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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).
- CH3CO2CH3 + CO â†’ (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 45â€“55 Â°C and low pressure (0.05â€“0.2 bar).
- H2C=C=O + CH3COOH â†’ (CH3CO)2O (Î”H = âˆ’63 kJ/mol)
Ketene is generated by dehydrating acetic acid at 700â€“750 Â°C in the presence of triethyl phosphate as a catalyst or (in Switzerland and the CIS) by the thermolysis of acetone at 600â€“700 Â°C in the presence of carbon disulfide as a catalyst.
- CH3COOH H2C=C=O + H2O (Î”H = +147 kJ/mol)
- CH3COCH3â†’ H2C=C=O + CH4
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:
- (CH3CO)2O + CH3CH2OH â†’ CH3CO2CH2CH3 + CH3COOH
- (CH3CO)2O + C6H6â†’ CH3COC6H5 + CH3CO2H
Ferrocene may be acetylated too:
- Cp2Fe + (CH3CO)2O â†’ 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 â†’ 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
- Carbon dioxide which reacts with water to produce carbonic acid.
- Sulfur dioxide, which does not form the non-existent sulfurous acid but does react with bases to form sulfites.
- Silicon dioxide, which does not react with water but will react with bases to form silicates
- Chromium trioxide, which reacts with water to form chromic acid. Chromic acid is a hypothetical acid. The monomer and dimer would respectively have structures similar to sulfuric and disulfuric acids.
From Yahoo Answers
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...
Answers:The answer is C. Examples include CO2, SO3, NO2, and P2O5. These will all make acids when they react with water.
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.
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.