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Acid / Base Extraction and Separation of Acidic and Neutral Substances

Experiment 3: Acid/base Extraction and Separation of Acidic and Neutral Substances


You will be given a mixture that contains three substances in equal amounts: benzoic acid, 2-naphthol and 1,4-dimethoxybenzene (p-dimethoxybenzene):

Your task: to separate these three compounds by taking advantage of differences in their acidity.


There are two terms we use when separating compounds from organic products:

1. To remove organic compounds (what you want) from aqueous solutions (or what you don’t want), we perform an “extraction”. For neutral organic compounds, we often add an organic solvent to dissolve a neutral organic compound to separate it away from inorganic, aqueous soluble “trash”. In today’s lab, we are adding a base to form an ionic salt from the organic compound, which will make it water soluble to separate it from the compound(s) still soluble in the ether solvent.

2. To remove inorganic unwanted compounds from what we want, we perform a “wash”. We add aqueous solutions to our organic compounds so they “wash” away  impurities.

• To remove acids, we add bases.

• To remove bases, we add acids.

• To remove salts, we just sometimes wash with water.

Today you will be extracting your compounds from a mixture. In their neutral, covalent forms, all three compounds are soluble in a slightly polar organic solvent such as diethyl ether (CH3CH2OCH2CH3) but are fairly insoluble in water. This is because they are composed of many non-polar C-C and C-H bonds and have only a couple polar covalent bonds. Generally they are relatively non-polar, and only relatively non-polar organic solvents want to surround them. However, benzoic acid and 2 naphthol are acidic due to their –OH groups and so will be converted to their ionic salt forms on reaction with an appropriate base.


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