Identification of Unknowns

Identification of Unknowns

Unknowns:

Each of you will receive two vials: one containing a liquid unknown and one a solid unknown. You will perform a number of experimental procedures on these compounds to gather data. The compounds are part of a finite number of compounds that are listed for you in order of increasing mp and bp. You will determine the actual structure of your unknowns by applying your experimental data to these lists, obtaining a shorter list of possible compounds and performing further experiments to make the final determination.

Since each of you has a different compound, you will have to work more independently than before and you will have to evaluate your data and the reliability of your data.

Solubility tests:

For these tests, you should use approx. 30 mg of your compound in 1 mL of solution to give you a 3% solution. The words soluble and insoluble are qualitative, not quantitative. We will use 3% as a determination of solubility. If your compound is not completely soluble as a 3% solution, you can call it insoluble or partially soluble. Moreover, some compounds which are not soluble at room temperature, may be soluble at higher temperature. Therefore, after obtaining data at room temperature, heat your solution in a water bath at 70oC or higher and observe any further changes. The results you record may have more meaning later when you know more about your unknown compound. You will test the solubility/reactivity of your compound in 4 different solutions.

Procedure: Into 4 labeled test tubes, add 1 mL of : water; 5% aq HCl; 5% aq NaHCO3; and 5% aq. NaOH. To each tube, add approx. 30 mg of the unknown. Shake or stir for a few seconds. Record your observations. It may take several minutes for the unknown to appear to dissolve or react. After suitable time, heat the four test tubes in a water bath and observe. Look for color changes, evolution of gas, any evidence of reaction such as precipitates, and enhanced solubility. Record your observations.

Repeat the solubility test for the other unknown.

Most organic compounds are not water soluble. Only low molecular weight and/or highly polar compounds will be water soluble to 3%. Salts of acids or bases will often be soluble.

If the compound is not soluble in water, solubility in acid suggests that it is basic (eg, amines), solubility in strong base suggests that it is at least weakly acidic (eg, phenols) and solubility in weak base (bicarbonate) suggests that it is a stronger acid (eg, carboxylic acids).

Whatever determination you make from this solubility test, you will corroborate that suggestion with other data including class tests and analysis of the compound's infrared spectrum.

Compounds which are insoluble in all of the above liquids can be tested for solubility and color change in concentrated sulfuric acid. Compounds giving a color change or showing solubility include neutral compounds such as alkenes, alkynes, alcohols, ketones, aldehydes, esters, ethers, amides and nitro compounds. Compounds that appear inert include alkanes, alkyl halides, and simple aromatic hydrocarbons. Dispose of the sulfuric acid solution carefully by pouring it into a beaker of cold water, carefully rinsing the test tube with water, and washing both down the sink.

Class tests

All class tests must be done in triplicate: known, unknown, and blank. The known gives you a positive test for reference and tells you if the reagents or procedure are faulty. The blank gives you the other extreme, a negative test. The results you obtain from your unknown can then be put into perspective. The triplicate tests should be done simultaneously thereby taking little extra time. For each test, use clean test tubes. For certain tests, the presence of even trace amounts of acetone can give false positives.

Do only those tests that you think will be useful based upon solubility results and your analysis of the infrared spectrum of the compound. If that analysis has not been done and you wish to do some class tests, do them in the order shown below.

Beilstein test for halogenated compounds. This test is very sensitive. Take a piece of copper wire with a loop on the end and heat it in the flame of a Bunsen burner until it glows red. Allow the wire to cool but avoid contaminating it especially if you put it down on the hood surface. Dip the cool loop into the known compound, and place it in the flame. You should observe a green flame after the first few seconds when your known compound burns (yellow flame). Burn off all remaining halogenated known compound, heat the loop until it glows red, then let the loop cool and run the test on you unknown solid or liquid. For knowns use bromobenzene or chlorbenzene for the liquid and a halogenated benzoic acid for the solid. This test should be done in a hood.

Ignition Test for Aromaticity. Place a small amount of compound on the end of a spatula or on a porcelain lid and apply the flame from a Bunsen burner. Highly unsaturated compounds such as aromatic compounds burn with a yellow, sooty flame. Use naphthalene for the solid unknown and toluene or xylene for the liquid known. This test should be done in a hood.

Bromine Test for Unsaturation: Alkenes and alkynes will readily add bromine across the multiple bond unless there are electron withdrawing groups on the multiple bond. One observes the rapid disappearance of the red-brown bromine color. Aromatic compounds can react with bromine more slowly to give bromine substitution and the formation of HBr, which can sometimes be observed by placing a piece of wet litmus paper over the mouth of the test tube. Warning: the reagent deteriorates with the formation of HBr so compare the results with a blank.

Into a dry, clean test tube, dissolve 0.1 mL of a liquid (or 50 mg of a solid) in 1 mL of methylene chloride. Add a 2% solution of bromine in a dichloromethane dropwise with agitation until the bromine color persists. A positive test requires five or more drops of bromine solution to reach a persistent red-brown color. Use cyclohexene, octene, or another simple alkene as the known. Use toluene as a known to test for aromaticity.

Permanganate Test for Unsaturation

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