CHM 556 ORGANIC CHEMISTRY II -BACHELOR (HONS) APPLIED CHEMISTRY, AS245EXPERIMENT 4: THE ALDOL CONDENSATION REACTION – PREPARATION OF BENZALACETOPHENONES (CHALCONES)
NAME: MOHAMAD NOR AMIRUL AZHAR BIN KAMIS (2014647344) PARTNER’S NAME: 1. MOHAMAD SHAFIQ BIN PARMAN 2. MOHAMAD WAFIY BIN MOHD ZULKIFLEE LECTURER’S NAME: PN. FARIDAHANIM BT MOHD JAAFAR DATE OF EXPERIMENT: 29/10/2014 DATE OF SUBMISSION: 21/11/2014
EXPERIMENT
4:
THE
ALDOL
CONDENSATION
REACTION
–
PREPARATION
OF
BENZALACETOPHENONES (CHALCONES). OBJECTIVE: Performing aldol condensation reaction between 3-nitrobenzaldehyde and acetophenone in the presence of base (ethanol) to produce an α, β-unsaturated ketone (3-nitrochalcone) and calculate the percentage yield of the crude and purified product and obtain the NMR spectrum for the pure product.
INTRODUCTION: An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugate enone. Benzaldehyde reacts with a ketone in the presence of base to give α, β-unsaturated ketones. This reaction is an example of a crossed aldol condensation where the intermediate dehydrates to produce the resonance-stabilized unsaturated ketone. Ketone loses a proton from an α-carbon to form an enolate ion, which attacks the carbonyl carbon of the aldehyde to yield, after protonation, α, β-hydoxy ketone. The intermediate is, for aromatic aldehyde at least, instable and undergoes base-catalysed dehydration to yield the unsaturated product.
PROCEDURE: 1. 0.7515g of 3-nitrobenzaldehyde was mix with 0.6mL acetophenone and 4mL of 95% ethanol in a conical flask. The mixture was stirred until the entire solid is dissolved. 2. 0.5mL of sodium hydroxide solution (6g NaOH solid in 10mL water) was added into the mixture and the stirring process continued. 3. 10mL of ice water was added and scratching was performed to induce to crystallization process. 4. The mixture then was transferred into a beaker containing 15mL of ice water. 5. The mixture was filtered to collect the solid by using Buchner funnel filtration set.
6. The solid collected was allowed to dry in oven with temperature of about 80°C for 30 minutes. The weight of the solid product was recorded. 7. The entire solid (1.0633g) was recrystallized with 42.5mL hot methanol solution, and the solid was filtered again. The solid was dried again in oven at temperature of about 80°C overnight to make sure complete drying process. 8. The dried solid product (3-nitrochalcone) was weighed and the percentage yield was calculated. 9. The melting point and NMR test were done with the solid product (3-nitrochalcone).
CALCULATION: Molar mass of acetophenone = 120g/mol Molar mass of 3-nitrobelzaldehyde = 151g/mol Molar mass of 3-nitrochalcone = 253g/mol Density of acetophenone = 1.03g/cm3 Mol of 3-nitrobenzaldehyde = = 0.004967mol 0.6mL of acetophenone = 0.6cm3 of acetophenone Mass of acetophenone = 0.6cm3 × 1.03g/cm3 = 0.618g Mol of acetophenone = = 0.00515mol Limiting reagent: 3-nitrobenzaldehyde Theoretical yield = 0.004967mol × 253g/mol = 1.2667g
RESULT AND OBSERVATION: i.
Observation
Colourless solution turns to reddish yellow and the precipitate form after the addition of sodium hydroxide solution.
Mass of product (3-nitrochalcone) = 0.7515g Melting point of product (3-nitrochalcone) =142 °C Percentage yield =
× 100%
Percentage yield of the product (3-nitrochalcone) =
× 100%
= 59.33% ii.
NMR result for chalcone
Signal
Chemical shift (ppm)
Frequency (Hz)
1H doublet
7.886-7.834
15.6
DISCUSSION: From the experiment, the limiting reagent is the 3-nitrobenzaldeyde. The theoretical mass is calculated by referring to the mole of the 3-nitrochalcone. From the result obtained, the product seems to be pure enough by comparing the actual melting point, 142°C with the theoretical melting point that around 146°C. Errors may occur during the filtration process by using the Buchner funnel vacuum filtration. The pressure used to collect the solid is too high until the filter paper form holes that can pass through the solid with the liquid into the conical flask. This then will make the actual yield mass will decrease. To make sure there is no error in weighing the product, the solid must be dry at least overnight to evaporate all the water moisture. From the NMR result the 1H doublet signal was observed with frequency of 15.6Hz. From the literature review, the frequency of signal for trans compound is between 12-18Hz, so that it is proven that the compound yield is that trans-compound. This frequency is compared to cis-compound frequency that is lower, 6-12Hz.
CONCLUSION: The product yield from this experiment is 3-nitrobenzaldehyde that proven with the H1 NMR spectrum. The percentage yield of the crude product obtained is 59.33% and the melting point of the product is 142 °C.
QUESTIONS: 1. Write the mechanism of the reaction. 1
O
OH-
O
CH 2
CH3
2
O
O
CH 2
O
-
O O
-
-
O
O
+
N
+
N
O
O
3
-
O
O
NaOH
+
O
N
+
N
O
O
-
O
OH
O 4
O
+
N
O
-H2O
+
N
O
OH
O
-
O
-
2. How do you confirm that the product is pure? Explain. To confirm the purity of the product yield, the melting point of the product were taken and been compared with the theoretical value. The closer value of the actual melting point to the theoretical value, the higher the purity of the product. The confirmation also been done by 1H NMR spectrum that will give the structure of the compound.
3. Why the ethanol is used during the recrystallization? Ethanol was used because it has lower boiling point than the water and the byproduct with impurities can dissolve in ethanol. When the ethanol is vaporized, the by-product together with the impurities will evaporate too and leaving only the pure product.
REFERENCES: 1. CHM 556 Organic Chemistry 2 lecture notes prepared by Prof. Faujan. 2. T.W. Graham Solomons, Craig B. Fryhle, Organic Chemistry, Asia, John Wiley and Sons, 2011. 3. Aldol condensation, 16/11/2014, http://en.wikipedia.org/wiki/Aldol_condensation. 4. Chalcone, 16/11/2014, http://en.wikipedia.org/wiki/Chalcone.
