ↑ Entry on 2,4,6-trimethyl-1,3,5-trioxane in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
5. ↑ Data sheet paraldehyde (PDF) from Merck , accessed on January 18, 2011.
6. ↑ HP Lacha; U. Kazmaier; HA Klein: Chemistry for Biologists, Springer Verlag 2005, ISBN 3-540-21161-6 , p. 515.
7. ↑ ^{a b c d e f} M. Eckert, G. Fleischmann, R. Jira, HM Bolt, K. Golka: Acetaldehyd , in: Ullmanns Enzyklopädie der Technischen Chemie , Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2012; doi : 10.1002 / 14356007.a01_031.pub2 .
8. ↑ ^{a b c} Pihlaja, K .; Tuomi, M .: Bond-Bond interactions in organic oxygen compounds. Part II. Anomalous stablization in cis-2,4,6-trimethyl-1,3,5-trioxane in Suom. Kemistil. 43 (1970) 224-226.
9. ↑ ^{a b} Clegg, GA; Melia, TP: Thermodynamics of Polymerization of Heterocyclic Compounds. Part IV. The heat capacity, entropy, enthalpy and free energy of paraldehyde in Makromol . Chem. 123 (1969) 194-202.
10. ↑ Hollmann, R .: Physical and natural equilibria between the modifications of aldehyde in Z. Phys. Chem., Stoechiom. Kinship 43 (1903) 129-159.
11. ↑ ES Domalski, ED Hearing: Heat Capacities and Entropies of Organic Compounds in the Condensed phase. Volume III. in: J. Phys. Chem. Ref. Data 25, 1996, pp. 1-525; doi: 10.1063 / 1.555985 .
12. ↑ RM Stephenson, S. Malanowski: Handbook of the Thermodynamics of Organic Compounds. Springer 1987, ISBN 94-010-7923-4 , doi: 10.1007 / 978-94-009-3173-2 .
13. ↑ Kewley, R .: Microwave spectrum of paraldehyde . In: Canadian Journal of Chemistry . 48 (5), 1970, pp. 852-855, doi : 10.1139 / v70-136 .
14. ↑ DC Carpenter, LO Brockway: The Electron Diffraction Study of Paraldehyde. In: Journal of the American Chemical Society. 58, 1936, pp. 1270-1273, doi : 10.1021 / ja01298a053 .
15. ↑ ^{a b c} E. Brandes, W. Möller: Safety-related parameters. Volume 1: Flammable Liquids and Gases. Wirtschaftsverlag NW - Verlag für neue Wissenschaft, Bremerhaven 2003.