The Extraction of Benzoic Acid

Topics: Solubility, Solvent, Chemistry Pages: 6 (2064 words) Published: October 8, 2012
Extraction is a purification technique used in organic chemistry to separate compounds from a mixture of two or more compounds. There are three different extraction techniques: liquid-liquid extraction, solid-liquid extraction and chemically active extraction. All three types of extraction follow the same principle. Organic molecules dissolve in organic solvents and polar molecules dissolve in aqueous solvents. This phenomenon is observed because of the intermolecular forces between solvent and solute molecules. For example, in aqueous solvents, the polar solute can interact with the solvent via hydrogen bonding, electrostatic forces, ion dipole forces, dipole-dipole forces and London forces. All of these can help a polar solute dissolve in an aqueous solvent. Polar molecules can be involved in these interactions because they have a partial charge. This partial charge is brought upon by the differences in electronegativity between the atoms in the molecule. The difference in electronegativity created a dipole in the molecule. The partially positive hydrogen atoms in H2O molecules are then able to interact with the partially negative part of the molecule and the partially negative oxygen atom in the H2O molecule is able to interact with the partially positive parts of the molecule. The exception is London forces, which is an interaction that nonpolar molecules can participate in as well due to the transient dipole moments that exist in them. In the solid-liquid extraction, a solvent is added to a mixture of solid compounds. A specific compound in the mixture will dissolve in that solvent. Vacuum filtration can then be used to separate the specific compound from the other compounds in the mixture, which are insoluble in that solvent. In the liquid – liquid extraction, you must have solvents that are immiscible (organic and aqueous). The solvents do not mix and separate to form two layers. The solvent with the lower density is on the top and the solvent with the higher density is in the bottom. A mixture of compounds is added to the solution. The compounds dissolve in their respective solvents (the polar compound in the aqueous and the less polar or nonpolar compound in the organic) and the compounds can be separated by separating the solvents from each other. In the chemically active extraction, organic acids and bases are separated from a mixture of compounds by converting the acid/base compound to a salt. The salt has a charge so it is soluble in aqueous solvents. The dissolved compound is then separated from the insoluble compounds using vacuum filtration. This technique can be used to separate acidic, alkaline and neutral organic compounds.

Extractions do not separate 100% of a compound. The distribution constant, Kd, is the equilibrium constant for extraction. It allows us to calculate how much of a solute will dissolve in a given pair of solvents based on the properties of the solvents used. The equation is: Kd=concentration in organic solventconcentration in aqueous solvent Organic molecules are mainly soluble in nonpolar organic solvents, and polar molecules are mainly soluble in aqueous solvents. However, traces of dissolved organic molecules can be found in aqueous solvents and traces of dissolved polar molecules can be found in nonpolar solvents. If Kd is greater than 1, the solution is more soluble in the organic solvent. If Kd is less than 1, the solution is more soluble in the aqueous solvent. If KD is equal to 1, the solution is equally soluble in the organic solvent and the aqueous solvent.

The purpose of this experiment was to extract benzoic acid from a crude mixture. The following is the extraction scheme that was used:

The crude mixture is dissolved in ether and the insoluble compounds are separated via vacuum filtration. Next, NaOH is added to the solution to generate the salt of the acid. The base removes the acidic hydrogen from the acid. Since NaOH is a strong base, it...
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