Synthesize Bezoic Acid from Benzaldehyde.

Experimant No 06 :-  To Synthesize Bezoic Acid from Benzaldehyde.

Reference: Vogel's Textbook of practical organic chemistry revised by B.  S.  Furniss, a.  J  Hannaford.  P.  W.  G.  Smith, A.  R.  Tatchell.  Fifth Edition.  ELBS.  1989.  page no - 1029. 

Requirements:

Apparatus - Beaker, Conical Flask, Water Bath Vessel, Reagent Bottle, Separatory Funnel, Funnel.  Measuring Cvlinder.  Pipette, Stirrer, Spatula, Etc

Chemicals - Potassium Hydroxide, Ice, Benzaldehyde, Ether, Sodium Metabisulphite, Sodium Carbonate, Hydrochloric Acid.  Instrument - Hot Air Oven.  Vacuum Pump, Melting Point Apparatus. 

Principle: maticaldehydes do not contain alpha hydrogen atomseg.  Formaldehyde and bonatachar under the influence of strong aqueous or al ammeous or alcoholic alkali undergoes simultaneous oxidation and on yielding the alcohol and corresponding carboxylate salt.

                                                                           

This dismutation or disproportionation reaction is known as the Cannizzaro reaction.  The mechanism of the reaction involves the production of the anion (1) which may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule.  The reaction is completed by a proton transfer toyield the carboxylate anion and the alcohol.

The reaction is illustrated here by the conversion of benzaldehyde into benzoic acid and benzyl alcohol. 

Reaction:-

Procedure:

I.                   Dissolve 29 g of potassium hydroxide in 27 ml of water in a beaker or conical flask, and cool the solution to about 20C in ice water.  Pour this solution in 250ml reagent bottle and add 32g (30 ml, 0. 3 mol) of pure benzaldehyde.  Cork bottle and shake it vigorously until it has been converted into thickemulsion and allow the bottle to stand overnight.

II.                 Add just sufficient water (about 105 ml) to dissolve the potassium benzoate which formed.  Pour this mixture into a separatory funnel, rinse reagent bottle with about 30 ml of ether and add this ether to the solution in the funnel. 

III.             Shake the solution in order to thoroughly extract the benzyl alcohol with the ether, separate the lower aqueous solution and carry out further extractions each with about 25 ml of ether.  Save the aqueous solution.

IV.              Add this aqueous solution into a mixture of 80 ml of concentrated hydrochloric acid and 80 ml of water and about 100 g of crushed ice. 

V.                Filter the precipitated benzoic acid at the pump, wash it with little cold water, drain and recrystallize from boiling water.

VI.              The yield of benzoic acid (colorless crystal), m.  p 121 ° C, is 13.  5 g (79%)

Result:-