Identification of an Unknown

In this experiment, you will be given an unknown organic compound and you will use a series of chemical and spectroscopic tests to determine its identity. You will only need to fill out the following form during lab; there is no prelab or report due. The possible main functional groups include: aldehydes, amines, carboxylic acids, ketones, and alcohols. Other functional groups may also be present. The structures and names of the possible unknowns will be posted in the laboratory. You will obtain a small vial of the unknown and should then carry out the following tests, although the order in which you carry out these tests is not important.

1. Classify the liquid by color and smell.
2. Perform chemical tests described below. Write the chemical reactions for any positive tests.
3. Obtain an IR spectrum and analyze it, labeling any pertinent stretches for functional groups on the spectrum.
4. Obtain an NMR spectrum and analyze it. Identify functional groups based on chemical shifts and alkyl groups based on integration and splitting.
5. Propose a structure. You will be allowed to leave when you get it correct.

Alcohols react with acetyl chloride, giving off heat.

Cautiously add about 10-15 drops of acetyl chloride, drop by drop, to about 0.5 mL of the liquid unknown in a small test tube. Evolution of heat and HCl gas indicate a positive test.

Place 2 mL of Lucas reagent in a small test tube and add three to four drops of the alcohol. Stopper the test tube and shake it vigorously. Tertiary, benzylic, and allylic alcohols give an immediate cloudiness as the insoluble alkyl halide separates from the aqueous solution. After a short time, the immiscible alkyl halide will form a separate layer. Secondary alcohols produce cloudiness after two to five minutes. Some secondary alcohols may have to be heated slightly to make the reaction proceed. Primary alcohols dissolve in the reagent to give a clear solution.

R-OH + HCl R-Cl + H₂O

Aldehydes and ketones (but not esters) react with 2,4-dinitrophenylhydrazine (DNP) to give an orange or red precipitate, which indicates a positive test.

Place one drop of the liquid unknown in a small test tube and add 1 mL of the 2,4-DNP reagent. Shake the mixture vigorously. Most aldehydes and ketones will give a yellow to red precipitate immediately, but some compounds will require up to 15 minutes (or gentle heating) to give a precipitate. Note: Many DNP derivatives are suspected carcinogens and should be handled with care.

Alcohols (primary and secondary) and aldehydes react with chromic acid (Jones reagent) to give a blue-green precipitate in this test. Aldehydes are oxidized to carboxylic acids. Secondary alcohols are oxidized to ketones, and primary alcohols are oxidized first to aldehydes and then to carboxylic acids. Ketones and tertiary alcohols are unreactive under the test conditions.

Dissolve one drop of a liquid unknown in 1 mL of acetone. Add several drops of the chromic acid reagent a drop at a time, while shaking the mixture. A green precipitate and loss of the orange color of the reagent indicate a positive test. The precipitate may form within 30 seconds or it may take a few minutes. If a precipitate forms and the solution remains orange, the test is negative. Note: Many compounds of chromium are suspected carcinogens and should be handled with care.

Unknown Report Sheet

Unknown Number

Color/Smell

Solubility Tests

Possible Functional Group(s)

Chemical Tests

Write out chemical reactions for any positive tests.

Possible Functional Group(s)

IR Analysis (Attach labeled spectrum.)

NMR Analysis (Attach labeled spectrum)

Proposed Structure