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.
Although, the above test was not carried out in this experiment, the type of carbonyl compound could also be guessed from the amount of 2,4-dinitrophenylhydrazine. Groups using carbonyl compound A obtained similar melting point with that of this carbonyl compound. However, this carbonyl compound required much more 2,4-dinitrophenylhydrazine to react and give crystal than carbonyl compound A. Aldehyde was more reactive than ketone. Because of positive inductive effect, the alkyl groups in pentan-3-one donate electrons to carbonyl carbon and made it less positive. Hence, it was less susceptible to nucleophilic attack. Besides, as the alkyl groups were more bulky than hydrogen atom. The higher steric effect hindered the close approach of nucleophile to the carbonyl compound. As pentan-3-one was less reactive, more 2,4-dinitrophenylhydrazine would be needed to react with it in order to give similar amount of crystal. Therefore, it was believed that there was a larger chance of the carbonyl compound to be pentan-3-one.
It should also be noted that the carbonyl compound could not be identified by matching with the melting point of different carbonyl compound. It was because the melting point of carbonyl compounds usually close to each other. The identification would be inaccurate as error may presented to change the melting point slightly and this slight difference may showed another compounds. As the melting point of 2,4-dinitrophenylhydrazone of carbonyl compounds were larger, it could be easier to identify particular compound.
By reacting carbonyl compound with hydroxylamine, oxime could be produced. The melting point of oxime could also be matched with melting point of oxime of different carbonyl compound. However, as oxime usually had lower melting and more soluble in the solvents. Reaction with 2,4-dinitrophenylhydrazone to form 2,4-dinitrophenylhydrazine was preferred.
Removal of impurities
The solid formed after reaction of carbonyl compound with 2,4-dinitrophenylhydrazine was not pure.
The impurities presented in the rough product would be mainly 2,4-dinitrophenylhydrazine. Excess 2,4-dinitrophenylhydrazine was added in order to consume all the carbonyl compound present for a larger yield. The remaining 2,4-dinitrophenylhydrazine would also be precipitated out after warming and cooling in hot and cold water bath respectively. This solid appeared to be the same as 2,4-dinitrophenylhydrazone. Some of the carbonyl compound or other impurities that originally present in the solution would also precipitate out. These precipitates mixed with the 2,4-dinitrophenylhydrazone and needed further process to remove them.
Then, the solid was dissolved in ethanol. If any insoluble impurities like pieces of filter paper were presented, they could be filtered out by funnel. Recrystallization then took place to purify the product. Only the compound which was soluble in hot ethanol and less soluble in cold ethanol (mainly2,4-dinitrophenylhydrazone) would be precipitated out as crystal. The compounds that were soluble in cold ethanol remained dissolved in it. Then, the solution was filtered to obtain the crystal. The crystal was washed by cold ethanol to remove the soluble compound in recrystallization that stick on the surface of the crystal. Finally, pure 2,4-dinitrophenylhydrazone was obtained and it’s melting point was determined. If the determination of melting point was carried out immediately without purification, an indistinct and inaccurate melting point would be obtained by this mixture of product.
Solvent used in recrystallization
The solvent used in recrystallization must not react with the 2,4-dinitrophenylhydrazone. Moreover, the solubility of 2,4-dinitrophenylhydrazone in this solvent must be high at high temperature and low at room temperature. Therefore, an appropriate choice would be ethanol. However, only small amount of ethanol should be used to dissolve and washed the solid. If too much ethanol was added to dissolve the solid, the time for recrystallization would be very long. It was because the solubility of a larger volume of solution would be larger than that of a smaller volume. If too much ethanol was added, longer time was needed for enough solvent to evaporate in order to give a concentrated solution. Only for a concentrated solution with saturated amount of solute could the solute start to precipitate. Added to the above, if too much ethanol was added, the solubility of the solution increased and very fine crystal would be formed.
On the other hand, if too much ethanol was used to washed the crystal, the 2,4-dinitrophenylhydrazone may be dissolved back in ethanol again and less 2,4-dinitrophenylhydrazone could be obtained.
Errors and improvements
As errors occurred in this experiment, leading to a very small yield of 2,4-dinitrophenylhydrazone.
Firstly, excess 2,4-dinitrophenylhydrazine should be added to the carbonyl compound to ensure all of the carbonyl compound was reacted. Otherwise, only a small amount of 2,4-dinitrophenylhydrazone would be produced. Secondly, the 2,4-dinitrophenylhydrazine should be mixed thoroughly with the carbonyl compound to increase contact between these two molecules and to allow reaction between them to take place. Thus, shaking was required until the solutions completely mixed together without seeing a clear boundary between two layers. Otherwise, the yield of 2,4-dinitrophenylhydrazone produced would be small again.
Errors also incurred in the recrystallization. If the solution was not warmed long enough, it may be too dilute for precipitation. Also, if the solution was not cooled enough, it was difficult to be saturated. The 2,4-dinitrophenylhydrazone still had a high solubility in this dilute, warm solution and little of them would precipitate out as crystal. To speed up the reaction and speed up the formation of precipitate in the condensation reaction, dilute sulphuric acid could be added as a catalyst.
Last but not least, as the yield of 2,4-dinitrophenylhydrazone was very small, the percentage loss of crystal during transfer was large. With this small amount of 2,4-dinitrophenylhydrazone, there was a higher possibility that this crystal would be impure or contaminated. Thus, the determination of melting point of 2,4-dinitrophenylhydrazone maybe less accurate.
As the melting point of the 2,4-dinitrophenylhydrazone matched with the melting point of 2,4-dinitrophenylhydrazones of pentan-3-one and propanal, the carbonyl compound may be a pentan-3-one or propanal. However, the possibility of being a propanal may be higher.
- Prepare 2,4-dinitrophenylhydrazone
- pour 2 cm3 carbonyl compound
- + 5 cm3 2,4-dinitrophenylhydrazone, shake test tube
- place in hot water bath for 3 min
- cool in water bath for 3 min
- suction filtration to obtain crystal
- Rinse with cold water
- Transfer crystal to beaker
- in hot water bath, dissolve with < 4 cm3 hot ethanol, add drop by drop
- filter with short steam funnel, connect by boiling tube
- place in hot water bath
- cool in ice-water bath
- suction filtration
- wash with ethanol
- transfer to water glass
- dry in oven
- determine mp