Question

Which alcohol can be oxidized by potassium dichromate but cannot be dehydrated?

Answer 1

Dehydration of Alcohols

1. Dehydration is the chemical reaction where the removal of water molecules takes place.
2. Dehydration of alcohol forms alkenes when treated with protic acids like ${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{H}}_3{\mathrm{PO}}_4$ or catalysts like anhydrous Zinc chloride and Alumina.
3. Dehydration of Ethanol takes place at a very high temperature with Conc.${\mathrm{H}}_2{\mathrm{SO}}_4$ form Ethene.
4. $$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\left(\mathrm{aq}\right)\underset{443\mathrm{K}}{\overset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2\left(\mathrm{g}\right) \\ \mathrm{Ethanol}\mathrm{Ethene} \end{gathered}$$
5. The general mechanism of dehydration of ethanol is shown below
6. The order of reactivity of alcohols towards dehydration is $3°>2°>1°$.
7. As ethanol is the primary alcohol so it is the least reactive to dehydration.

Oxidation of Alcohol

1. Oxidation of alcohol leads to dehydrogenation which is the removal of dihydrogen$\left({\mathrm{H}}_2\right)$.
2. During oxidation, the cleavage of hydrogen bonds and formation of double bonds between carbon and oxygen.
3. Ethanol when treated with strong oxidizing agents like potassium permanganate $\left({\mathrm{KMnO}}_4\right)$and potassium dichromate$\left({\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\right)$ form ethanal and further undergoes oxidation leading to the formation of carboxylic acid which is ethanoic acid.

1. The reactivity order of alcohols towards oxidation is $1°>2°>3°$.
2. As the primary alcohols are more reactive and ethanol is the primary alcohol so it can undergo oxidation.
3. Therefore, ethanol can be oxidized by potassium dichromate but cannot be dehydrated.