Experiment 3
Identification of a Carbonyl Compound by Preparing its Derivative
Introduction
Crystalline derivatives of many carbonyl compounds can be formed by condensation reactions with compounds such as
phenylhydrazine and 2,4dinitrophenylhydrazine. These derivatives can usually be isolated in relatively pure forms which have
well defined melting points. Phenylhydrazine forms derivatives (phenylhydrazones) readily with aromatic aldehydes, but in general
2,4dinitrophenylhydrazine is preferred because its derivatives (2,4dinitrophenyihydrazones) have higher melting points and are
less soluble. These derivatives are useful in identification of carbonyl compounds.
Chemicals
2,4dinitrophenylhydrazine in methanol, ethanol, methanol, carbonyl compound labelled X (different groups of students may work
on different carbonyl compounds), dilute H2S04
Apparatus
Beaker, 100 cm3 measuring cylinder, apparatus for melting point determination, apparatus for suction filtration, rubber rings (cut
from a rubber tubing of appropriate diameter), ice bath
Procedure
[Hazard Warning: Methanol solution of 2,4dinitrophenylhydrazine is flammable and toxic, methanol, ethanol and many
carbonyl compounds are flammable, and bench dilute sulphuric(V1) acid is corrosive.]
A. Preparation of 2,4dinitrophenylhydrazone of compound X
1. Add 10 drops of compound X to 5 cm3 of 2,4dinitrophenyihydrazine solution in a 50 cm3 beaker. If crystals are not formed,
add about 1 cm3 of dilute sulphuric(V1) acid. If they are still not formed, warm the mixture gently, then cool with scratching
in ice water.
2. Filter off the crystals by suction filtration. While still on suction, wash the crystals with 1 cm3 of methanol.
3. Recrystallize the crystals from hot ethanol as follows:
(a) Transfer the crystals to a 100 cm3 beaker standing on a steam bath (or in a 250 cm3 beaker of hot water).
(b) Dissolve the crystals in the minimum amount of hot ethanol.
(c) When the crystals have dissolved, cool the solution in an ice-water mixture until crystals reappear.
(d) Filter the crystals by suction. If necessary, rinse the beaker with the filtrate (not the extra solvent) to complete the transfer.
Finally, wash the crystals with a few drops of cold ethanol. Dry the crystals by drawing air through them for a few minutes.
4. Spread the crystals on a dry watch glass and leave overnight in a desiccator for drying.
B. Determination of the melting point of the 2,4dinitrophenylhydrazone of compound X
1. Introduce a small amount of the crystals into a melting point tube until a total length of about 0.5 cm is compacted at the
bottom of the tube.
2. Attach the prepared melting point tube to the thermometer, as shown in Figure 1.
3. Half-fill a boiling tube with paraffin oil, and position the thermometer with attached tube and the stirrer as shown in Figure 2.
4. Position the apparatus over a low Bunsen flame and gauze and gently heat the apparatus, stirring the paraffin oil all the time
by moving the stirrer up and down.
5. Note the temperatures when the crystals start to melt and when the melting is completed.
Figure 1
Figure 2
Carbonyl derivative / page 1
6. Compare the melting point of the crystals with the values given in the following table and thus identify compound X.
Name
Formula
Melting point of
Boiling point/C
2,4-dinitrophenylhydrazone/C
Aldehydes
methanal
HCHO
167
21
ethanal
CH3CHO
146, 164 (2 forms)
21
propanal
CH3CH2CHO
156
48
butanal
CH3CH2CH2CHO
123
75
(CH3) 2CHCHO
187
2methylpropanal
64
C6H5CHO
237
benzaldehyde
178
Ketones
propanone
CH3COCH3
56
128
butanone
CH3CH2COCH3
80
115
pentan-2-one
CH3CH2CH2COCH3
102
141
pentan-3-one
CH3CH2COCH2CH3
102
156
hexan-2-one
CH3CH2CH2CH2CO2CH3
128
107
(CH
)
CHCH
COCH
117
95
4-methylpentan2one
3 2
2
3
cyclohexanone
156
162
Discussion
1. What soluble impurities may be present in your product before recrystallization? How can they be removed in the
recrystallization process?
2. What factors should be considered in selecting a suitable solvent in the recrystallization step?
3. In the recrystallization procedure, why were the crystals dissolved in only the minimum amount of hot ethanol?
4. If the sample contains insoluble impurities such as pieces of filter paper, cork, etc., suggest how they can be removed.
5. If the melting point of the 2,4dinitrophenylhydrazone is 156C, suggest how you can confirm whether compound X is propanal
or pentan3one.
Carbonyl derivative / page 2