Nucleophilic addition to carbonyl Grignard reaction with an aldehyde PDF

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Nucleophilic addition to carbonyl Grignard reaction with an aldehyde

Purpose The purpose of this experiment is to synthesize the grignard reagent isopropyl magnesium bromide and react this Grignard reagent with 4-methoxybenzaldehyde to form secondary alcohol, 1-(4-methoxyphenyl)-2-methylpropan-1-ol. The product will then be characterized using infrared spectroscopy and nuclear magnetic resonance spectroscopy

2-bromopropane

Introduction The Grignard reaction was the first reaction to generalize the use of carbon as a nucleophile to make carbon-carbon bonds. Grignard reagents are made up of an alkyl group, a magnesium atom, and a halogen. In a Grignard reaction the carbon carries a slight negative charge so it is really easy for it to react with another carbon that got a slight positive charge. The most important thing while conducting a Grignard reaction is that there must be no traces of moisture in the reaction at all because this will break down the Grignard reagent quickly so all the glassware that’s going to be used have to be placed in an oven for 24 hours. A Few extra precautions taken to ensure there is no moisture includes the reaction flask being flame-dried Before adding solvent; a drying tube keeps moisture from entering the apparatus; iodine is vaporized in the flask to tie up traces of water and to activate the surface of the magnesium; the diethyl ether solvent is specially dried and packaged to guarantee it is anhydrous. A Grignard

reagent as a formula of RMgX where X is a halogen and R is an alkyl group. The reaction of a Grignard reagent with formaldehyde produces primary alcohol. With any other aldehyde, the Grignard reaction produces secondary alcohol. With a ketone, the Grignard reaction produces tertiary alcohol.

Procedure Two 25 mL Erlenmeyer flask was taken from a drying oven. A drying tube was packed with anhydrous CaCl2. A dry 50 mL round-bottom flask was clamped to a support stand. A magnetic stir bar was added. Using a medium flame from a bunsen burner all outer surfaces of the flask were heated starting from the bottom and working up until there were no more water vapor condenses on the flask. The drying tube was inserted into the round bottom flask, the flask was cooled for 5 mins and the drying tube was removed. 0.36g of Mg turning was weighed. The turning was added, along with 3-4 crystals of Iodine to the flask. The dry condenser was inserted into the round bottom flask. The drying tube was placed in the top of the condenser. The bunsen burner flask was used to gently heat the round bottom flask until the iodine vapor fills the flask. The apparatus was allowed to cool to room temperature. When the apparatus was cooled, a magnetic stir bar was placed under the round bottom flask of the apparatus. Tubing was then attached to the condenser and the tap water was turned on slowly to let it flow through the condenser. In a cool dry 25 mL Erlenmeyer flask, 1.48 g of 2-bromopropane was weighed and immediately 10 mL of anhydrous diethyl ether was added by pouring directly from the ether container. The drying tube from the top of the reflux condenser was removed. The 2-bromopropane solution was poured through the condenser into the round bottom flask. When rapid boiling was observed the magnetic stirrer was adjusted to slow or medium speed. When the boiling began to subside, a hot water bath was prepared by filling a 250 ml beaker with hot tap water. The bath was placed in the round bottom flask. The flask was immersed until the water level in the bath was even with the reaction mixture level in the flask. The flask was warmed for 10-15 min to complete the formation of the Grignard reagent. The hot water bath was removed and the solution was allowed to cool to room temperature 0.68g of 4-methoxybenzaldehyde was weighed into a cool, dry 25 mL Erlenmeyer flask. Approximately 10 mL of anhydrous diethyl ether was added by pouring from the container. The drying tube was removed from the top of the reflux condenser. Over 5-10 mins a Pasteur pipet was used to add the 4-methoxybenzaldehyde solution in approximately 0.5 mL portions through the condenser into the round bottom flask. The drying tube was replaced after each addition. The solution was added at such a rate that the stirred reaction mixture refluxes gently. After all of the solutions were added, the stirred mixture was warmed with hot water for 10 min. The reaction was allowed to cool to room temperature. An ice water mixture was prepared by filling a 250 mL Erlenmeyer flask with ice to the 50 mL

mark. Deionized water was then added to the water to the 50 mL mark. The Erlenmeyer flask containing the ice water was set on the magnetic stirrer, a stirrer bar was added and the stirrer was adjusted to a rapid rate. The round bottom flask was removed from the apparatus. Gradually the reaction mixture was poured into the ice water. The round bottom flask was rinsed with 4-5 mL of 1 M H3PO4 and the rinse was added to the reaction mixture in the ice water. The round bottom flask was rinsed with 10 mL of solvent grade diethyl ether. This rinse was added to the reaction mixture. The mixture was stirred rapidly and gradually 1M H3PO4 was added until the mixture is acid to the pH paper. A filter funnel was placed in the top of a 125 mL separatory funnel. A loose piece of cotton was placed in the filter funnel and the filter funnel was removed. The layers were allowed to separate. The aqueous layer was drained from the separatory funnel into a 100 mL beaker. The ether layer was poured into a 50 mL Erlenmeyer flask and a stopper was placed on the flask. The aqueous layer was returned to the empty separatory funnel and a second 10 mL portion of ether was extracted. The aqueous layer was drained into the beaker. And the aqueous layer was poured into the container labeled “acidic aqueous layer”. The beaker was rinsed with water. The original ether solution was added to the second ether layer into the separatory funnel. The combined ether layer was washed with 10 mL of 5% aqueous NaOH. The NaOH layer was drained into the 100- mL beaker. The NaOH layer was poured into the container labeled “ recovered 5% NaOH”. The beaker was rinsed with water. The ether layer was washed with 10 mL of saturated NaCl solution. The aqueous layer was drained off into the 100 mL beaker. The ether solution was transferred to a dry 50 mL Erlenmeyer flask. Enough anhydrous magnesium sulfate was added to the solution. A stopper was placed on the flask and the solution was allowed to dry for 5 min. The solution was filtered through a fluted filter paper into a tared 50 mL round bottom flask. A simple distillation apparatus was assembled in the fume hood. A 50 mL round bottom flask was used containing the product as the distilling flask. A boiling chip was added and a hot water bath was used to distill the ether from the product and the ether was collected in a 50 mL beaker.

Data 0.361 mg 2-bromopropane 1.481g (1.13mL) P-anisaldehyde : 0.693g The initial round bottom flask was 39.401g, after distillation mass 40.085g 4-methoxy benzaldehyde was 0.69g.

0.361 g Mg / 24.305g/mol = 0.0148 mol Mg 1.481 ml* (1 g/ 1.31ml) *( 1mol /122.99g) = 0.00918 mol 2-bromopropane

0.693g * (1 mol/136.15g ) = 0.00508 mol p-anisaldehyde

Theoretical yield 0.00508 mol p - anisaldehyde * 1mol / 1 mol * 150 g/ 1mol =0.762 g 2-methyl-1-(4-methylphenyl propan-1-ol ) Percentage yield 0.69 4-methoxy benzaldehyde / 0.762 *100 = 90%

Quality of results A few errors that were made within this experiment was exposing certain chemicals to the air. The measuring process was taking too long and the chemicals were exposed to the air longer than expected. It was challenging to keep up with the series of steps but the experiment was successful. No water vapor was exposed during the experiment, the stoppers were placed on the flasked when needed, the flask was dried evenly to remove condensation when needed. Because everything was conducted accurately the percentage yield was a high 90%.

Conclusion Grignard reagents must be kept from reacting with any water for the desired product to be formed. This experiment was successful with a percentage yield of 90%....