Acetic anhydride

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Acetic anhydride
Acetic anhydride
Acetic anhydride
Acetic anhydride electron density.PNG
Identifiers
CAS number 108-24-7 YesY
PubChem 7918
ChemSpider 7630 YesY
UNII 2E48G1QI9Q YesY
EC number 203-564-8
ChEBI CHEBI:36610 YesY
RTECS number AK1925000
Jmol-3D images Image 1
Image 2
Properties
Molecular formula C4H6O3
Molar mass 102.09 g mol−1
Appearance colorless liquid
Density 1.082 g cm−3, liquid
Melting point −73.1 °C (−99.6 °F; 200.1 K)
Boiling point 139.8 °C (283.6 °F; 412.9 K)
Solubility in water 2.6 g/100 mL, see text
Refractive index (nD) 1.3901
Pharmacology
Legal status Poison (S6)(AU)


Hazards
MSDS ICSC 0209
EU Index 607-008-00-9
EU classification Corrosive (C)
R-phrases R10, R20/22, R34
S-phrases (S1/2), S26, S36/37/39, S45
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Flash point 49 °C (120 °F; 322 K)
Explosive limits 2.7–10.3%
Related compounds
Related acid anhydrides Propionic anhydride
Related compounds Acetic acid
Acetyl chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, formed by its reaction with the moisture in the air.

Structure and properties[edit]

Acetic anhydride, like most acid anhydrides, are flexible molecules with nonplanar structures. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. The energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.[1]

Like most acid anhydrides, the carbonyl carbon of acetic anhydride is a potent electrophile as the leaving group for each carbonyl carbon (a carboxylate) is a good electron-withdrawing leaving group. The internal asymmetry may contribute to acetic anhydride's potent electrophilicity as the asymmetric geometry makes one side of a carbonyl carbon more reactive than the other, and in doing so tends to consolidate the electropositivity of a carbonyl carbon to one side (see electron density diagram).

Production[edit]

Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric Gerhardt (1816-1856) by heating potassium acetate with benzoyl chloride.[2]

Acetic anhydride is produced by carbonylation of methyl acetate:[3]

CH3CO2CH3 + CO → (CH3CO)2O

The Tennessee Eastman acetic anhydride process involves the conversion of methyl acetate to methyl iodide and an acetate salt. Carbonylation of the methyl iodide in turn affords acetyl iodide, which reacts with acetate salts or acetic acid to give the product. Rhodium chloride in the presence of lithium iodide is employed as catalysts. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions. In contrast, the Monsanto acetic acid process, which also involves a rhodium-catalyzed carbonylation of methyl iodide, is at least partially aqueous.

To a decreasing extent, acetic anhydride is also prepared by the reaction of ketene (ethenone) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar).[4]

H2C=C=O + CH3COOH → (CH3CO)2O (ΔH = −63 kJ/mol)

Ketene is generated by dehydrating acetic acid at 700–750 °C in the presence of triethyl phosphate as a catalyst or (in Switzerland and the CIS) by the thermolysis of acetone at 600–700 °C in the presence of carbon disulfide as a catalyst.[4]

CH3COOH is in equilibrium with H2C=C=O + H2O (ΔH = +147 kJ/mol)
CH3COCH3 → H2C=C=O + CH4

The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922,[5] when the demand for acetic anhydride increased due to the production of cellulose acetate.

Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.

Reactions[edit]

Acetic anhydride is a versatile reagent for acetylations, the introduction of acetyl groups to organic substrates.[6] In these conversions, acetic anhydride is viewed as a source of CH3CO+. Alcohols and amines are readily acetylated.[7] For example, the reaction of acetic anhydride with ethanol yields ethyl acetate:

(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH

Often a base such as pyridine is added to function as catalyst. In specialized applications, Lewis acidic scandium salts have also proven effective catalysts.[8]

Aromatic rings are acetylated by acetic anhydride. Usually acid catalysts are used to accelerate the reaction. Illustrative are the conversions of benzene to acetophenone and ferrocene to acetylferrocene:[9]

(C5H5)2Fe + (CH3CO)2O → (C5H5)Fe(C5H4COCH3) + CH3CO2H

A former industrial route to vinyl acetate involved the intermediate ethylidene diacetate. This geminal diacetate was obtained by reaction of acetaldehyde and acetic anhydride in the presence of a ferric chloride catalyst:[10]

CH3CHO + (CH3CO)2O → (CH3CO2)2CHCH3

Hydrolysis[edit]

Acetic anhydride dissolves in water to approximately 2.6% by weight.[11] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give carboxylic acids. In this case, acetic acid is formed:[12]

(CH3CO)2O + H2O → 2 CH3CO2H

Applications[edit]

As indicated by its organic chemistry, Ac2O is mainly used for acetylations leading to commercially significant materials. Its largest application is for the conversion of cellulose to cellulose acetate, which is a component of photographic film and other coated materials. Similarly it is used in the production of aspirin (acetylsalicylic acid), which is prepared by the acetylation of salicylic acid.[13] It is also used as a wood preservative via autoclave impregnation to make a longer lasting timber.

