Acetal

Synthesis

It can be synthesised by the following steps:A mixture of ketone 236 (50 g, 0.19 mol), 2,2-dimethyl-1,3- propanediol (22.08 g, 0.21 mol) and PPTS (4.85 g, 0.019 mol) was  refluxed in benzene in a round bottom flask fitted with Dean-Stark  apparatus for 7 hours to remove water. The reaction mixture was washed with aq. NaHCO3 solution, and then thoroughly with water, dried over anhydrous Na2SO4 and filtered.  Evaporation of the solvent under reduced pressure followed by crystallization from EtOAc  furnished a colourless solid (67.27 g, 95%).

Description

Acetal (full name: Acetaldehyde diethyl acetal/1,1-Diethoxyethane) is a major flavoring component of distilled beverages, especially malt whisky and sherry.
Acetaldehyde diethyl acetal is used as a flavoring agent to provide fruit, nut, rum, and whiskey flavors. It can react with diketene to form ethyl 5-ethoxy-3-oxohexanoate in the presence of titanium chloride. It can also be used to synthesize mixed acetal glycosides via transacetalation.

As a flavor ingredient

Identification:

Synthesis

Acetaldehyde diethyl acetal can be obtained by the reaction between ethyl alcohol and acetaldehyde in the presence of anhydrous calcium chloride.

Preparation

To a pressure bottle containing 20 gm (0.18 mole) of anhydrous calcium chloride is added 105 gm of 95% (2.17 moles) ethanol and the mixture cooled to 8°C. Then 50 gm (1.14 moles) of cold acetaldehyde is slowly poured down the wall of the bottle. The bottle is closed and shaken vigorously for 5-10 min, with cooling if necessary. The mixture is allowed to stand at room temperature with intermittent shaking for 24 hr. The upper layer, which has separated, weighs 128-129 gm. It is washed three times with 30-40 ml of water. The organic layer is dried over 3 gm of anhydrous potassium carbonate and distilled through a 1 ft column, to afford 70-72 gm (59-60%), b.p. 101-103.5°C. The low-boiling fractions are washed again with water, dried and again fractionally distilled to give another 9.0-9.5 gm (7.9-8.1%), b.p. 101-103.5°C. Therefore, the total yield amounts to 79-81.5 gm (67-69%).

References

• Maarse, H. (1991). Volatile Compounds in Foods and Beverages. CRC Press. p. 553. ISBN 0-8247-8390-5.
• Zea, Luis; Serratosa, María P.; Mérida, Julieta; Moyano, Lourdes (2015). "Acetaldehyde as Key Compound for the Authenticity of Sherry Wines: A Study Covering 5 Decades". Comprehensive Reviews in Food Science and Food Safety. 14 (6): 681–693.

Description

Acetal is a clear, colourless, and extremely flammable liquid with an agreeable odour. The vapour is susceptible to cause flash fire. Acetal is sensitive to light and, on storage, may form peroxides. In fact, it has been reported to be susceptible to autoxidation and should, therefore, be classified as peroxidisable. Acetal is incompatible with strong oxidising agents and acids.

Chemical Properties

Acetal.has.a.refreshing,.pleasant,.fruity-green.odor

Chemical Properties

Acetal, an aldehyde, is a clear, volatile liquid with an agreeable odor

Chemical Properties

Acetal is a clear, colorless, and extremely fl ammable liquid with an agreeable odor. The vapor may cause fl ash fi re. Acetal is sensitive to light and on storage may form peroxides. In fact, it has been reported to be susceptible to autoxidation and should, therefore, be classifi ed as peroxidizable. Acetal is incompatible with strong oxidizing agents and acids.

Occurrence

Present.in.some.liquors.(e.g.,.sake,.whiskey.and.cognac);.also.detected.and.quantitatively.assessed.in.rums.. Found.in.apple.juice,.orange.juice,.orange.peel.oil,.bitter.orange.juice,.strawberry.fruit,.raw.radish,.Chinese.quince.fruit,.Chinese. quince.flesh,.udo.(Aralia cordata Thunb.)

Uses

Acetaldehyde diethyl acetal is used as a flavoring agent to provide fruit, nut, rum, and whiskey flavors.

Uses

Solvent; in synthetic perfumes such as jasmine; in organic syntheses.

Preparation

From.ethyl.alcohol.and.acetaldehyde.in.the.presence.of.anhydrous.calcium.chloride.or.small.amounts.of.mineral. acids.(HCl).

Definition

A type of organic compound formed by addition of an alcohol to an aldehyde. Addition of one alcohol molecule gives a hemiacetal. Further addition yields the full acetal. Similar reactions occur with ketones to produce hemiketals and ketals.

Aroma threshold values

Detection:.4.to.42.ppb

General Description

A clear colorless liquid with a pleasant odor. Boiling point 103-104°C. Flash point -5°F. Density 0.831 g / cm3. Slightly soluble in water. Vapors heavier than air. Moderately toxic and narcotc in high concentrations.

