Ethanoate esters, formation

The opposite process to ester formation is the formation of an acid and an alcohol from the ester by hydrolysis (i.e. the action of water) in the presence of either an alkali or acid (eg. the hydrolysis of ethyl ethanoate in Figure 7.1.13). 

Ester R—c f OR OR HCOOCH3 CH3COOCH3 methyl methanoate methyl ethanoate methyl formate methyl acetate... 

The names of acyl compounds are based on the parent carboxylic acid. The lUPAC systematic naming protocol for esters (R—COOR ) has the R group named as the carboxylic acid and the ic acidis replaced with the suffix ate. So the root word becomes alkanoate. The R group is named as the appropriate alkyl group and it is written as a separate word in front of the alkanoate. Therefore, the generic ester is alkyl alkanoate. The carbonyl carbon of the ester gets top priority in the naming scheme and substituents are numbered with respect to the carbonyl carbon as position 1 (Rg. 18.3). The common names for methanoate and ethanoate are formate and acetate, respectively, so we will frequently encounter alkyl (methyl, ethyl, etc.) formates and acetates. 

The reactions of 2-hydroxy-2-(3-indolyl)ethanoamides and of ethyl 2-hydroxy-2-(2-pyrrolyl) ethanoates with trialkyl ortho-esters follow similar routes with the initial formation of the enol ethers (295) and (297), followed by an ortho- Claisen rearrangement to give the 2,3-disubstituted indoles (296) and pyrroles (298) (79JOC1885,80TL4335). 

Formation of esters using ethanol and higher alcohols. Mix 1 cm3 of ethanol and 1 cm3 of glacial ethanoic acid. Add 2 drops of concentrated sulfuric acid and warm the mixture for a few minutes. Then pour the mixture into a beaker of cold water. The distinctive smell of an ester can be detected. Repeat the procedure using higher alcohols - propanol, butanol and pentanol. 

The equation for the formation of an ester by the reaction of an alcohol with a carboxylic acid (e.g. ethanoic acid), in the presence of some concentrated sulfuric acid to provide hydrogen ions as a catalyst, is ... 

Esters are mildly polar and have pleasant aromas. Many esters are found in natural foodstuffs banana oil (3-methylbutyl ethanoate common name, isoamyl acetate), pineapples (ethyl butanoate common name, ethyl butyrate), and raspberries (isobutyl methanoate common name, isobutyl formate) are but a few... 

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. 

More generally, reduction of cyclic anhydrides such as phthalic anhydride (benzene-1,2-dicarboxylic acid anhydride) with zinc (Zn) in ethanoic acid (acetic acid, CH3CO2H) results in the formation of cyclic esters (lactones) (Scheme 9.114). As shown, the zinc (Zn) metal is oxidized (to zinc oxide [Zn02]), while the... 

Scheme 10.9. A representation of the Polonovski reaction in which the A-oxide of A-methyl-A-phenylaniline is treated with ethanoic anhydride (acetic anhydride) and rearrangement occurs with migration of oxygen from N C and formation of the acetate ester of N,N-diphentylaminomethanol. The aminoester, on hydrolysis, produces acetate, methanal (formaldehyde) and iV-phenylaniline. See Lounasmaa, M. Koskinen, A. Heterocycles, 1984,22,1591 and Grierson, D. The Polonovski Reaction, in Paquette, L. A. (ed.). Organic Reactions, Vol. 39, Wiley, Hoboken, NJ, 1990.

From Figure 10.32 we can see how the structure of an ester is huilt up. However, it is more conventional to draw the condensed or full structural formulas of esters the other way round. Thus, ethyl ethanoate is usually written CH3COOC2H5 in chemical equations, for instance. Figure 10.33 shows the structures of some esters (propyl methanoate, methyl ethanoate and ethyl propanoate) written or drawn in this more usual format. 

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