Question

In an extraction experiment, an aqueous solution of propionic acid will be extracted with hexane and with ethyl acetate. Which solvent should extract more of the propionic acid? Explain.

Answer 1

Extraction refers to the recovery of a required substance from a multiple mixture,and is a separation technique, which is based on differences in solubilities of substances in two immiscible solvents (generally water (aqueous) and a water insoluble organic solvent). A solvent chosen for extraction usually has a low b.p. so that it can be easily removed if the dissolved substance is to be recovered (If there is no other dominant factors are absent as explained below). Propionic acid is slightly polar and soluble in polar solvents like ethyl acetate.

Extraction of the carboxylic acids follow the trend

alcohols > oxygenated diluents > aromatic hydrocarbons > aliphatic hydrocarbons > long chain aliphatic hydrocarbons mixture.

Oxygenated solvents are organic solvent, molecules of which contain oxygen. Examples of oxygenated solvents: alcohols, glycol ethers, methyl acetate, ethyl acetate, ketones, esters, and glycol esters. This suggests that alcohols and oxygenated diluents are most effective in physical extraction of propionic from dilute solutions. Their hydrogen bond donor ability is the reason for higher extractions in these diluents.

Hence in view of above, ethyl acetate solvent can extract more of the propionic acid than hexane solvent. Since ethyl acetate contain carbonyl oxygen which has ability to form weak hydrogen bonding with propionic acid in contrast to hexane, which is only aliphatic hydrocarbon (not able to form hydrogen bonding with propionic acid).

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