| Demonstration of Microscale Projection Experiments -
Chemistry en miniature Objective: Nucleophilic Substitution - SN1 and SN2 Peter Keusch |
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. Hazards and safety precautions:
Safety glasses and protective gloves should be worn. Good ventilation required. Experimental procedure: The temperature of the water in the cuvette is approx. 60 °C (water bath). 1 mL of the blue test solution is pipetted into each of three test tubes. Then, using an Eppendorf pipette, 50 mL of the alkyl halide are added to the solutions in T1, T2 and T3 (see table below). A Pasteur pipette is placed in each of the test tubes. The solutions are simultaneously 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 bromide is liberated in the reaction. The color change of the indicator allows to demonstrate the different reactivity of the alkyl halides. A heterolytic fission of the C - Br bond occurs during the the hydrolysis process. The reactivity increases in the order 1-bromobutane < 2-bromobutane < tertiary butylbromide. The three hydrolysis reactions have the same nucleophile and the same leaving group. Hence, the rates of the SN reactions will depend only on the different structures of the butylbromide isomers. On the basis of mechanistical investigations can be proven: · The hydrolysis of primary halides proceeds by a SN2-mechanism. The reaction is bimolecular, i.e. two species are involved in the rate-determining step. The reaction rate depends on both the alkyl halide's (R - X) and the nucleophile's (Nu) concentration: v = k [ R - X] [ Nu ].  The SN2 (bimolecular nucleophilic substitution) involves rear-side attack of a nucleophile at the carbon atom, opposite to the leaving group being displaced. The incoming group replaces the leaving group in one step. Bond making and breaking occurs simultaneously. The "pentacoordinated" transition state of the SN2 reaction is a trigonal bipyramid with the nucleophile and the leaving group located at the tops of the pyramids and the three remaining substituents located at the corners of the trigonal base. As the incoming nucleophile begins to bond with the carbon, the leaving group is departing with the bonding electrons. As a result of the mechanism, the three remaining substituents are rejected. The inversion of configuration resembles the way an umbrella turns inside out in a strong gust of wind (1). If the substrate under nucleophilic attack is chiral, this leads to an inversion of stereochemistry, called the "Walden Inversion". · 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 an 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 (2). 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. This effect results in a mixture of two enantiomers (mirror image isomers). · The hydrolysis mechanism of the secondary butylbromide depends very strongly on the reaction conditions. 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
Rod Beavon SN2 Nucleophilic substitution - bimolecular - Animation Rod Beavon SN1 Nucleophilic substitution unimolecular - Animation General experimental instructions and index of experiments 
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