Acetic anhydride, also known as ethanoic anhydride, is a chemical compound, that has the formula (CH3CO)2O having a common abbreviation Ac2O. This is widely used as a reagent for organic synthesis. Acetic anhydride is a colorless liquid that has a powerful smell of acetic acid, that is formed reacting with the moisture present in air. The synthesis of acetic anhydride, was first done by Charles Frederic Gerhardt through heating potassium acetate with benzoyl chloride. Carbonylation of methyl acetate produces acetic anhydride.
The Tennessee Eastman acetic anhydride process involves the conversion of methyl acetate to methyl iodide producing acetate salt. Further, carbonylation of methyl iodide gives acetyl iodide that reacts with acetate salts or acetic acid to provide the product. Rhodium chloride, in presence of lithium iodide is used as a catalyst, because of non-stability of acetic anhydride in water, thus the process of conversion occurs under anhydrous conditions.
Acetic anhydride is a very versatile reagent in acetylation, which introduces acetyl groups to organic substrates, in these kind of conversions, acetic anhydride is used as a source of CH3CO+
Amines and alcohols are readily acetylated, in the reaction of acetic anhydride with ethanol makes ethyl acetate
(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH
Bases such as pyridine are added to function as a catalyst.
Aromatic rings are acetylated using acetic anhydride, where acid catalysts are used for accelerating the reaction. There are examples to demonstrate the conversion of benzene to acetophenone and ferrocene to acetylferrocene
C5H5)2Fe + (CH3CO)2O → (C5H5)Fe(C5H4COCH3) + CH3CO2H
Dicarboxylic acid is converted to anhydrides by treatment with acetic anhydride. This is used in the preparation of mixed anhydrides, by reactions with nitric acid, acetyl nitrate.
Aldehydes on reaction with acetic anhydrides, when an acidic catalyst is present is used to give germinal diacetate, the germinal diacetate obtained from acetaldehyde and acetic anhydride is
CH3CHO + (CH3CO)2O → (CH3CO2)2CHCH3
There are other roles of acetic anhydride like acetylations, which create significant materials for commercial purposes. The largest application of acetyl anhydride is the conversion of cellulose acetate that is a part of photographic film and for coating other materials also used for the manufacturing of filters used in cigarettes. This is also used in the production of aspirin (acetylsalicylic acid), that is prepared from the acetylation of salicylic acid. The acetic anhydride is also used for making wood preservatives using autoclave impregnation that creates a timber both longer and lasting.
The acetic anhydride, just like most of the acid anhydrides is a non-planar structured flexible molecule. The pi system linkage, from the central oxygen offers weak resonance stabilization in comparison with the dipole-dipole repulsion in between the two carbonyl oxygens. The barriers of energy that bond the rotation between each optimal aplanar conformations are significantly low. The carbonyl carbon of acetic anhydrides comprises of electrophilic characteristics, because the leaving group is carboxylate. The potent electrophilicity of the acetic anhydrides is due to the internal asymmetry that makes one side of the carbonyl carbon much more reactive than the other, and thus it tends to consolidate electropositivity of a carbonyl carbon to one side.
The melting point of acetic anhydride is -73.10 C and the boiling point is 1400 C. Density of acetic anhydride is 1.087, vapor density is 3.5 (vs air) and vapor pressure is 10 mm Hg (360C). The refractive index is n20/D 1.390 (lit.). Flash point for acetic anhydride is 1300 F and for storing the anhydride the temperature should be set at room temperature. Solubility of anhydride that it is miscible with ether, chloroform and benzene. The anhydride is liquid in form and it is colorless as well with a specific gravity of 1.082. This is a very strong smelling, pungent, vinegar-like characteristic odor. This is highly sensitive to moisture. Acetic anhydride is flammable and incompatible with strong oxidizing agents, water, alcohols and strong bases.
To calculate the total number of moles of acetic anhydride, from the volume ( cell D2 ), there is a need to find the density of acetic anhydride, which is (1.08 g/ml) and the formula mass (102.09 g/ mol).
There is a need to write the balanced chemical equation for the chemical process, then calculate the number of moles of each reactant in order to identify the limiting reactant ( the smallest number of required moles) then the theoretical yield is calculated of the product based on the moles of the limiting reactant.
The balanced reaction for this case is given by:
So, there is a requirement of equal number of moles of acetic anhydride and salicylic acid. The products formed are acetylsalicylic acid (aspirin) and acetic acid.
This carboxylic anhydride is commonly used as a acetylating agent for amines and alcohols. Acytic anhydrides participate in the Friedel-Craft’s intramolecular cyclization reaction of cyclopropyl ketones to afford the tetralones. There has been reported to promote the Knoevenagel condensation of various aldehydes and activated methylene compounds. There are various uses of acetic anhydrate, in reactant synthesis like,
3-acetoxybenzoic acid by reacting with 3-hydroxybenzoic acid
Acetic anhydride was used for the following studies:
Lipozyme TL IM (commercial immobilized lipase from Thermomyces lanuginosus) catalyzed acetylation of essential clove oil.
Acetyl donor to investigate the lipase catalyzed acetylation of nanofibrillated cellulose (NFC).
Synthesis of N-substituted pyrazolines.
Acetylation of Barnyardgrass (Echinochloa crus-galli) starch.
This is the only acetic anhydride which, is used as a sample from other reservoirs when the samples are acetylated for 13 hours at near about 1000 C. The samples are then evaporated to dryness under N2 and chloroform is added using a third reservoir.
The molecular weight of acetic anhydride is 102.1 g/mol, which boils at a temperature of 1390 C and the density of acetic anhydride is 1.08 g/cm3.
Acetic anhydride is irritable and a combustible liquid. Due to the reactivity with water, alcohol, foam or carbon dioxide are used for suppressing fire. However, the vapour form of acetic anhydride is harmful and can cause severe damage in contact with the skin. Precautionary measures must be taken while working with acetic anhydride.
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