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.)
Fatty acid metabolism Acetyl-CoA is formed into malonyl-CoA by acetyl-CoA carboxylase, at which point malonyl-CoA is destined to feed into the fatty acid synthesis pathway. Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, and is subject to both phosphorylation and allosteric regulation. Regulation by phosphorylation occurs mostly in mammals, while allosteric regulation occurs in most organisms. Allosteric control occurs as feedback inhibition by palmitoyl-CoA and activation by citrate. When there are high levels of palmitoyl-CoA, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-CoA carboxylase to prevent a build-up of fatty acids in cells. Citrate acts to activate acetyl-CoA carboxylase under high levels, because high levels indicate that there is enough acetyl-CoA to feed into the Krebs cycle and produce energy.[40]