Identification of carbonyl compounds by preparing their derivatives

Authors Avatar

S.K.H. Lam Woo Memorial Secondary School

F.7 Chemistry Laboratory Report

Name: Chan Ching Wai     Class: F.7H (2)

Experiment 3: Identification of carbonyl compounds by preparing their derivatives

Date of Experiment: 15-10-2010


To identify an unknown carbonyl compound by determining the melting point of it’s derivative formed from the reaction with 2,4-dinitrophenylhydrazine solution.


In the experiment, carbonyl compound underwent condensation reaction with 2,4-dinitrophenylhydrazine solution to form 2,4-dinitrophenylhydrazone. The solid formed was then purified by recrystallization in order to give an accurate, distinct melting point. As the 2,4-dinitrophenylhydrazone formed from different carbonyl compound would have different boiling points, by matching the measured boiling point with that stated officially, the type of carbonyl compound could be identified.

Carbonyl compound contained a C=O functional group which was polar. The electron cloud was displaced to the more electronegative oxygen atom and made the carbonyl carbon partial positive. The carbonyl carbon was hence electron deficient and susceptible to attack by electron rich nucleophile.

Carbonyl compound could undergo condensation with derivative of ammonia. In this case, carbonyl compound could serve as electrophiles and nucleophile at the same time. Water was eliminated as a by-product and 2,4-dinitrophenylhydrazone was produced.

Equation of reaction:

Mechanism of condensation


Melting point of crystal = 1560C

Melting point of propanal = 1560C

Melting point of pentan-3-one= 1560C

As the melting point of the crystal matched the 2,4-dinitrophenylhydrazone formed from propanal and pentan-3-one, thus the carbonyl compound was a propanal or pentan-3-one.

To determine whether the carbonyl compound was a propanal or pentan-3-one, the carbonyl compound

Join now!

should be tested before the addition of 2,4-dinitrophenylhydrazine.

It could be tested by Fehling’s solution and red precipitate would be formed for propanal (an aldehyde)

but no precipitate would be formed for pentan-3-one (ketone).

CH3CH2COH + Cu2+ + OH-   CH3CH2COO- + Cu2O +3H2O

It could also be tested by Tollen’s reagent. Silver mirror would be formed for propanal but

not for pentan-3-one.

CH3CH2COH + Ag(NH3) 2OH   CH3CH2COO- NH4+ + Ag + NH3 + H2O

Testing with acidified potassium dichromate solution was also possible. Propanal would reduced the

solution to change from orange to green but pentan-3-one would not. ...

This is a preview of the whole essay