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Expt 4 Recrystallization of Benzoic Acid
organic- bio chemistry (CHEM153L)
Xavier University - Ateneo de Cagayan
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EXPERIMENT 4 - Purification - Recrystallization of Benzoic acid
Purpose: a) To purify samples of organic compounds that are solids at room temperature b) To dissociate the impure sample in the minimum amount of an appropriate hot solvent Equipment / Materials: hot plate 125 - mL Erlenmeyer flask ice stirring rod spatula Büchner funnel impure benzoic acid weighing paper digital scales rubber tubing (hose) benzoic acid boiling stones (chips) filter paper 25 mL graguated cylinder 50 mL beaker Mel-temp apparatus Discussion: The products of chemical reactions can be impure. Purification of your products must be performed to remove by-products and impurities. Liquids are customarily purified by distillation, while solids are purified by recrystallization (sometimes called simply "crystallization"). Recrystallization is a method of purifying a solid. There are two types of impurities: those more soluble in a given solvent than the main component and those less soluble. (If there are any impurities that have the same solubility as the main component, then a different solvent needs to be chosen.) When organic substances are synthesized in the laboratory or isolated from plants, they will obviously contain impurities. Several techniques for purifying these compounds have been developed. The most basic of these techniques for the purification of organic solids is recrystallization, which relies on the different solubilities of solutes in a solvent. Compounds, which are less soluble, will crystallize first. The crystallization process itself helps in the purification because as the crystals form, they select the correct molecules, which fit into the crystal lattice and ignore the wrong molecules. This is of course not a perfect process, but it does increase the purity of the final product. The solubility of the compound in the solvent used for recrystallization is important. In the ideal case, the solvent would completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in that solvent at room temperature or at zero oC. In addition the impurity either would be completely insoluble in the particular solvent at the high temperature, or would be very soluble in the solvent at low temperature. In the former case, the impurity could be filtered off at high temperature, while in the latter case the impurity would completely stay in solution upon cooling. In the real world, this will never happen and recrystallization is a technique that has to be practiced and perfected. Regardless of crystallization method, the purity of the solid can be verified by taking the melting point. A good (suitable) recrystallization solvent will dissolve a large amount of the impure compound at temperatures near the boiling point of the solvent. Small amount of compound being purified should remain in solution at low temperatures, between approximately 25 and – 5 oC. Low solubility at low temperatures minimizes the amount of purified compound that will lose during recrystallization. A suitable recrystallization solvent should also be partially volatile in order to be easily removed from the purified crystals. The solvent should not react with the compound being purified and it should have the boiling point below the melting point of the compound being purified because solid melts before dissolves (oiling out). In selecting a good recrystallization solvent one should also consider flammability, toxicity, and expense. In selecting a solvent consider that like likes like. Polar compounds dissolve polar compounds and non-polar compounds dissolve non-polar compounds. The most commonly used recrystallization solvents are presented in the following table. solvent formula polarity boiling point ( 0 C) water H 2 O very polar 100 ethanol CH 3 CH 2 OH polar 78 methanol CH 3 OH polar 65 dichloromethane CH 2 Cl 2 slightly polar 40 diethyl ether (CH 3 CH 2 ) 2 O slightly polar 35
