<P> A generic mechanism for an acid Fischer esterification is shown below . </P> <P> The primary advantages of Fischer esterification compared to other esterification processes are based on its relative simplicity . Straightforward acidic conditions can be used if acid - sensitive functional groups are not an issue; sulfuric acid can be used; softer acids can be used with a tradeoff of longer reaction times . Because the reagents used are "direct," there is less environmental impact in terms of waste products and harmfulness of the reagents . Alkyl halides are potential greenhouse gases or ozone depletors and possible ecological poisons . Acid chlorides evolve hydrogen chloride gas upon contact with atmospheric moisture, are corrosive, react vigorously with water and other nucleophiles (sometimes dangerously); they are easily quenched by other nucleophiles besides the desired alcohol; their most common synthesis routes involve the evolution of toxic carbon monoxide or sulfur dioxide gases (depending on the synthesis process used). </P> <P> Acid anhydrides are more reactive than esters because the leaving group is a carboxylate anion--a better leaving group than an alkoxide anion because their negative charge is more delocalised . However, such routes generally result in poor atom economy . For example, in reacting ethanol with acetic anhydride, ethyl acetate forms and acetic acid is kicked out as a leaving group, which is considerably less reactive than an acid anhydride and will be left as a byproduct (in a wasteful 1: 1 ratio with the ester product) if product is collected immediately . If conditions are acidic enough, the acetic acid can be further reacted via the Fischer esterification pathway, but at a much slower pace . However, in many carefully designed syntheses, reagents can be designed such that acid anhydrides are generated in situ and carboxylic acid byproducts are reactivated, and Fischer esterification routes are not necessarily mutually exclusive with acetic anhydride routes . (Examples of this include the common undergraduate organic lab experiment involving the acetylation of salicylic acid to yield aspirin .) </P> <P> Fischer esterification is primarily a thermodynamically - controlled process: because of its slowness, the most stable ester tends to be the major product . This can be a desirable trait if there are multiple reaction sites and side product esters to be avoided . In contrast, rapid reactions involving acid anhydrides or acid chlorides are often kinetically - controlled . </P>

The leaving group in the fischer esterification is
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