(Solved):   PRACTICAL MANUAL EXPERIMENT 4 (TO BE DONE ON) THE BARBIER REACTION, SYNTHESIS OF UNSATURAT ...

 

PRACTICAL MANUAL EXPERIMENT 4 (TO BE DONE ON) THE BARBIER REACTION, SYNTHESIS OF UNSATURATED ALCOHOLS Experimental Procedure Place zinc powder \( (5.85 \mathrm{~g}, 0.09 \mathrm{~mol}) \) in a \( 500 \mathrm{ml} \) Erlenmeyer flask containing a fairly large (ca. \( 3 \mathrm{~cm} \) ) magnetic stirrer bar. Add saturated aqueous ammonium chloride solution (75 \( \mathrm{ml} \) ) and stir the mixture. Prepare in advance a solution of a benzaldehyde \( (8 \mathrm{ml}, 0.075 \mathrm{~mol}) \) in THF (15 \( \mathrm{ml} \) ) and add this to the stirred zinc suspension. Continue to stir vigorously while adding allyl bromide \( (7.8 \mathrm{ml}, 0.09 \mathrm{~mol} \) ) in four or five portions from a dropping funnel over about \( 5-10 \) minutes (Note: the reaction heats up considerably while adding the allyl bromide and may foam or even overflow the flask if you are not cautious). Stir the mixture for an hour. When the mixture has cooled to room temperature, add diethyl ether ( \( 15 \mathrm{ml} \) ). Filter the mixture through a small plug of cotton wool to remove excess zinc and any precipitate that may have formed. Rinse the precipitate with fresh diethyl ether \( (15-20 \mathrm{ml}) \), then transfer the combined filtrates to a separatory funnel and separate the organic layer. Extract the aqueous layer with a further portion of diethyl ether (about \( 15 \mathrm{ml} \) ). Dry the combined organic phases over magnesium sulphate, filter off the drying agent and remove the solvent on a rotary evaporator. Run an NMR analysis of your product. Spectra Benzaldehyde \( \delta_{\mathrm{H}}\left(\mathrm{CDCl}_{3}\right) 10.03(\mathrm{~s}, 1 \mathrm{H}), 7.88(\mathrm{~m}, 2 \mathrm{H}), 7.63(\mathrm{~m}, 1 \mathrm{H}), 7.53(\mathrm{~m}, 2 \mathrm{H}) \mathrm{ppm} \). \( 8 \mathrm{C}\left(\mathrm{CDCl}_{3}\right) 191.4 \) (C), \( 136.9 \) (C) \( 134.5(\mathrm{CH}), 129.9(2 \times \mathrm{CH}), 129.2(2 \times \mathrm{CH}) \mathrm{ppm} \). Allyl bromide \( \delta_{\mathrm{H}}\left(\mathrm{CDCl}_{3}\right) 5.99(\mathrm{~m}, 1 \mathrm{H}), 5.33(\mathrm{dd}, 1 \mathrm{H}), 5.17(\mathrm{dd}, 1 \mathrm{H}), 3.97(\mathrm{~d}, 2 \mathrm{H}) \mathrm{ppm} \). \( \delta \mathrm{C}\left(\mathrm{CDCl}_{3}\right) 134.5(\mathrm{CH}), 118.4\left(\mathrm{CH}_{2}\right), 32.4\left(\mathrm{CH}_{2}\right) \mathrm{ppm} \).