Chemicals:

1-bromobutane (M.W. = 137.03, d =1.2785)

2-bromobutane (M.W. = 137.03, d = 1.2585)

2-bromo-2-methylpropane (M.W. = 137.03, d = 1.221)

ethanol 95 %

0.1 M AgNO3 solution

If the butylbromides are not perfectly colorless, then they must be distilled. (1-bromobutane, b.p. 101.6°C; 2-bromobutane, b.p. 91.2°C; 2-bromo-2-methylpropane, b.p. 73.3°C). The butylbromides are to be stored in brown bottles.

heating mantle

thermometer

3 conical measures, graduated, 500 mL

3 glass stirring rods

round bottom flask 1.5 L

3 snap-cap vials 10 mL

measuring cylinder 500 mL

measuring cylinder 25 mL

Experimental procedure:

1.2 L of ethanol are heated to 60°C using a heating mantle and a round bottom flask.

Measured quantities of the butylbromide isomers are available in sap-cap vials:

Vial 1: 8.8 g (64 mmole) 1-bromobutane

Vial 2: 8.8 g (64 mmole) 2-bromobutane

Vial 3: 8.8 g (64 mmole) 2-bromo-2-methylpropane

20 mL of aqueous 0.1 M AgNO3 solution are added to each of three conical measures containing 400 mL of ethanol heated to 60°C. 1-bromobutane is poured into the first conical measure while stirring with a glass rod. 2-bromobutane and tertiary butylbromide, respectively, is added simultaneously with stirring to the alcoholic solutions of the conical measures 2 and 3, respectively.

Results:

Reaction is indicated by the formation of a pale yellow precipitate. The tertiary halide 2-bromo-2-methylpropane immediately forms a light yellow precipitate. 2-bromobutane reacts next - a turbidity of the reaction mixture can be observed within a few of seconds. After a couple of minutes, 1-bromobutane begins to react with AgNO3 solution.

· The rate, at which the heterolytic fission of the C-Br bond occurs, rises in the order 1-bromobutane < 2-bromobutane < tertiary butylbromide.The three reactions have the same nucleophile and the same leaving group. Hence, the rates of the SN-reactions will depend on the different structures of the butylbromide isomers.
· On the basis of mechanistical investigations can be proven: The primary halide reacts according to the SN2-mechanism (1), the tertiary halide according to the SN1-mechamism (2). The hydrolysis mechanism of the secondary butylbromide depends very strongly on the reaction conditions.