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A mixture of toluene, benzoic acid, aniline and phenol was dissolved in diethyl ether and extracted with 5% $\ce{NaHCO3}$ solution? This extract was acidified until a white crystalline compound appeared. What is the identity of this compound?

I had this question in my exam today. The answer is benzoic acid but I wanted to know how.

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Well what you are doing here is trying to extract it by making it slightly alkaline using water, as water and diethyl ether seperate anything that is only soluble in alkaline water will be extracted from the organic phase. Of course toluene and aniline won't do much if $\ce{NaHCO3}$ is added. For phenol and benzoic acid it's just about the base you are using. $\ce{NaHCO3}$ is not strong enough to deprotonate the phenol. For benzoic acid, the acid is strong enough to be deprotonated by $\ce{NaHCO3}$. When it is acidified the benzoic acid is formed again and precipitates.

The problem here is that many people forget how poorely soluble benzoic acid and salicylic acid really are. I made the same mistake many years ago. Although they only feature a quite small organic fragment they are insoluble in water although they are acids. Their ionic salts however are soluble. So adjusting the pH value either dissolves or precipitates them in water.

Edit: With the same method you could also probably extract the aniline using acidified water (instead of alkaline) as a soluble anilinium forms.

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