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Question
Give a simple chemical test to distinguish between the following pair of compounds:
Phenol and Benzoic acid
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Solution 1
Phenol and benzoic acid can be distinguished by ferric chloride test. In the ferric chloride test, phenol reacts with neutral FeCl3 to make an iron-phenol complex that turns violet.
\[\ce{\underset{Phenol}{6C6H5OH} + FeCl3 -> \underset{(Violet colour)}{\underset{Iron-phenol complex}{[Fe(OC6H5)6]^3-}} + 3H+ + 3Cl-}\]
But benzoic acid reacts with neutral FeCl3 to give a buff coloured ppt. of ferric benzoate.
\[\ce{\underset{Benzoic acid}{3C6H5OH} + FeCl3 ->\underset{(Buff coloured ppt)}{\underset{Ferric benzoate}{(C6H5COO)3Fe}} + 3HCl}\]
Solution 2
Benzoic acid reacts with NaHCO3 to release CO2 gas with effervescence, whereas phenol does not.
Phenol Br2 decolourises water to give white precipitate but does not give benzoic acid.
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