Reaction of Butylbromide Isomers with Silver Nitrate

Demonstration Experiment on Video

Reaction of Butylbromide Isomers with AgNO₃

Objective: Nucleophilic Substitution - S_N1 and S_N2

Peter Keusch

German version

Chemicals:
1-bromobutane (M.W. = 137.03, d = 1.2785)
2-bromobutane (M.W. = 137.03, d = 1.2585)
tert-butylbromide (2-bromo-2-methyl propane) 98 % (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-methyl propane, b.p. 73.3 °C). The butylbromides are to be stored in brown bottles.

Apparatus and glass wares:

heating mantle
thermometer
3 conical measures, graduated, 500 mL
measuring cylinder 25 mL
measuring cylinder 500 mL
round bottom flask 1.5 L
3 snap-cap vials 10 mL
3 glass stirring rods

Hazards and safety precautions:

Butylbromide isomers are harmful if inhaled or swallowed. Eye and skin irritant.
Silver nitrate is poisonous. Long-term exposure can cause permanent blue-grey staining of eyes, mouth, throat and skin, (argyria) and may cause eye damage.

Ethanol is highly flammable.

Safety glasses and protective gloves should be worn. Good ventilation required.

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) tert-butylbromide

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.

Video clip (Download RealPlayer .rm file)

Discussion:

· The butylbromide isomers react with AgNO₃ solution to form a precipitate of AgBr.

· The rate, at which the heterolytic fission of the C - Br bond occurs, rises in the order 1-bromobutane < 2-bromobutane < tert-butylbromide. The three reactions have the same nucleophile and the same leaving group. Hence, the rates of the S_N 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 S_N2 mechanism (1) , the tertiary halide according to the S_N1 mechamism (2). The hydrolysis mechanism of the secondary butylbromide depends very strongly on the reaction conditions.

References:
Rod Beavon S_N2 Nucleophilic substitution bimolecular - Animation Rod Beavon S_N1 Nucleophilic substitution unimolecular - Animation Demonstration Experiment on Video Hydrolysis of Butyl Bromide Isomers
Demonstration Experiment on Video Hydrolysis of tertiary Butyl Halides

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