pHarmacist
(Hive Addict)
03-02-03 23:22
No 413172
      Benzaldehyde to Benzyl Alcohol
(Rated as: excellent)
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pHarmacist's voice: Both benzaldehyde and benzyl alcohol are valuable precursors in underground chemistry. My personal experience is that benzaldehyde is easier to come across than benzyl alcohol, at least where I live. Somebody else might disagree, anyhow, I'm still going to introduce to you a neet, relatively OTC preparation of benzyl alcohol in nearly 87% yield from benzaldehyde. This synth also yields benzoic acid that can come handy when times get rough. Following is a typical Cannizzaro reaction:


VOGEL'S Textbook Of Practical Organic Chemistry [Fifth Edition] page 1029, Experiment 6.133


Ph-CHO + Ph-CHO ---KOH---> Ph-CH2OH + Ph-CO2-K+


Benzyl Alcohol:

Dissolve 29 g of potassium hydroxide in 27 mL of water contained in a beaker or conical flask, and cool the solution to about 20C in ice-water. Pour the solution into a 250-mL reagent bottle, and add 32 g (30 mL, 0.3 mol) of pure benzaldehyde; cork the bottle secuerly and shake the mixture vigorously until it has been converted into a thick emulsion. Allow the mixture to stand overnight or for 24 hours in the stopped bottle. Add just sufficient water (about 105 mL) to dissolve the potassium benzoate. Pour the liquid into a separatory funnel, rinse out the bottle with about 30 mL of ether and add this ether to the solution in the funnel. Shake the solution in order to thoroughly extract the benzyl alcohol with the ether, separate the lower aqueous solution and carry out two further extractions each with about 25 mL of ether. Save the aqueous solution. Combine the ether extracts and distil the ether from a water bath (rotary evaporator) until the volume is about 25 mL. Cool and shake the ether solution twice with 5 mL portions of saturated sodium metabisulphite solution in order to remove any benzaldehyde which may be present. Separate the etheral solution, wash it with 10 mL of 10 % sodium carbonate solution (to ensure complete removal of the bisulphite), then with 10 mL of water, and dry with anhydrous magnesium sulphate or anhydrous potassium carbonate. Remove the ether on a water bath, and distil the residual liquid from an air bath; replace the water condenser by an air condenser or empty the water completely from the condenser jacket. Collect the benzyl alcohol at 204-207C (the pure compound boils at 205.5C). The yield is 13 g (86.5%).

Benzoic acid (optional):

Pour the aqueous solution remaining from the ether extraction with stirring into a mixture of 80 mL of concentrated HCl, 80 mL of water and about 100 g of crushed ice. Filter the percipitated benzoic acid at the pump, wash it with little cold water, drain and recrystallise from boiling water. The yield of benzoic acid (colourless crystals), m.p. 121C, is 13.5 g (79%)

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