Question

What is the action of the following reagents on ethanol?
(a) Lucas reagent
(b) Phosphorus pentachloride
(c) Red phosphorus and bromine
(d) Acidified potassium dichromate.

Answer 1

(a) The Lucas reagent is an aqueous solution of strong acid (HCl) and zinc chloride $\left(ZnC{l}_2\right)$. Primary alcohols such as ethanol react with Lucas reagent very slowly on heating to give alkyl halide i.e., chloro-ethane.
$\underset{Ethanol}{C{H}_{3}C{H}_{2}OH}+\underset{Conc.}{HCl}\stackrel{AnhydrousZnC{l}_2}{\to }\underset{Chloroethane}{C{H}_{3}C{H}_{2}Cl}+H_{2}O$.
(b) Ethanol when refluxed with $PC{l}_5$ forms ethyl chloride.
$\underset{Ethanol}{C{H}_{3}C{H}_{2}OH}+\underset{\underset{pentachloride}{Phosphorus}}{PC{l}_5}\to \underset{Ethylchloride}{C{H}_{3}C{H}_{2}Cl}+HCl+\underset{\underset{oxychloride}{Phosphorus}}{POC{l}_3}$
(c) Alkyl bromides are prepared by the action of bromine in presence of red phosphorus on alcohols in situ. In this reaction phosphorus tribromide is formed as an intermediate which in turn reacts with ethanol and forms corresponding alkyl bromide, i.e., ethyl bromide.
$2P+3B{r}_2\to \underset{\underset{tribromide}{Phosphorus}}{2PB{r}_3}$
$\underset{Ethanol}{3C{H}_{3}C{H}_{2}OH}+PB{r}_3\to \underset{Ethylbromide}{3C{H}_{3}C{H}_{2}Br}+H_{3}P{O}_3$
(d) Ethanol on reaction with acidified potassium dichromate gets oxidised to corresponding aldehyde which on further oxidation gives carboxylic acid.
$\underset{Ethanol}{C{H}_{3}C{H}_{2}OH}\stackrel{K_{2}C{r}_2{O}_7}{\underset{dil.H_{2}SO4}{\to }}\underset{Ethanal}{C{H}_3\stackrel{\stackrel{O}{||}}{C}-H}\stackrel{\left[O\right]}{\to }\underset{Ethanoicacid}{C{H}_{3}COOH}$.