Organic compounds with one polar functional group and a low number of carbon atoms such as methanol, ethanol, and n- propanol are highly soluble (miscible) in water. These alcohols form hydrogen bond with water due to the polar – OH functional group. As the number of carbons per polar functional group increase, solubility decreases. The solubility of alcohols with four to five carbons is given in the following table. alcohol formula Solubility (g/100 ml H 2 O) n-butanol CH 3 CH 2 CH 2 CH 2 OH 8 n-pentanol CH 3 CH 2 CH 2 CH 2 CH 2 OH 2 n-hexanol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH 0. n-pentanol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH 0. Compounds with six or more carbons for each polar group will not be very soluble in polar solvents but will be soluble in non-polar solvents such as benzene and cyclohexane. If a single solvent cannot be found that is suitable for recrystallization, a solvent pair often used. The solvents must be miscible in one another. Some commonly used solvent pairs are water-ethanol, acetic acid – water, ether-acetone. Typically, the compound being recrystallized will be more soluble in one solvent than the other. The compound is dissolved in a minimum amount of the hot solvent in which it is more soluble. The following formulas used in solubility problems. % lost in cold solvent = (solubility in cold solvent/solubility in hot solvent) x % recovery of solid = [g (solid ) – g (solid lost)] x 100 / g (solid) Example (1)- The solubility of solid “X” in hot water (5 g/100 ml at 100 oC) is not very great, and its solubility in cold water (0 g/100ml at 0 oC) is significant. What would be the maximum theoretical percent recovery from crystallization of 5 g of solid “X” from 100 ml water? Assuming the solution is chilled at 0 oC. Percent solid lost in cold water = (solubility in cold water/ solubility in hot water) x100 = (0.53/5) x100 = 9% grams solid lost in cold water = grams mass of original solid x percent lost = 5 g x 9% = 0 g g (solid recovered) = g (solid) – g (solid lost) = 5 – 0 = 4 g % recovery = g (solid recovered) x100 / g (solid) = (4.52/5) x100 = 90 % Example (2) – The solubility of compound “X” in ethanol is 0 g per 100 ml at 0 oC and 5 g per 100 ml at 78oC. What is the minimum amount of ethanol needed to recrystallize a 12 g sample of compound “X”? How much would be lost in the recrystallization, that is, would remain in the cold solvent? amount of ethanol needed at 78 oC = (12 g)( 100 ml/5 g) = 240 ml amount of sample remaining in the cold solvent at 0 oC = (240 ml)(0 g/100 ml) = 1 g or % lost = (0.80/5) x100 = 16 % Î 12 x 16% = 1 g The actual laboratory we will do is the recrystallization of benzoic acid from water using the temperature gradient method. Benzoic acid is not very soluble in cold water, but it is soluble in hot water. The purpose of this experiment is to learn the technique of recrystallization by purifying benzoic acid.
benzoic acid Buchner funnel vacuum(suction) filtrate
Fig. 2 – Büchner funnel and suction flask
Pour the chilled mixture into the Buchner funnel. The water should filter quickly - if not, check for vacuum
leaks. Get all the crystals out of the flask using a spatula or stirring rod. Rinsing with 1 or 2 mLs of cold water
helps get the crystals out of the flask, and rinsing helps remove impurities.
Let the aspirator run for a few minutes to start air-drying the crystals. Then use a spatula to lift the filter paper
and crystals out of the Buchner funnel, then press them as dry as possible on a large clean paper towel (hand
dry), allow them to dry completely, and transfer the dry sample to a pre-weigh weighing paper. Determine the
weigh the DRY crystals of recovered benzoic acid.
Calculate the percent recovered using the following written formula and determine the melting point of
your recrystallized benzoic acid.
Weight of benzoic acid obtained after recrystallization
% Recovered = x
Weight of benzoic acid before recrystallization
Note: Submit product to the instructor in a properly labeled container.
Course & Year _________________ Group No. _____ Date Performed _________________________ Name (Surname, First Name. MI) Signature 1. Date Submitted _________________________ 2. Instructor: ML B Salingay Instructor’s Signature
3.
4.
5.
6.
Rating _____________________ 7.
1. Sample name ____________________________
2. Data on the impure Benzoic acid
a. Mass of the benzoic acid + weighing paper ________ g
b. Mass of weighing paper ________ g
c. Mass of impure benzoic acid ________ g
3. Data for recrystallized benzoic acid
a. Mass of recrystallized benzoic acid + weighing paper ________g
b. Mass of weighing paper ________ g
c. Mass of recrystallized benzoic acid ________g
d. Calculation of percentage recovery
(show calculation)
________%
d. Melting point of recrystallized benzoic acid ________
o
C
e. Structural formula of the benzoic acid
Q&A: Answer in space provided.
1. What is the ideal solvent for crystallization of a particular compound? What is the primary consideration in
choosing a solvent for crystallizing a compound?
Expt 4 Recrystallization of Benzoic Acid
Course: organic- bio chemistry (CHEM153L)
University: Xavier University - Ateneo de Cagayan
