Objective: Separate and purify acetanilide from a mixture by recrystallization.
Compare the melting points of crude and recrystallized acetanilide.
Introduction : The most common method of purifying solid organic compounds is by recrystallization. In this technique, an impure solid compound is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools. As the compound crystallizes from the solution, the molecules of the other compounds dissolved in solution are excluded from the growing crystal lattice, giving a pure solid.The first consideration in purifying a solid by recrystallization is to find a suitable solvent. There are four important properties that you should look for in a good solvent for recrystallization.
1. The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature.
2. The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent
3. The solvent should not react with the compound being purified.
4. The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized.
Theory:
The best way to remove insoluble material is to filter the hot mixture, while the desired material is dissolved. Insoluble debris will be trapped on the filter paper. The colored, resinous impurities are usually large, relatively polar organic molecules which have a strong tendency to be adsorbed on surfaces. Filtration of the hot mixture must be done rapidly to avoid crystallization of material on the filter paper or in the funnel.
The filtrate should be collected in an appropriately sized Erlenmeyer flask. Some crystals may have formed in the flask by the end of the filtration, but these are often very small and not well formed. They should be dissolved by warming the filtrate. The best crystals are obtained by slow cooling without agitation...
To study the inter conversion of two geometric isomers, maleic acid (cis isomer) to fumaric acid (trans isomers), the differences in physical properties between this pair of cis-trans isomers and determine the stereochemistry of addition of bromine to butenedioic acid.
Maleic acid and fumaric acid are geometric isomers of butenedioic acid. Each of these isomers has its own distinctive properties such as melting point, solubility, density and stability.
In part A of the experiment, Maleic acid could be converted to the more stable fumaric acid when heated with hydrochloric acid as Fumaric acid is less soluble in water than maleic acid. The hydrochloric acid served as an acid-catalyst of the reaction. Thus, the π bond was broken. Rotation about the sigma bond occurred readily. Loss of a proton could be lead to the formation of either the cis or the trans isomer. Under equilibrium conditions, the thermodynamically favoured product predominated.
(Q2) Maleic acid has a lower melting point than fumaric. Although maleic acid has a stronger dipole moment, the formation of intra molecular hydrogen bond reduces the formation of intermolecular
hydrogen bond. This reduces the extent of the hydrogen formed between adjacent molecules in the crystal and causes a lower melting point than that of fumaric acid. And, the trans-isomer is more symmetrical in shape and can be packed more regularly in the solid crystal.
(Q1) Fumaric acid is more stable than maleic acid (7 kcal lower heat of combustion per mole), the equilibrium lied much towards the fumaric acid side. This is because in fumaric acid, the two bulky carboxyl groups are further apart and experience less steric repulsion. As we started with a solution of maleic acid, then as fumaric acid was formed it precipitates from solution.
