Question

Organic chemistry post-lab question: "Why is important to add the alkyl halide dropwise in your reaction? (Hint: your reaction is not heated, but you still need a water-cooled condenser!)" Please answer in detail!

Here is a copy of the experiment, thank you!

The Grignard reaction is an important synthetic process by which a carbon-carbon bond is formed. Magnesium metal is first reacted with an organic halide. The resultant organo-magnesium halide (Grignard reagent) is then combined with a carbonyl compound, ultimately forming an alcohol. Strict anhydrous conditions must be used for the formation of the Grignard reagent to be successful. G. W. Breton and C. A. Hughey (J. Chem. Ed., 1998, 75, 85) have designed a Grignard-like synthetic procedure in which zinc metal is used to mediate the reaction between allyl bromide (3-bromopropene) and benzaldehyde. In contrast to the Grignard reaction, this carbon-carbon bond formation is carried out in saturated aqueous ammonium chloride solution. Thus, no special drying of solvents, reagents, or glassware is required. The zinc metal is apparently oxidized during the reaction. ОН ~ Br + ſ Zn° sat. aq. NH4Cl THF EXPERIMENTAL PROCEDURE Caution: Allyl bromide is toxic and flammable. Gloves should be worn during its handling, and all manipulations should be carried out in the fume hood. Place 0.156 g of powdered zinc powder and 1.0 mL of saturated aqueous ammonium chloride into a 10- mL round-bottomed flask, along with a stir bar. Prepare a solution of 0.102 mL of benzaldehyde and 0.5 mL of tetrahydrofuran (THF) and add it to the flask. What two solvents are now in the flask? Which is organic and which aqueous? Equip it with a condenser, set up the pump and ice water bath to cool the condenser, and clamp the apparatus safely. Stir the mixture vigorously, and use a syringe to add 0.200 mL of allyl bromide dropwise through the top of the condenser. The bromide should be added so that each drop falls directly into the mixture; DO NOT let it run down the wall of the condenser. An immediate reaction should take place, with the disappearance of the zinc powder. Stir the mixture for 30 minutes after you have added all of the allyl bromide. Check that the reaction is complete by preparing a TLC plate. Your plate should compare your substrate to the reaction mixture (mobile phase: 30% ether- hexane, double run, meaning you should develop the plate once, let it dry, and then let the mobile phase run up it again to push and separate the compounds further). Visualize the spots with UV light and KMnO4 solution. Which compounds should be visible under the UV light? Should the substrate or the product have a higher Rf value? Sketch the TLC plate in your notebook, and use your results to determine whether you should let the reaction run longer. Add 1.0 mL of diethyl ether to the reaction mixture, and filter the mixture through a plug of cotton in a Pasteur pipet to remove excess zinc and any solid (zinc salts) that may have formed. Rinse the flask with 1.0 mL of ether, and filter it through the cotton plug. Collect all liquids in a small test tube and cap it. Use another Pasteur pipet to carefully remove the top layer from the bottom. Why shouldn't you use a separatory funnel for this, even though it is an extraction? Hint: The smallest separatory funnels hold about 25 mL. Separate the organic layer, and extract the aqueous layer with another 1.0 mL of ether. Combine the organic layers in another small test tube, and dry them over anhydrous sodium sulfate. Using a Pasteur pipet, transfer the organic solution (no solids) to a pre-weighed 5-mL thick wall flat bottom conical flask from the microkit (including a spin vane). Rinse the sodium sulfate with 0.5 mL of ether, and transfer this ether to the 5-mL flask. NOTE: evaporation of solvent should be done only in the fume hood). Place the flask on an aluminum block on a hot plate/stir plate, and stir with a spatula with gentle warming to evaporate the solvents. Finally remove traces of solvent using vacuum tubing. The crude product remains in the flask. Weigh and determine the % yield of the product. Obtain and analyze the IR spectrum of the product. Discuss these results in your report.

Answer 1

Since this reaction need to be carried out using cold condensor, it implies that the reaction is exothermic in nature. To avoid wastage of solvent through evaporation in air due to heat generated and thus recycling the solvent through reflux, condensor is being used.

So the alkyl halide has to be adddd drop by drop so that the heat generation due to the exothermic reaction is controlled and slow. Otherwise sudden addition of large amount of alkyl halide in the reaction mixture will lead to generation of very large amount of heat and thus reaction becomes uncontrolled and can lead to explosion or solvent can catch fire.