Test for Aldehydes

Four qualitative tests are used to identify an aldehyde, three of them (Tollens', Fehling's, Benedict's) exploiting the aldehyde's own easy oxidisability (Section 12.5.B.1) by pairing it with a mild metal-ion oxidant that gets visibly reduced, and the fourth (Schiff's) a colour-restoration test specific to the aldehyde functional group. (i) TOLLENS' REAGENT TEST: Tollens' reagent is an ammoniacal silver nitrate solution; when an aldehyde is warmed with it, the aldehyde reduces Ag+ to metallic silver, depositing a bright silver mirror on the inside of the test tube -- hence the alternative name, the silver mirror test. Example: CH3CHO + 2 [Ag(NH3)2]+ + 3 OH- gives CH3COO- + 4 NH3 + 2 Ag (the mirror) + 2 H2O. (ii) FEHLING'S SOLUTION TEST: Fehling's solution is prepared fresh by mixing equal volumes of Fehling's A (aqueous copper sulphate) and Fehling's B (an alkaline solution of sodium potassium tartrate, i.e. Rochelle salt, which complexes and stabilises the Cu2+ in alkaline solution); when an aldehyde is warmed with it, the deep-blue Cu2+ solution is reduced and changes to a brick-red precipitate of cuprous oxide, Cu2O. (iii) BENEDICT'S SOLUTION TEST: Benedict's solution is a related, single-bottle mixture of CuSO4, sodium citrate and NaOH (citrate playing the role tartrate plays in Fehling's), and gives the identical blue-to-red-precipitate colour change with an aldehyde, via the same Cu2+-to-Cu2O reduction. (iv) SCHIFF'S REAGENT TEST: Schiff's reagent is rosaniline hydrochloride dissolved in water, whose characteristic red/magenta colour has been decolourised by passing SO2 gas through it; a dilute solution …