JOURNAL OF CHEMICAL EDUCATION
VARIATION IN REACTIVITY-A
DEMONSTRATION
DOUGLAS M. EOWEN Dartmouth College, Hanover, New Hampshire
T H E wide variation in reactivity that may occur among members of the same homologous series, perhaps even closely related isomers, frequently seems incomprehensible to students beginning the study of organic chemistry. Still harder to grasp is the fact that even the sequence of reactivities among several homologous compounds may depend upon the reaction and the experimental conditions selected. The student fails to appreciate and perhaps to believe, unless shown, that although n-butyl alcohol is essentially unaffected by refluxing with hydrochloric acid and t-butyl alcohol reacts very rapidly a t room temperature, quite the opposite is true when the two alcohols are treated with metallic sodium. The isomeric halides normal and tertiary butyl bromides may beused conveniently in three simple demonstrations as rather striking examples of varying reactivity. The reactions involved in these experiments are : (1) The displacement of bromine by iodine from sodium iodide dissolved in acetone RBr + I - + Na+- RI + Na+, Br-4
suspension after ten to fifteen seconds; an immediate and copious precipitate of silver bromide will form on addition of the tertiary bromide to the second tube. The latter precipitation is so rapid that the student may suspect the presence of traces of hydroeen bromide; that such is not the actual case is shown by the third test. Reaction (8). Add the required amount of a suitable indicator of the universal type' to a solution of 30 ml. of water in 100 ml. of acetone; then add enough 0.1 N sodium hydroxide solution dropwise to bring the solution, as shown by the indicator, to a pH of about 9. Divide the resulting solution into two approximately equal portions and add to each 0.5 ml. of the appropriate bromide. The color of the solution of n-butyl bromide will remain unchanged for a t least one hour, but after a second or two the solution containing the tertiary bromide will pass fairly rapidly (as indicated by the succession of color changes) through neutrality to a pH of about 5. Since the original color of the second solution will not change instantaneously the possibility of any significant amount of hydrogen bromide in the alkyl bromide can he rejected. The color (2) The displacement of bromine by a nitrate group cycle, furthermore, can be repeated frequently by addfrom silver nitrate dissolved in alcohol ing quickly enough sodium hydroxide solution to restore the original basicity; almost a t once the solution RBr + NOa- + AgC RNOI Ag", Br-4 will again become acidic. (3) The hydrolysis by hydroxide ion or water moleFor the purposes of a beginning class the instructor cules in aqueous acetone solution will probably find it inadvisable to give an explanation RBI + OHROH + Br-; of these differences in reactivity other than an indicaRBr + 2H,O ROH + HsO+ Br- tion that the reaction mechanisms vary considerably. The students may be curious regarding the rapid Reaction (1). To a test tube containing 5 ml. of hydrolysis of t-butyl bromide in acidic solution. More sodium iodide-acetone solution (15 g. of sodium iodide advanced students may be interested in the actual in 100 ml. of pure acetone) add 0.50 ml. of n-butyl bro- mechanisms involved; an excellent discussion has been mide. Do likewise with t-butyl bromide. After one to given by Hamrnett.% The first two of these reactions two minutes the solution of the primary halide will he- have been used for the characterization of organic come turbid and then deposit a crystalline precipitate of halide^.^ sodium bromide. At least one hour will be required for the precipitation of a detectable amount of sodium bro1 Amend Universal Indicator is very satisfactory (Amend Drug rtnd Chemical CC, New York 10, N. Y.). midein the second tube. ¶ H m ~ ~ rL.r P., , "Physical Organic Chemistry," McGrawReaction (8). Use the same procedure but for the Book Co., New York, 1940. substitution of an alcoholic solution of silver nitrate Hill 8 SHRINER, R. L., ~ L N DR. C. FUSON, "The Systematic Identifi(2% solution in 95% ethanol). Normal butyl bromide cation of OrganicCompounds," 3rd ed., John Wiley and Sons. will give a faint turbidity almost a t once and a milky New York, 1948.
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