Lab+5+Extraction+and+Separation+of+a+Mixture

__**Intro:**__

Using acid base chemistry in the separation of substances can be a quite useful technique in organic chemistry. A basic extraction can be carried out by applying a hot solvent such as water to a dry substance like coffee. This will allow for the extraction of caffeine and flavor from the coffee beans. Acid base chemistry can be useful in separating neutral, basic, and acidic organic substances by controlling the pH of the aqueous layer. In acid base extraction a solvent such as ether is mixed with an aqueous solution of a different pH. This will aid in ionizing the intended compound and separate it from the mixture. Once the molecule is ionized it will precipitate out of the ether solvent and dissolve into the aqueous layer. The aqueous layer can then be removed and the pure substance can be obtained through crystallization.

In this week's lab a mixture of carboxylic acid, phenol, and a neutral substance were separated from an ether solution. First, the carboxylic acid was ionized with a weak base and removed. A strong base was then applied to the mixture to allow the phenol to ionize and precipitate into the aqueous layer. This layer of aqueous solution was then extracted which left the neutral compound in the ether solvent. The three separated substances were then crystallized for melting point and identification.


 * __Procedure:__**

Chapter 8: Experiment 1. Seperatin of a Carboxylic Acid, a Phenol, and a Neutral Substance Williamson K.L., 2003. Macroscale and Microscale Organic Experiments 4th Edition. Boston (MA): Houghton Mifflin Company, p. 138-140

__**Data:**__



1.) The experiment began by dissolving 0.202g of a mixture into approximately 2mL of MTBE (methyl tert-butyl ether) followed by 1mL of a aqueous solution of sodium bicarbonate to test tube #1. The contents of the tube were thoroughly mixed. Upon separation of mixture into 2 distinct layers, the aqueous layer was removed using a pipette and placed into test tube #2. Following the removal of this aqueous layer, another 0.15mL of aqueous sodium bicarbonate was added to test tube #1 mixed and poured off into the lower layer of tube #2. 2.) An addition of 1.0mL of 3M aqueous sodium hydroxide was poured into test tube #1. Five drops of deionized water was added to break up crystallization and the bottom layer was again removed using a pipette then placed in tube #3. 0.5mL of NaCL was added to test tube #1 following this removal.



3.) 0.15mL of MTBE was added to tube #2 as drops of hydrochloric acid were dropped into the tube carefully. Excess solvent was removed with a Pasteur pipette as crystals were liquefied in a hot sand bath. Following the observed change from solid to liquid, test tube #2 was allowed to cool slowly at room temperature and placed into an ice bath. Crystals were collected on a Hirsch funnel using the procedure utilized in the crystallization lab. 4.) The procedure for test tube #2 in step 3 was replicated as well for test tube #3. For both tubes, individual filter papers were tared on a scale and dried crystals were placed on the paper. Weight was calculated to assess percentage of recovery. 5.) Melting points were determined for all three crystallized products to assess purity. This was accomplished using the Mel Temp apparatus as comparisons were made to standard melting points.


 * EXPERIMENTAL MELTING POINTS FOR THE NEUTRAL SUBSTANCE, CARBOXYLIC ACID, AND PHENOL: **
 * Table of possible compounds and pka's: **

__**Analysis/ Discussion:**__

When executing experimentation of this particular lab, it is crucial to understand the potential results from the acid base chemistry of the compounds. A successful extraction can then occur as long as those concepts are understood. It is beneficial to know which compounds dissolve into distinct solutions. The two solutions used in lab were an ether layer of t-butyl methyl ether and an aqueous layer of water accompanied by a base. The compounds in the mixture all dissolved in the ether solution but were insoluble in the aqueous layer until further conditions allowed for solubility. The reason for such a reaction in the neutral compound, carboxylic acid and phenol is that all compounds are covalent solids in ether. Water is a poor solvent of covalent molecules but rapidly dissolves ionic compounds.

