Ethanoic propanoic anhydride

Ethanoic propanoic anhydride (acetic propionic anhydride)... 

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Acid anhydrides also react with alcohols, in the presence of amine bases, to give esters. The reaction is essentially the same as that for acid chlorides, and triethylamine is commonly added as a base. This is most useful when symmetrical anhydrides such as acetic anhydride are used. Heating acetic anhydride and isopropanol to reflux gives an 80% yield of isopropyl acetate (19 in Section 20.2). When fert-butyl alcohol 59 is heated with acetic anhydride, ester 62 is formed, along with acetic acid. Mixed anhydrides are expected to give a mixture of all possible ester products, so this reaction is done most often with symmetrical anhydrides such as acetic (ethanoic) anhydride or propanoic anhydride. When acetic anhydride reacts with methanol, for example, the products are methyl acetate and acetic acid. Formation of esters by reaction of an alcohol and an anhydride will be used only when a symmetrical anhydride can be used as a starting material. 

Acid anhydrides have structure 58 and are named by recognizing that the structure consists of two carboxylic acid units. The lUPAC rules demand that the two acids be named sequentially, followed by the word anhydride. For anhydride 63 in Table 16.3, one component is derived from butanoic acid and the other from propanoic acid. The names of the acid are listed alphabetically, so the name of 63 is butanoicpropanoic anhydride. For a general rule of naming, list R and R in 63 alphabetically, followed by the word anhydride). Anhydride 63 is an unsymmetrical anhydride because the two acid components are different. If both acid components are identical, it is a symmetrical anhydride such as 64. Using both acid names for this compound leads to diethanoic anhydride, or simply ethanoic anhydride When the single name is used, the anhydride is understood to be symmetrical. Anhydride 64 is derived from ethanoic acid, which has the common name acetic acid, so the common name acetic anhydride is often used for 65. 

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