<unk>  is a special type of<unk>  and the process of forming an ester by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. Most carboxylic acids are suitable for the reaction, but the alcohol should generally be a primary or secondary alkyl. Tertiary alcohols are prone to elimination, and phenols are usually too unreactive to give useful yields. Commonly used catalysts for a<unk>  include  sulfuric acid, tosic acid, and lewis acids such as scandium(III) triflate. For more valuable or sensitive substrates (for example, biomaterials), dicyclohexylcarbodiimide is often used. The reaction is often carried out without a solvent (particularly when a large reagent excess e.g. of MeOH is used) or in a non-polar solvent (e.g. toluene) to facilitate the Dean-Starck method. Typical reaction times vary from 1-10 hours at temperatures of 60-110°C.
Direct acylations of alcohols with carboxylic acids is preferred over acylations with anhydrides (poor atom economy) or acid chlorides (moisture sensitive). The main disadvantage of direct acylation is the unfavorable chemical equilibrium that must be remedied e.g. by a large excess of one of the reagents, or by the removal of water (for example by Dean-Stark distillation, the use of molecular sieves, or the use of a stoichiometric quantity of concentrated sulfuric acid as the catalyst). In one study  it is found that Tetrabutylammonium tribromide (TBATB) is a very effective catalyst. For example the acylation of 3-phenyl propanol with glacial acetic acid and TBATB at reflux generates the ester in 15 minutes in a 95% chemical yield without the need to remove water. It is believed that hydrobromic acid released by TBATB protonates the alcohol over the acid making the carboxylate the actual nucleophile in a reversal of the standard<unk>  mechanism.
Fischer esterification
Edman degradation
Glucuronidation
Hydroxylation
Carboxylation
Bergmann degradation
Palmitoylation
Alkyne metathesis