Question

In: Chemistry

Which reagent(s) may be used to distinguish between each of the following pairs of compounds? Select...

Which reagent(s) may be used to distinguish between each of the following pairs of compounds? Select a test that gives positive results with one compound of the pair and negative results with the other compound.Explain what will be observed in each case.

(a) 1-butanol and butanal

(b) toluene and acetophenone

Solutions

Expert Solution

(a) Use Tollens reagent

The reagent consists of a solution of silver nitrate and ammonia. A positive test on butanal with Tollens reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel. The diamminesilver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia.

The ionic equation of the overall reaction is shown below where R refers to an alkyl group.

2 [Ag(NH3)2]+ + RCHO + H2O → 2 Ag(s) + 4 NH3 + RCO2H + 2 H+

  (Silver mirror)

(b) Use DNPH, Brady's reagent

2,4-Dinitrophenylhydrazine (DNPH) is a red to orange solid with chemical formula C6H3(NO2)2NHNH2. It can be used to qualitatively detect the carbonyl functionality of a ketone or an aldehyde functional group. A positive test on acetophenone is indicated by a red precipitate (known as a dinitrophenylhydrazone.).

The reaction between 2,4-Dinitrophenylhydrazine and a ketone is shown below:

RR'C=O + C6H3(NO2)2NHNH2 → C6H3(NO2)2NHNCRR' + H2O

(Red ppt.)

queen_honey_blossom answered 2 years ago
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