Question

An unknown compound D is first oxidised to an aldehyde and then to acetic acid by a dilute solution of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\mathrm{and}{\mathrm{H}}_2{\mathrm{SO}}_4$. The unknown compound 'D' is:

• A. ${\mathrm{CH}}_3\mathrm{CHO}$
• B. ${\mathrm{CH}}_2{\mathrm{CH}}_3\mathrm{OH}$
• C. ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$
• D. ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_3$

Answer 1

The correct option is C

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$

Explanation for correct option:

(C) ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$

• The oxidation of primary alcohol gives aldehyde which on further oxidation gives acid.
• When secondary alcohol on oxidation gives ketone.
• The unknown compound D is ethanol which on oxidation gives aldehyde which on further oxidation gives acetic acid.
• The oxidizing agent used here is employed for oxidation is dilute potassium dichromate with sulphurc acid when concentrated is used the alcohol is directly oxidized to acid.
• $$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\left(\mathrm{l}\right)\underset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\overset{\mathrm{dil}.{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7}{\to }}{\mathrm{CH}}_3\mathrm{CHO}\left(\mathrm{aq}\right)\underset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\overset{\mathrm{dil}.{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7}{\to }}{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right) \\ \left(\mathrm{Ethanol}\right)\left(\mathrm{Acetaldehye}\right)(\mathrm{Acetic}\mathrm{acid}) \end{gathered}$$

Explanation for incorrect option:

(A) ${\mathrm{CH}}_3\mathrm{CHO}$

• Aldehyde on oxidation does not give aldehyde. So compound D cannot be this. This is an incorrect option.

(B) ${\mathrm{CH}}_2{\mathrm{CH}}_3\mathrm{OH}$

• The carbon valences are not balanced well. This, compound cannot exist. So this is an incorrect option.

(D)${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_3$

• Alkane could not give aldehyde on oxidation. So, this is an incorrect option.

Hence, the correct option is (C) ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$