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Acetic anhydride

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.

Formic anhydride is an even simpler acid anhydride, but it spontaneously decomposes, especially once removed from solution.


Contrary to what its Lewis structure seems to predict, acetic anhydride, like many other acid anhydrides that are free to rotate, has experimentally been found to be aplanar. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. However, the energy barriers to bond rotation between each of the optimal aplanar conformations are quite low. .

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).


Acetic anhydride is produced by carbonylation of methyl acetate:

CH3CO2CH3 + CO → (CH3CO)2O

This 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 iodide and lithium iodide are 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 with acetic acid at 45–55 Â°C and low pressure (0.05–0.2 bar).

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

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

CH3COOH H2C=C=O + H2O (ΔH = +147 kJ/mol)

The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922, 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.


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


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

Aromatic rings are acetylated, usually in the presence of an acid catalyst. Illustrative is the conversion of benzene to acetophenone:

(CH3CO)2O + C6H6→ CH3COC6H5 + CH3CO2H

Ferrocene may be acetylated too:

Cp2Fe + (CH3CO)2O → CpFe(C5H4COCH3)


Acetic anhydride dissolves in water to approximately 2.6% by weight. Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give acetic acid:

(CH3CO)2O + H2O → 2 CH3CO2H


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 ph

Acidic oxide

An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an acid anhydride) is an oxide that either

Examples include:

Acidic oxides are oxides of either nonmetals or of metals in high oxidation states.

From Yahoo Answers

Question:Give me some examples please. What process creates excessive amounts of acid anhydrides in our environment? What environmental problem does this create? What's the balanced equation for that reaction? This is a homework problem. Thanks

Answers:An acid anhydride is a synthetic (man-made) chemical (reagent) that is used to acylate amines, alcohols, or thiols. A common example is acetic anhydride, which has the following structure: CH3-C(=O)-O-C-(=O)-CH3 Here's a little more informaiton about acetic anhydride (from wikipedia)... "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. (Formic anhydride spontaneously decomposes, especially once removed from solution.) It is a colorless liquid that smells strongly of acetic acid, formed by its reaction with the moisture in the air." Look at the wikipedia link given below to learn more about anhydrides...

Question:A. a metal and oxygen. B. a nonmetal and hydrogen. C. a nonmetal and oxygen. D. oxygen and hydrogen

Answers:The answer is C. Examples include CO2, SO3, NO2, and P2O5. These will all make acids when they react with water.

Question:What do you get when you add ethanoic anhydride with 3-chlorobenzoic acid? Any hints would help.

Answers:Carboxylic acid + anhydride < --> anhydride.+ carboxylic acid A carboxylic acid will be acylated by an acid anhydride. More often than not, the carboyxlic acid and anhydride have different substituents (R1-CO2H and (R2-CO)2O. The anhydride is usually symmetrical. When this happens, the product is a mixed anhydride (R1-CO2-CO-R2). In your case R1 = 3-chlorophenyl and R2 = acetyl. The product is acetic 3-chlorophenyl anhydride (mixed anhydrides are named with the two acyl groups listed separately, in alphabetical order, followed by the word anhydride.

Question:I'm really lost right now in chemistry. How can you predict the acidic or basic nature of anhydrides? Some questions I have on my worksheet are: -CO2, and it's acidic because it's organic? Am I in the right here? One question I couldn't get right was: SeO2. Why is it acidic? I'm also having difficulty getting how to write the formulas of anhydrides. Some questions would be like: -Na2O -CaO -N2O5 I didn't get any questions right on this part. Please don't just give me the answer; explain why you got the answer, thanks

Answers:Anhydrides, BO , are generally more acidic the further to the right (more electronegative) in the periodic table B is and the larger the number x is. If B is to the right of the Zintl line (the zig-zag line in the periodic table*), the anhydride is generally acidic. Thus, N O (~NO ) is the anhydride of a strong acid: N O + 2 H O 2 HNO ). N is electrongative and the oxidation state of N is +5 (and that is because x is the largest it can be for NO ). Likewise, SO is the anhydride of a strong acid: SO + H O H SO . S is fairly electronegative and the number if oxygens (x) is again as high as it can be. CO and SeO react with water to give weaker acids: H CO and H SeO (note that very little H CO actually forms, but what does form is acidic). C and Se are closer to the Zintl line and because x is only 2 instead of 3, SeO forms an acid that is weaker than that formed by SeO (+ H O H SeO a strong acid). Na O and CaO are basic anhydrides - Na and Ca are electropositive: Na O + H O 2NaOH(aq) ; CaO + H O Ca(OH) (aq) NaOH is a very strong base, Ca(OH) is fairly strong. Al O is on the borderline because Al borders the Zintl line. Al O is said to be amphoteric (reacts with strong acids like a base, reacts with strong bases like an acid) - not completely like an acid or a base.

From Youtube

ACID ANHYDRIDE ESTER :A few lessons on your basic acid anhydrides and esters =]