Sodium Iodide in Acetone Test for aliphatic halides

Another method for distinguishing between primary secondary, and tertiary halides makes use of sodium iodide dissolved in acetone. This test complements the alcoholic silver nitrate test, and when these two tests are used together, is possible to determine fairly accurately the gross structure of the attached alkyl group. The test depends on the fact that both sodium chloride and sodium bromide are not very soluble in acetone, whereas sodium iodide is. The reactions that occur are SN2 substitutions in which iodide ion is the nucleophile; the order of reactivity is primary > secondary > tertiary.

With the reagent, primary bromides give a precipitate of sodium bromide in about 3 min at room temperature, whereas the primary and secondary chlorides must be heated to about 500C before reaction occurs. Secondary and tertiary bromides react at 50C, but the tertiary chlorides fail to react in a reasonable time. It should be noted that this test is necessarily limited to bromides and chlorides.

SODIUM IODIDE IN ACETONE (for alkyl halides that can undergo Sn2 reactions) – Primary and some secondary alkyl chlorides or bromides will give a precipitate of sodium iodide in the reagent. Alkyl iodides will not give the precipitate. Aryl or vinyl halides do not react.

Procedure

In a test tube place 0.25 mL or 0.2 g of your unknown. Add 2 mL of a 15% solution of sodium iodide in acetone, noting the time of addition. After the addition, shake the test tube well to ensure adequate mixing of the unknown and the solution. Record the time needed for any precipitate to form. After about 5 minutes, if no precipitate forms, place the test tube in a 50oC water bath. Be careful not to allow the temperature of the water bath to go above this temperature since the acetone will evaporate, giving a false positive result. After 6 minutes more in the bath, if no precipitates are visible, remove the test tube and let it cool to room temperature. Note any change that might indicate that a reaction has occurred. Continue slow reactions for up to 45 minutes at room temperature.

Positive Test

The formation of a white precipitate indicates the presence of halides.

Complications

1.When the sodium iodide solution is added to the unknown, a precipitate of sodium iodide might occur leading to a false positive test. Upon mixing, the precipitate of sodium iodide should dissolve.

2.Excessive heating can cause the loss of acetone and the production of solid salt leading to a false positive test.