In starch industry, acetic anydride is a common acetylation compound, used for the production of modified starches (E1414, E1420, E1422)

Because of its use for the synthesis of heroin by the diacetylation of morphine, acetic anhydride is listed as a U.S. DEA List II precursor, and restricted in many other countries.[14]

Safety[edit]

Acetic anhydride is an irritant and combustible liquid. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression.[15] The vapour of acetic anhydride is harmful.[16]

When mixed with hydrogen peroxide, an excess of acetic anhydride reacts with one of the reaction products peracetic acid and forms highly shock sensitive and explosive diacetyl peroxide.[17]

References[edit]

  1. ^ Wu, Guang; Van Alsenoy, C.; Geise, H. J.; Sluyts, E.; Van Der Veken, B. J.; Shishkov, I. F.; Khristenko (2000), "Acetic Anhydride in the Gas Phase, Studied by Electron Diffraction and Infrared Spectroscopy, Supplemented with ab Initio Calculations of Geometries and Force Fields", The Journal of Physical Chemistry A 104 (7): 1576, doi:10.1021/jp993131z. 
  2. ^ Charles Gerhardt (1852) “Recherches sur les acides organiques anhydres” (Investigations into the anhydrides of organic acids), Comptes rendus … , 34 : 755-758.
  3. ^ Zoeller, J. R.; Agreda, V. H.; Cook, S. L.; Lafferty, N. L.; Polichnowski, S. W.; Pond, D. M. (1992), "Eastman Chemical Company Acetic Anhydride Process", Catal. Today 13 (1): 73–91, doi:10.1016/0920-5861(92)80188-S 
  4. ^ a b Arpe, Hans-Jürgen (2007-01-11), Industrielle organische Chemie: Bedeutende vor- und Zwischenprodukte (6th ed.), Weinheim: Wiley-VCH, pp. 200–1, ISBN 3-527-31540-3 .
  5. ^ Milestones in the history of WACKER, Wacker Chemie AG, retrieved 2009-08-27 .
  6. ^ "Acid Anhydrides", Understanding Chemistry, retrieved 2006-03-25 .
  7. ^ Shakhashiri, Bassam Z., "Acetic Acid & Acetic Anhydride", Science is Fun… (Department of Chemistry, University of Wisconsin), retrieved 2006-03-25 .
  8. ^ Macor, John; Sampognaro, Anthony J.; Verhoest, Patrick R.; Mack, Robert A. (2000), "(R)-(+)-2-Hydroxy-1,2,2-Triphenylethyl Acetate", Org. Synth. 77: 45 ; Coll. Vol. 10: 464 
  9. ^ Taber, Douglass F., Column chromatography: Preparation of Acetyl Ferrocene, Department of Chemistry and Biochemistry, University of Delaware, retrieved 2009-08-27 .
  10. ^ G. Roscher "Vinyl Esters" in Ullmann's Encyclopedia of Chemical Technology, 2007 John Wiley & Sons: New York. doi:10.1002/14356007.a27_419
  11. ^ Acetic Anhydride: Frequently Asked Questions, British Petroleum, retrieved 2006-05-03 .
  12. ^ Acetic Anhydride: Material Safety Data Sheet (PDF), Celanese, retrieved 2006-05-03 .
  13. ^ Acetic anhydride, SIDS Initial Assessment Report, Geneva: United Nations Environment Programme, p. 5 .
  14. ^ UN Intercepts Taliban's Heroin Chemical in Rare Afghan Victory, Bloomberg, retrieved 2008-10-07 .
  15. ^ "Data Sheets". International Occupational Safety and Health Information Centre. Retrieved 2006-04-13. 
  16. ^ "NIOSH". Pocket Guide to Chemical Hazards. Archived from the original on 22 April 2006. Retrieved 2006-04-13. 
  17. ^ "Chemical Safety: Synthesis Procedure". Chemical & Engineering News 89 (2): 2. 2011-01-10. 

External links[edit]