Question

What is the product of controlled oxidation of methanol?
A) Methanoic acid
B) Methanal
C) Carbon dioxide and water
D) methane

Answer 1

Hint: To solve such problems,we should have proper knowledge about oxidation and alcohols and what is formed from alcohols after oxidation. Primary alcohol can be oxidized to aldehyde which is done by reagent Collin's reagent and aldehyde can be further oxidized to carboxylic acid.


Complete solution:
Remember, oxidation is a process involving the gain of oxygen, loss of hydrogen or loss of electrons. Oxidation of alcohols is oxidation in terms of hydrogen transfer. The alcohol is oxidised by loss of hydrogen. Oxidation and reduction in terms of hydrogen transfer is common in hydrocarbon chemistry.
Alcohols can be oxidised by a variety of oxidising agents. Sodium or potassium dichromate acidified with dilute sulphuric acid can bring about oxidation in straight chained alcohols. Straight chained alcohols with one alkyl group or primary alcohols as they are referred to can be oxidised to form aldehydes.
Also remember that the oxidation of the alcohol to an aldehyde is indicated by the colour change of the dichromate solution as it is reduced from the orange colour of chromate ion to the green of chromium(III) ions. In this reaction the methanol is oxidised to methanal by removing two hydrogen atoms. The aldehyde needs to be removed from the reaction mixture as soon as possible otherwise the resulting aldehyde can undergo further oxidation to a carboxylic acid. This is done by using an excess of alcohol and distilling off the aldehyde as soon as it forms.
 Also remember that Methanol can also be oxidised by passing a mixture of methanol vapour and air over a silver catalyst at 500 degree. Producing methanol through this process is important in industry as methanal is used in the production of plastics.
Hence we can conclude the answer to shorter words with r=the reaction:
Methanol is a primary alcohol ,which on controlled oxidation gives methanol as:
$C{H}_{3}OH+\left[O\right]\underset{dil{H}_{2}S{O}_4}{\overset{K_{2}C{r}_2{O}_7}{\to }}C{H}_{3}CHO$

Thus, the correct option is (B) .


Note: Methanol is oxidised by sodium or potassium dichromate acidified in dilute sulphuric acid to form the aldehyde methanal. If however, an excess of the oxidising agent is added and the reaction is allowed to continue with no removal of the aldehyde further oxidation to a carboxylic acid takes place.