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Identification of carbonyl compounds by preparing their derivatives

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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 Objective: To identify an unknown carbonyl compound by determining the melting point of it's derivative formed from the reaction with 2,4-dinitrophenylhydrazine solution. Introduction: 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 Results: 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. ...read more.


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. Discussion: 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. ...read more.


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. Conclusion 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. (A) Prepare 2,4-dinitrophenylhydrazone 1. pour 2 cm3 carbonyl compound 2. + 5 cm3 2,4-dinitrophenylhydrazone, shake test tube 3. place in hot water bath for 3 min 4. cool in water bath for 3 min 5. suction filtration to obtain crystal 6. Rinse with cold water (B) Recrystallization 1. Transfer crystal to beaker 2. in hot water bath, dissolve with < 4 cm3 hot ethanol, add drop by drop 3. filter with short steam funnel, connect by boiling tube 4. place in hot water bath 5. cool in ice-water bath 6. suction filtration 7. wash with ethanol 8. transfer to water glass 9. dry in oven 10. determine mp ...read more.

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