Preparation of benzoic acid by cannizzaro reaction -pptx.pdf

Preparation of benzoic acid by Cannizzaro reaction

1. MEDICINAL
CHEMISTRY PRACTICE
4
th
STAGE
Preparation of benzoic acid
by Cannizzaro reaction
Dr.NarminHamaaminhussen
Lab.7
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2. Aim:
To synthesize benzoic acid from benzaldehyde and
determine its % yield.
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3. THEORY:
Stanislao Cannizzaro is an Italian Chemist who first discovered this
reaction.
The disproportionation reaction of aldehydes without α-hydrogens in presence
of a strong base to produce an alcohol and a carboxylic acid is
calledCannizzaro reaction.
One molecule of aldehyde is reducedto the correspondingalcohol, while a
second one isoxidizedto thecarboxylic acid.
* The applicability of Cannizzaro reaction in organic synthesis is limited as the
yield is not more than 50% for either acid or alcohol formed.
*In case of aldehydes that do have α-hydrogen, thealdol condensationreaction
takes place preferentially.
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4. 4
Cannizzaro reaction is limited to aldehydes such as formaldehyde and
benzaldehyde, which has no hydrogen on the carbon next to the –CHO group
(no α hydrogen).
Examples of Cannizzaro reaction:
1)Formaldehyde is disproportionated to formic acid and methyl alcohol in
strong alkali.
2)Benzaldehyde can be converted to benzoic acid and benzyl alcohol.

5. Reaction:
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6. Uses of benzoic acid:
As an inactive ingredient in thepharmaceuticalindustry, it
isusedas antimicrobial preservative, antifungal, and tablet
and capsule lubricant.Benzoic acidhas beenusedin
combination withsalicylic acid, as in Whitfield's ointment,
foruseas an antifungal for athlete's foot and ringworm.
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7. Procedure:
Dissolve 7 gm of KOH in 7 ml D.W. in a beaker. Cool the solution
and pour into a 125ml reagent bottle.
Add 7.5 ml benzaldehyde, cork the bottle securely and shake the
mixture vigorously until it has been converted to a thick emulsion.
Allow the mixture to stand for 24hr or longer (next week). By which
the time the reaction should have been completed.
After this period, add about 30ml water to completely dissolve
potassium benzoate.
Transfer everything to a separatory funnel, rinse the bottle with
10ml of ether and again add to the solution in the separatory
funnel.
Shake the separating funnel well, separate the lower aqueous layer,
repeat this addition of ether and extraction of aqueous layer for
two times.
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8. 8
Now there’s two separated solution:
Aqueous solution containing Benzoic acid.
Ethereal solution containing Benzyl alcohol.
Extraction of Benzoic acid from aqueous layer:
Prepare a mixture of (20ml of conc. HCl + 20ml water + Crushed ice).
Pour the aqueous solution into this the mixture with stirring. Benzoic acid
precipitates in this time.
Filter the precipitated benzoic acid. To remove the remaining ppt., wash
the flask with cold water.
Dry the filtrate and weigh your product.

9. Calculation:
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