Air & Water Reactions

Highly flammable. Forms heat-sensitive explosive peroxides on contact with air. Slightly soluble in water.

Reactivity Profile

Acetal can react vigorously with oxidizing agents. Stable in base but readily decomposed by dilute acids. Forms heat-sensitive explosive peroxides on contact with air. Old samples have been known to explode when heated due to peroxide formation [Sax, 9th ed., 1996, p. 5].

Health Hazard

May irritate the upper respiratory tract. High concentrations act as a central nervous system depressant. Symptoms of exposure include headache, dizziness, drowsiness, abdominal pain, and nausea.

Health Hazard

Mild irritant to skin and eyes; acute toxicityof low order; narcotic at high concentrations;4-hour exposure to 4000 ppm lethal to mice;the oral LD50 value for mice is 3500 mg/kg.

Health Hazard

Exposures to acetal cause irritation to the eyes, skin, gastrointestinal tract, nausea, vomit- ing, and diarrhea. In high concentrations, acetal produces narcotic effects in workers.

Fire Hazard

Highly flammable; flash point (closed cup) -21°C (-6°F); vapor density 4.1 (air = 1), vapor heavier than air and can travel some distance to a source of ignition and flash back; autoignition temperature 230°C (446°F); vapor forms explosive mixtures with air, LEL and UEL values are 1.6% and 10.4% by volume in air, respectively (DOT Label: Flammable Liquid, UN 1088). .

Flammability and Explosibility

Flammable

Industrial uses

Acetal homopolymer resins have high tensilestrength, stiffness, resilience, fatigue endurance,and moderate toughness under repeatedimpact. Some tough grades can deliver up to 7times greater toughness than unmodified acetalin Izod impact tests and up to 30 times greatertoughness as measured by Gardner impact tests.
Automotive applications of acetal homopolymerresins include fuel-system and seat-beltcomponents, steering columns, window-supportbrackets, and handles. Typical plumbingapplications that have replaced brass or zinccomponents are showerheads, ball cocks, faucetcartridges, and various fittings. Consumer itemsinclude quality toys, garden sprayers, stereocassette parts, butane lighter bodies, zippers,and telephone components. Industrial applicationsof acetal homopolymer include couplings,pump impellers, conveyor plates, gears, sprockets,and springs.

Safety Profile

Moderately toxic by ingestion, inhalation, and intraperitoneal routes.A skin and eye irritant. A narcotic. Dangerous fire hazard when exposed to heat or flame; can react vigorously with oxidizing materials. Forms heat-sensitive explosive peroxides on contact with air. when heated to decomposition it emits acrid smoke and fumes. See also ETHERS and ALDEHYDES.

Potential Exposure

Used as a solvent; in synthetic perfumes, such as jasmine, cosmetics, flavors; in organic synthesis.

Metabolism

When acetal was fed at a level of 5% in the diet for 6 days, availability of energy was 64% in chicks and 29% in rats (Yoshida et al. 1970 & 1971). Acetal is rapidly hydrolysed in the stomach(Knoefel, 1934). The resulting acetaldehyde is readily oxidized to acetic acid and eventually to carbon dioxide and water(Williams, 1959).

Shipping

UN1088 Acetal, Hazard Class: 3; Labels: 3-Flammable liquid. UN1988 Aldehydes, flammable, toxic, n.o.s., Hazard Class: 3; Labels: 3-Flammable liquid, 6.1-Poisonous materials, Technical Name Required

Purification Methods

Dry acetal over Na to remove alcohols and H2O, and to polymerise aldehydes, then fractionally distil. Or, treat it with alkaline H2O2 at 40-45o to remove aldehydes, then saturate with NaCl, separate, dry with K2CO3 and distil it from Na [Vogel J Chem Soc 616 1948]. [Beilstein 1 IV 3103.]

Incompatibilities

Aldehydes are frequently involved in self-condensation or polymerization reactions. These reactions are exothermic; they are often catalyzed by acid. Aldehydes are readily oxidized to give carboxylic acids. Flammable and/or toxic gases are generated by the combination of aldehydes with azo, diazo compounds, dithiocarbamates, nitrides, and strong reducing agents. Aldehydes can react with air to give first peroxo acids, and ultimately carboxylic acids. These autoxidation reactions are activated by light, catalyzed by salts of transition metals, and are autocatalytic (catalyzed by the products of the reaction). The addition of stabilizers (antioxidants) to shipments of aldehydes retards autoxidation. Presumed to form explosive peroxides on contact with air and light. May accumulate static electrical charges, and may cause ignition of its vapors.

Waste Disposal

Dissolve or mix the material with a combustible solvent and burn in a chemical incinerator equipped with an afterburner and scrubber. All federal, state, and local environmental regulations must be observed.