Question

When liquid ‘A’ is treated with a freshly prepared ammoniacal silver nitrate solution, it gives bright silver mirror. The liquid forms a white crystalline solid on treatment with sodium hydrogensulphite. Liquid ‘B’ also forms a white crystalline solid with sodium hydrogensulphite but it does not give test with ammoniacal silver nitrate. Which of the two liquids is aldehyde? Write the chemical equations of these reactions also.

Answer 1

Liquid ‘A’ forms a bright silver mirror on treatment with ammoniacal \[\ce{AgNO3}\] solution, therefore, liquid ‘A’ is an aldehyde.

Liquid B does not give test with ammoniacal \[\ce{AgNO3}\] solution, therefore, liquid ‘B’ must be a methyl ketone.

Chemical equations for these reactions are:

\[\begin{array}{cc}
\phantom{........}\ce{R}\phantom{......................}\ce{R}\phantom{.......}\ce{SO3Na}\phantom{}\\
\phantom{...}\backslash\phantom{......................}\backslash\phantom{...}\phantom{..}/\\\
\phantom{}\ce{C = O + NaHSO3 -> \phantom{..}C}\\
\phantom{.............}/\phantom{......................}/\phantom{.....}\backslash\phantom{..........}\\
\phantom{.....}\ce{\underset{Aldehyde(A)}{H}\phantom{................}\underset{(White crystalline solid)}{\phantom{.}H\phantom{.......}OH}}\phantom{..}
\end{array}\]

\[\begin{array}{cc}
\phantom{........}\ce{R}\phantom{......................}\ce{R}\phantom{.......}\ce{SO3Na}\phantom{}\\
\phantom{...}\backslash\phantom{......................}\backslash\phantom{...}\phantom{..}/\\\
\phantom{}\ce{C = O + NaHSO3 -> \phantom{..}C}\\
\phantom{.............}/\phantom{......................}/\phantom{.....}\backslash\phantom{..........}\\
\phantom{....}\ce{\underset{Methyl ketone (B)}{H3C}\phantom{.............}\underset{(White NaHSO3 adduct)}{\phantom{.}H3C\phantom{.......}OH}}\phantom{.....}
\end{array}\]

\[\ce{\underset{Aldehyde (A)}{R - CHO} + \underset{Tollens reagent}{2[Ag(NH3)2]+} OH- ->[Warm] RCOONH4 + \underset{(Silver mirror)}{2Ag + 3}NH3 + H2O}\]

\[\begin{array}{cc}
\ce{O}\phantom{...............................}\\
||\phantom{...............................}\\
\ce{\underset{Methyl ketone (B)}{R - C - CH3} ->[Tollens reagent] No silver mirror}
\end{array}\]