Question

What is the test shown by aldehydes with Fehling's solution?

Answer 1

Fehling's solution:

• Fehling's solution comprises a mixture of two separate solutions.
• Fehling's solution A is the first solution which is Copper sulphate(${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$) and is blue in colour.
• Fehling's solution B is a colourless solution consisting of Potassium Sodium tartrate or Rochelle salt (${\mathrm{KNaC}}_4{\mathrm{H}}_4{\mathrm{O}}_6.4{\mathrm{H}}_2\mathrm{O}$) and a strong alkali like Sodium hydroxide($\mathrm{NaOH}$).
• Fehling's solution is used to differentiate between Aldehyde and Ketone Functional groups as well as a generic test for the monosaccharides.

Test of Aldehydes using Fehling's solution:

• The presence of Aldehydes($\mathrm{R}-\mathrm{CHO}$) in a solution can be tested using Fehling's solution.
• Oxidation of Aldehydes takes place and its presence is confirmed by the formation of red-colored Cuprous oxide(${\mathrm{Cu}}_2\mathrm{O}$).
• If the color of the solution does not change from its blue color, it indicates the absence of the Aldehyde group.

Chemical Reactions involved:

The above phenomena can be represented by the following chemical reactions:

$\mathrm{RCHO}\left(\mathrm{aq}\right)+2{\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+5{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{RCOO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

(Aldehyde) (Carboxylic acid) (Red Cuprous oxide)

$\mathrm{RCHO}\left(\mathrm{aq}\right)+2\mathrm{Cu}{{\left({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{O}}_6\right)}_2}^{2-}\left(\mathrm{aq}\right)+5{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{RCOO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+4{\mathrm{C}}_4{\mathrm{H}}_4{{\mathrm{O}}_6}^{2-}\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

(Copper tartrate complex) (Tartrate)