While benzoic acid has a pKa of 4.17, it is a stronger acid then 4-//tert//-Butylphenol with a pKa of 10.17. If there are two distinctly different pKa’s, then ionization will occur only to one of the acids. This will leave the other suspended in the ether solution. Weak bases have the ability to ionize strong acids. An aqueous solution of sodium bicarbonate (a weak base) was added to the ether in the reaction tube. This allowed for benzoic acid to ionize as a solid sodium benzoate. The resulting sodium benzoate is, in turn, soluble in water. The final result of this addition will leave the neutral compound and the Phenol (weak acid) in the ether layer. The ionized sodium benzoate will precipitate out of the ether and dissolve in the aqueous layer below. The Aqueous layer was then removed by a pipette leaving only the phenol and the neutral compound suspended in the ether solution.

In the process of extraction a judgment of the densities of the two solutions must take place to ensure proper removal from the solution. //T-//butyl methyl ether as a density of 0.7404 g/ml while water has a density of 1g/ml. Since ether has a lower density than water, this layer will always reside on top of the aqueous layer.

Phenol was ionized by the addition of sodium hydroxide to the solution. Strong bases are successful at removing the proton from weak acids. The result of this addition was a phenolate anion that precipitated into an aqueous layer. A distinct amount of crystals were observed on top of the ether layer indicating a saturation of this layer. The addition of 5 drops of water allowed for all ionic phenol molecules to dissolve in water. The aqueous layer was then removed leaving behind the neutral compound and the ether.

Once all three compounds were separated into reaction tubes the process of crystallization began. In the first reaction tube containing only the neutral compound and the ether was placed on a hot bath to evaporate the ether leaving behind the crystals of the neutral compound. Crystals were observed to be clear and plate like leaving enough recovery for melting point calculations. The second and third reaction tube contained the aqueous solution of the base and the anions of the intended recovery. Before crystallization was preformed, hydrochloric acid (a strong acid) was placed into the reaction tubes. The addition of the strong acid lowered the pH of the reaction tubes and the phenol and carboxylic anions reacted. This resulted in a precipitate of the carboxylic acid and phenol. To aid in extracting the solid phenol and carboxilic acid, calcium chloride pellets (an effective drying agent) was added to the reaction tubes to absorb any remaining water, leaving behind only the precipitate desired

Possible sources of error:

The order of extraction occured chronologically starting with benzoic acid, phenol and the neutral substance. The melting point readings indicated that benzoic acid ,which re-crystallized, was indeed pure and the melting point matched the known melting range of benzoic acid. Both phenol and the neutral substance had melting ranges that proved inconsistent with known melting ranges for both compounds. Therefore, accurate identification and purity of the neutral substance is debatable. Since phenol and the neutral substance were extracted after the benzoic acid, it is possible that the layers could have mixed and contaminated the extracted layer from the pipette.

A procedure known as backwashing was used in an effort to remove any possible contaminates (organic material specifically) from both the neutral substance and phenol solutions. By adding extra ether to each solution an exagerated layer seperating the intitial solutions are created. Any contaminates present will transfer to the ether layer and then can be removed via pipette. Though the experiment only called for a single backwash, multiple attempts would have resulted in a more pure solution. This could have gained closer accuracy in melting ranges and reflected a more pure substance.

The precision required to pipette a specific layer out of a reaction tube becomes more accurate with experience. Some level of contamination can be anticipated when multiple lab technicians are participating in this procedure which results in an experiment with a lower degree of control. In addition, glassware from the micro-lab kit utilized during experimentation could have contaminated samples of phenol and the neutral substance. Such contamination could have fluctuated melting point ranges from the expected temperature readings.

The final procedure of collecting the crystals yielded from the re-crystallization for analysis of melting ranges could have resulted unwanted contaminates that affected the melting ranges. Possible contaminates could have originated from the laboratory equipment used to harvest the crystals or the surface used to collect crystals into the capillary tubes. Either situation could potentially result in a lower then expected melting range. Yes, your results leave something to be desired, but you obviously get the point of the lab, and you have written it up very well. Nice job. Really nice job.

This lab earned the following scores for: format (2/2) style (2/2) data (3/3) quality of the result (1/1) quality of the reported data (1/1) conclusion (2/2) error (1/1) post-lab Q (2 free points) for a total of 14/14. A pleasure to read!

Density of MTBE found at: [] Density of water: http://www.wolframalpha.com/input/?i=density+of+water