52nd International Chemistry Olympiad, Istanbul, TURKEY
Preparatory problems: PRACTICAL
Problem P5. Cannizzaro Reaction Aldehydes are widespread in nature. Most of them are known for their sweet odors. For instance, vanillin exists in vanilla with its distinct odor. Aldehyde functional group-containing molecules are often used in perfumes because of their pleasant fragrance. Benzaldehyde provides the distinctive smell of almonds. Stanislao Cannizzaro discovered in 1853 that benzaldehyde on treatment with a base gave equimolar quantities of benzoic acid and benzyl alcohol. In this task, you are asked to prepare p-chlorobenzoic acid and p-chlorobenzyl alcohol through a Cannizzaro reaction of p-chlorobenzaldehyde.
Cannizzaro is a disproportionation reaction in which an internal oxidation/reduction reaction occurs. The reaction of an aldehyde with a strong base takes place only if the aldehyde has no alpha hydrogen atoms. The mechanism of the Cannizzaro reaction involves hydride ion (H๏ญ) transfer, which takes place only in the presence of a strong nucleophile. Chemicals Substance Name ClC6H5CHO p-Chlorobenzaldehyde
State Solid
KOH
Potassium hydroxide
Solid
(C2H5)2O
Diethyl ether
Liquid
HCl
Hydrochloric acid
Aqueous solution
NaHSO3
Sodium bisulfite
Aqueous solution
GHS Hazard Statement H302, H315, H317, H319, H411, P273, P280, P302 + P352, P305 + P351 + P338 H290, H302, H314, P260, P280, P301 + P312 + P330, P301 + P330 + P331, P303 + P361 + P353, P305 + P351 + P338 + P310 H224, H302, H336, H412, P210, P273, P301 + P312 + P330, P403 + P233 H290, H314, H335, P260, P280, P303 + P361 + P353, P304 + P340 + P310, P305 + P351 + P338 + P310 H302, P301 + P312 + P330
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