| Demonstration of Microscale Projection Experiments -
Chemistry en miniature Objective: Nucleophilic Substitution - SN1, Effect of the Leaving Group on Rate Peter Keusch |
If the butyl halides are not perfectly colorless, then they must be distilled (2-chloro-2-methylpropane, b.p. 51-52 °C; 2-bromo-2-methylpropane, b.p. 73.3 °C; 2-iodo-2-methylpropane, b.p. 130-131 °C). The butylhalides are to be stored in brown bottles. Hazards and safety precautions:
Safety glasses, protective gloves and good ventilation required. Experimental procedure: The temperature of the water in the cuvette is approx. 6 0°C (water bath). 1 mL of the blue test solution is pipetted into each of three test tubes. Then, using an Eppendorf pipette, the specified volume of the alkyl halide is added to the solutions in T1, T2 and T3.
A Pasteur pipette is placed in each of the test tubes. The solutions are mixed by gentle squeezing of the pipette bulb. Results: 
|  Discussion: · Bromothymol blue (transition range: pH 6.0 - 7.8) is an acid-base indicator that appears blue in an alkaline (base) medium, green in neutral, and yellow in an acidic solution. The solvolysis of the butyl bromide isomers is revealed by the indicator change from blue to yellow as hydrogen halide is liberated in the reaction. · The solvolysis of the tertiary butyl halides in water takes place by a SN1 mechamism . The reaction is unimolecular - only one species is involved in the slow step of the reaction. The reaction rate depends only on the concentration of the alkyl halide (R - X), not the nucleophile: v = k [ R - X ]. In a SN1 reaction the key step is the loss of the leaving group to form the intermediate carbocation. This step is the slow, rate determining step of the reaction. The carbocation is then attacked by a nucleophile in a fast second step to form the product. The more stable the carbocation is, the easier it is to form, and the faster the SN1 reaction will be. The planar, trigonal carbocation may be attacked equally well from either side by a nucleophile. As a consequence, a SN1 reaction leads to a racemization, in which both retention and inversion of configuration at a chiral center occur to the same extent. Optically active tertiary haloalkanes produce a mixture of two enantiomers (mirror image isomers). 
 · In the rate-determining step of SN1 reactions, the alkyl halide (R - X) is cleaved into a positively charged carbocation and a negatively charged leaving group. The reaction rate depends not only on the polarity of the solvent and on the stability of the carbocation, but also on the stability of the leaving group. The more stable the leaving group is, the more easily the C - X bond is also cleaved, the higher the reaction rate is. Conjugated bases of strong acids are good leaving groups. The experiment above shows that bromide ion is a better leaving group than the chloride ion. Bromide is a weaker base than chloride. The weaker base is more stable and thus more easily formed. References:  Demonstration Experiment on Video
Hydrolysis of Butyl Bromide Isomers
Demonstration Experiment on Video
Reaction of Butylbromide Isomers with Silver Nitrate
Demonstration Experiment on Video
Hydrolysis of tertiary Butyl Halides
Holger Schickor Nukleophile Substitution nach SN 2 - Animation Rod Beavon SN 1 Nucleophilic Substitution unimolecular - Animation General experimental instructions and index of experiments 
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