the leaving group in the fischer esterification is
Fischer–Speier esterification Acid anhydrides are more reactive than esters because the leaving group is a carboxylate anionâ€”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.)
Metabolism The most common set of catabolic reactions in animals can be separated into three main stages. In the first stage, large organic molecules, such as proteins, polysaccharides or lipids, are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Finally, the acetyl group on the CoA is oxidised to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.