Kathleen Armstrong
Foothill College Chemistry

Project 2

This lab requires you to separate a mixture of two organic compounds, purify them, and then characterize them from their melting points. The purity of the isolated compounds will be assessed by TLC (Thin Layer Chromatography) and each will be identified as one of 8 possible unknowns by mixed-melting point.

Acid-Base Extraction:

You will separate the mixture using Acid-Base Extraction. Extraction is the most efficient method for separation of organic components that differ substantially in their water solubility. In extraction, a solute is partitioned between two immiscible layers according to its solubility in each. Most neutral organic compounds are water insoluble but soluble in an immiscible organic solvent like Diethyl ether. As a result, an extraction procedure can effect the separation of an organic product from any relatively polar inorganic compounds or salts.

Acid-base Extraction is a sub-category of Extraction in which the water layer is made acidic or basic in order to convert one component of the mixture to an ionic compound making it water soluble so that it may be separated from other relatively hydrophobic organic compounds.

Most organics are soluble in polar organic solvents like dichloromethane or diethylether, but only very small organics (≤ six carbons) or very polar neutral organic compounds are soluble in water. Therefore a mixture containing a relatively polar carboxylic acid (RCOOH) like benzoic acid and a relatively non-polar hydrocarbon like naphthalene (C10H8) both will dissolve in organic solvent and not water. If the mixture contains, instead an organic salt (like sodium benzoate NaC7H5O2) and a neutral organic like naphthalene (C10H8), the two can be easily separated by adding sufficient water to make the sodium benzoate dissolve, leaving the naphthalene behind.

In order to be sure that we don't add so much water that even the naphthalene dissolves (given enough solvent anything will dissolve), we would first dissolve both components of the mixture in an organic solvent, then we would add a small portion of water to allow the ionic salt to dissolve while allowing the napthalene to stay behind in the organic layer.

Consider the solubility chart below. Why is it that benzoic acid dissolves in base but not in water? The answer is that the base is mostly water, but contains solvated hydroxide which will instantly deprotonate the benzoic acid on contact and will thereby create the ionic sodium benzoate. The benzoic acid disappears into the aqueous layer because it has become a salt and is now soluble in water.

Solubility
 
Ether
Water
1 M HCl
1M NaOH

Benzoic Acid
PhCOOH

–––
–––

Napthalene
C10H8

–––
–––
–––
Aniline
PhNH2
–––
–––
Ethanol
CH3CH2OH

Project 2.1

You will be assigned an unknown to separate. Your unknown contains two of the components listed in the table below. Keep track of which layer is aqueous and which is organic. Label the lower layer each time it is drained from the Separatory funnel. One of the components in your mixture will be soluble in water following its deprotonation by NaOH. The other does not have an acidic proton and so it will remain in the organic layer. Once the components are separated, they each must be neutralized and then purified by recrystallization.

Project 2.2:

You will be taking the melting points of your recrystallized products once they are dry. Once you have two readings that agree within ±1°, you will identify your unknown as one of the following:

Compound Melting Point (°C)
2-nitrobenzoic acid 146-148
trans-stilbene 122-125
benzoic acid 121-122
phenanthrene 100-101
3-methylbenzoic acid 108-111
fluorene 116-117
2-methylbenzoic acid 101-104
benzoin 135-137

You will be graded on the correct identification of your unknowns, so be sure to follow the proper technique, heating slowly and taking a mixed-melting point if necessary.

In order to prepare for this lab, you will need to read Project 2.1 and 2.2 (beginning with page 243 in "Modern Projects and Experiments in Organic Chemistry") carefully. Also you must review Mohrig's "Techniques" text on Extraction, Recrystallization, Drying, Vacuum Filtration and Melting Points. See the correlation guide for the appropriate sections of the 3rd edition. If you have the second edition, please review the Table of Contents. Please also review the brief tutorials provided under the Techniques tab on the LabNotes page here.

Once you have read the text, you will need to prepare your notebook with the following:

  1. Purpose

  2. Net Reaction
    Show the acid-base reactions that are relevant to this extraction procedure. Since you do not know the specific carboxylic acid, use the notation RCOOH for the carboxylic acid and RH for the alkane.

  3. Table of Reactants/ Products/ Solvents
    Possible Unknowns are given in the table below. Include these in your table along with any solvents (density, bp and handling info).


    Compound

    MW

    Lit MP(°C)

    Source

    Primary Hazard

    2-nitrobenzoic acid

     

     

     

     

    benzoin

     

     

     

     

    fluorene

     

     

     

     

    2-methylbenzoic acid
    (o-toluic)

     

     

     

     

    phenanthrene

     

     

     

     

    stilbene

     

     

     

     

    3-methylbenzoic acid (m-toluic)

     

     

     

     

    benzoic acid

     

     

     

     
  4. Theoretical Yield Calculation in mL
    N/A

  5. Procedure
    Do not recopy the entire procedure into your notebook. Rather make this an abbreviated outline of the procedure that will enable you to move quickly through the procedure. It should detail what is to be done not how each step is to be done. Save room to the right of your procedural outline for data and observations corresponding to each step. Begin the procedure on a separate page so that you have both the original and the copy in your notebook during the lab (you will be turning in the earlier pages of your prelab at the beginning of the lab lecture).



During the lab:

Write down your unknown number before you begin. You will lose points if you do not record data and observations during the lab. Remember to date every entry, record all weights, observations, corrections and melting points IN PEN IN YOUR NOTEBOOK. You will be marked off if I catch you recording data on scratch paper, or if you do not have your notebook with you when you take a measurement. Both partners must record data. Do not leave the lab until all data is recorded in your own notebook.

Before you leave, turn in your data sheet (yellow copy).

Following Lab:

Complete the following Summary Sheet (.doc) and turn it in along with the Thermometer Calibration lab. Keep a copy of both summary sheets in your notebook.

This Lab is due one week from the day that it is completed unless an earlier date is specified by your instructor

 

 


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©Kathleen Armstrong July, 2013