Purpose: To separate and purify a solid carboxylic acid and a liquid neutral com
ID: 1031926 • Letter: P
Question
Purpose: To separate and purify a solid carboxylic acid and a liquid neutral compound dissolved in ether. Procedure: 1. Separation and purification of the carboxylic acid Wash the ether solution twice Transfer the ether solution to a separatory e. with 10 mL portions of 10% NaOH. (what is the purpose of this step?) Combine aqueous solutions and cool in ice bath. Add 6 M HCl while swirling to the cooled aqueous solution until it is acidic. (What is the purpose of this step, and what should you expect to see?) Make sure the solution remains acidic as sometimes the pH changes as the reaction progresses. Cool the acidified solution in the ice bath for another 5-10 minutes. Collect the crude acid by vacuum filtration. Recrystallize the carboxylic acid using water as the solvent. Measure the melting point of this solid after is has been dried for a while.Explanation / Answer
Firstly, the ether layer contains both the carboxylic acid and the neutral compound. When NaOH is added to the ether-water solvent system, due to the immiscibility of ether with water, after neutralization of the acid with NaOH takes place, a sodium salt of the carboxylic acid is formed which is extremely polar and hence will dissolve into the water layer leaving only the neutral compound in the ether layer. This neutral compound can now be isolated in its pure form by simple evaporation of the ether layer set aside, devoid of any water.
Next, to take the carboxylic acid back into the organic layer to get that acid in its pure form, HCl is added. This neutralizes the basic water solution containing the carboxylic acid which in the process also protonates the carboxylate anion giving the ether-soluble carboxylic acid. This layer of freshly added ether is again set aside to recrystallize the acid. When HCl is added to the water layer alone, without any ether one can observe turbidity or precipitation from the aqueous medium which is the precipitation of the carboxylic acid in its neutral form arriving our of the water layer due to its insolubility.
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