Q3P - Organic Chemistry (Mcmurry)

Question

Assume you have a mixture of naphthalene and benzoic acid that you want to separate. How might you take advantage of the acidity of one component in the mixture to effect a separation?

Step-by-Step Solution

Answer

Benzoic acid is separated from the mixture of naphthalene and benzoic acid by taking advantage of the solubility of benzoate.

Step 1: Identify whether the given compound is polar or non-polar and their solubility in polar and non-polar solvents.

Naphthalene is a benzene-based aromatic hydrocarbon with two fused rings. It is soluble in non-polar solvents such as ether and insoluble in polar solvents such as water. Since it is a non-polar aromatic hydrocarbon. It is neither acidic nor basic.

Benzoic acid is a monobasic aromatic carboxylic compound. It is soluble in benzene, ethanol and ether but slightly in water. The conjugate base of benzoic acid is benzoate which is very soluble in water but insoluble in non-polar solvents. Because benzoate is an anionic compound. The mixture of non-polar and ionic compound is separated by using the solubility method.

Step 2: Apply the solubility rules to extract the compound.

The given mixture of naphthalene and benzoic acid is dissolved in ether. Add aqueous ${}^{\mathbf{NaOH}}$ solution to this mixture. The aqueous layer is separated with a separating funnel that contains the benzoate salt after neutralising benzoic acid. Now the separated aqueous layer is acidified with dilute mineral acid such as ${}^{\mathbf{HCI}}$ . Ether is added to the acidified solution to extract benzoic acid.