Relative Reactivities of Primary, Secondary and Tertiary Alcohols

Equipment

Three small screw top vials, pipettes w/ bulbs, overhead projector.

Reagents

1-butanol, 2-butanol, 2-methyl-2-propanol, Lucas reagent (16 g anhydrous zinc chloride dissolved in 10 mL concentrated hydrochloric acid).

Presentation

1. Add 5 drops of an alcohol to a vial.
2. Repeat step one with the other two alcohols and separate vials.
3. Add three squeezes of the Lucas reagent to the primary alcohol vial, screw the lid on, shake vigorously, and lay the vial on its side on the center of the overhead projector.
4. Repeat step 2 for the secondary alcohol and then the tertiary alcohol.

Hazards

Vapors of these three alcohols are mildly irritating to eyes, nose, throat, and are toxic upon inhalation. Liquid contact with eyes is irritating and may produce burns. Repeated contact with skin may dry (defat) and crack skin.
Hydrochloric acid can irritate the skin. Hydrochloric acid vapors are extremely irritating to the eyes and respiratory system. Therefore, it should be handled only in well-ventilated area.

Discussion

The tertiary alcohol will react immediately to produce an insoluble alkyl halide. This makes the contents of the vial cloudy and the vial will appear black on the screen. The secondary alcohol reacts more slowly to produce the corresponding alkyl halide. The time varies from approximately 1-3 minutes depending upon the temperature. The screen image will initially appear clear, but will gradually darken to black. The primary alcohol will not react, so its screen image is clear and remains so.

References

1. Pavia, Lampman, Kriz, Introduction to Organic Laboratory Techniques, 2^nd edition, Saunders College pub., 1982, p. 427.

Equipment

Three 50 mL beakers w/ stir rods, pipettes w/ bulbs, overhead projector.

Reagents

1-butanol, 2-butanol, 2-methyl-2-propanol, chromic acid (1 g chromium (VI) oxide dissolved in 1 mL of concentrated sulfuric acid and this diluted with 3 mL of DI water).

Presentation

1. Add enough of the three alcohols to separate beakers to cover the bottoms.
2. Place the beakers on the overhead projector.
3. Add ~2 drops of the chromic acid solution (enough to give the mixture a definite yellow color) to a beaker and stir the contents.
4. Repeat step three for the other two beakers.

Hazards

Vapors of these three alcohols are mildly irritating to eyes, nose, throat, and are toxic upon inhalation. Liquid contact with eyes is irritating and may produce burns. Repeated contact with skin may dry (defat) and crack skin.
Because sulfuric acid is both a strong acid and a powerful dehydrating agent, it must be handled with great care. The dilution of concentrated sulfuric acid is a highly exothermic process and releases sufficient heat to cause burns. Therefore, when preparing dilute solutions from the concentrated acid, always add the acid to the water, slowly, with stirring and cooling the receiving beaker.
Both chromic oxide (VI) and chromic acid are powerful oxidizing agents and suspected human carcinogens. Contact with solid chromic anhydride may result in severe burns of eyes with possible permanent vision loss. Skin contact with solid chromic anhydride or with concentrated solutions of chromic acid causes burns with potential for major skin damage.

Discussion

Primary and secondary alcohols will be oxidized to carboxylic acids and ketones respectively. The tertiary alcohol will not react. When the alcohol is oxidized, the Cr⁶⁺ (yellow/orange) is reduced to Cr³⁺ (blue/green). Therefore the appearance of a blue green color is indicative of a primary or secondary alcohol. The beakers that have reactions take place may become opaque from a variety of precipitates. The colors can still be viewed from the side.

References

1. Pavia, Lampman, Kriz, Introduction to Organic Laboratory Techniques, 2^nd edition, Saunders College pub., 1982, p. 427.