The use of an alcohol and hydrogen chloride

This is the classical Fischer-Speier procedure. Originally the acid was dissolved in the alcohol and boiled under reflux while anhydrous hydrogen chloride was bubbled 

A few milligrams of the acid are treated with 1ml of the alcohol containing 3 M HCl, and heated at 60 °C for methanol or at higher temperature for the higher alcohols. Reaction times for maximum yields need to be established experimentally. For methyl or ethyl estens, the alcohol is evaporated off, leaving the ester as residue. Addition of 5% by volume of benzene to the alcohol may help to entrain the water formed in the reaction. Alternatively, the reaction mixture is poured into 5 ml of water, the mixture is made alkaline with 3 M potassium bicarbonate and the esters are extracted into 3 x 1 ml portions of hexane. The hexane solution may be dried over anhydrous sodium sulphate or, where absorption losses on the desiccant are undesirable, concentrated into a small volume for analysis. 

One or two milligrams of the acid in a small vial are heated with 100 pi of the alcohol containing 3 M HCl 

Amino acid(s) at the milligram level are treated with 2 ml of 4 M methanolic HCl at 70 °C for two hours. The excess methanol is evaporated to dryness is a stream of nitrogen. Traces of water may be entrained from the dried residue by addition of 100 y of methylene chloride and re-evaporation. The residue consists of the hydrochlorides of the methyl esters of the amino acid(s) [27, 28]. 

Oxepinac in biological fluids was esterified in n-propanol by a classical procedure. The extract from the fluid was dissolved in 1 ml of n-propanol and saturated with gaseous HCl at 60 °C for an hour. After evaporation under reduced pressure, the residual esters were treated with 1 ml of 3% sodium bicarbonate and 5 ml of chloroform for 2 minutes. The chloroform layer was evaporated to dryness and the residue was dissolved in ZOOfil of n-propanol for analysis [29], 

---