Separation of Aromatic Compounds and Comprehensive Exercises

Separation of mixed aromatic compounds by solvent extraction is a frequently tested comprehensive problem. Differences in acidity, basicity, and neutrality are used systematically.

Basic Knowledge

Classification of organic compounds (acid-base properties):
・Strongly acidic: sulfonic acids (-SO₃H)
・Carboxylic acidity: -COOH (converted to a salt by aqueous NaHCO₃, liberating CO₂)
・Weakly acidic: phenols (do not react with NaHCO₃; converted to a salt by aqueous NaOH)
・Neutral: hydrocarbons, esters, ethers, aldehydes, ketones, alcohols, and nitro compounds
・Weakly basic: amines (converted to a salt by aqueous HCl)

Example of systematic separation using a separatory funnel (using distribution between an ether layer and an aqueous layer):
Example: Separate a mixed ether solution of benzoic acid, phenol, aniline, and toluene (neutral).
① Add aqueous HCl: aniline becomes anilinium chloride and moves into the aqueous layer → recover aniline.
② Add aqueous NaHCO₃ to the ether layer: benzoic acid becomes its sodium salt and moves into the aqueous layer (phenol remains) → recover benzoic acid.
③ Add aqueous NaOH to the ether layer: phenol becomes sodium phenoxide and moves into the aqueous layer → recover phenol.
④ Only toluene remains in the ether layer.

After each separation, neutralize the aqueous layer with a suitable acid or base to liberate the desired compound, and extract it again with ether if necessary for isolation.

Further Study (Principles and Applications)

Partition coefficient: When a compound dissolves in two layers, the ratio of its concentrations in the two layers is called the partition coefficient K. Ionic salts distribute strongly into the aqueous layer, while neutral organic compounds distribute strongly into ether (the organic layer). Maximizing this difference is the essence of systematic separation.
Practical applications: This technique is routinely used in pharmaceuticals, natural-product chemistry, and environmental analysis. For example, after a pharmaceutical synthesis, by-product salts are transferred to the aqueous layer and the desired product to the organic layer for purification.
When the partition coefficient is small, efficiency is improved by multiple extractions: extraction efficiency=1-(K·V_water/(K·V_water+V_organic))ⁿ (n=number of extractions).
Chromatography (column, TLC, HPLC, and GC) is an extension of this distribution concept and is a core technology in modern analytical chemistry.
In environmental analysis, the standard workflow for POPs (persistent organic pollutants) is organic-solvent extraction from environmental samples (water and soil) → concentration → quantitative analysis by GC-MS.

Points of Caution (Common Confusions and Frequent Errors)

① The key is distinguishing between NaOH and NaHCO₃. NaHCO₃ captures only carboxylic acids (substances stronger than carbonic acid), whereas NaOH captures both phenols and carboxylic acids. Using them successively separates carboxylic acids from phenols. ② Taking aniline first with aqueous HCl works when the ether layer is neutral to acidic. In a basic solution, aniline remains in its free form and stays in the ether layer. ③ The order matters: the standard order is amine first, followed by carboxylic acid, phenol, and then the neutral compound. ④ Phenols with high steric hindrance, such as 2,4,6-tribromophenol, may be sparingly soluble even in aqueous NaOH and are exceptions.

Practice

1. Write the procedure for sequentially separating benzoic acid, p-cresol, aniline, and nitrobenzene (neutral) from a four-component ether solution.
2. Explain why phenol does not react with aqueous NaHCO₃ by comparing acid strengths.
3. Explain the concept of the partition coefficient together with the need for multiple extractions.