Question

Which has higher
oxidising property Tollen's reagent or Fehiling solution?

Answer 1

In benzaldehyde, the carbonyl group is an electron withdrawing group so the carbonyl group pulls the electron from the electron-rich benzene ring. Due to this, the polarity of the C−HC−H bond in the carbonyl group is reduced because the C−HC−H bond now has a higher electron density. Thus, the C−HC−H bond becomes stronger (the lesser the polarity of a bond, the stronger the bond ). So Fehling's solution (comparatively a weaker oxidizing agent than Tollen's reagent) can't oxidize benzaldehyde (an aromatic aldehyde). However, Fehling's solution can oxidize an aliphatic aldehyde..

Answer 2

Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are alpha-hydroxy-ketones. ... A negative result is the absence of the red precipitate; it is important to note that Fehling's will not work with aromatic aldehydes; in this case Tollens' reagent should be used.