Question

Read the experiment and briefly describe what you would predict to be your result if you were to perform this experiment and why.

Part A: Determination of Factors Affecting the Relative Rates of Sy2 Reactions All test tubes must be dry. Effect of Structure of the Alkyl Halide Measure 2 mL of 15% sodium iodide in acetone into each of three clean, dry 10-cm test tubes. Add 2 drops of 1-bromobutane (butyl bromide) to the first test tube; add 2 drops of 2-bromobutane (sec-butyl bromide) to the second test tube; and add 2 drops of 2- methyl-2-bromopropane (tert-butyl bromide) to the third test tube. Stopper the tubes with corks. (Do not use rubber stoppers.) Shake the tubes to mix. Observe closely dur ing the first 15-20 minutes, then at intervals throughout the lab period. Observe the test tubes for a sign of cloudiness or precipitation. Record your observations. Steric Effects Measure 1 mL of 15% sodium iodide in acetone into each of three clean, dry 10-cm test tubes. Into one tube, add 2 drops of 1-bromobutane; into the second test tube, add 2 drops of 1-bromo-2-methylpropane (isobutyl bromide); into the third test tube, add 2 264 Chapter Eight Introduction to Nucleophilic Substitution Reactions drops of 1-bromo-2,2-dimethylpropane (neopentyl bromide). Stopper the tubes and shake. Observe closely. Record your observations. Effect of the Leaving Group Measure 1 mL of 15% sodium iodide in acetone into each of two clean, dry 10-cm test tubes. Add 2 drops of 1-bromobutane into one test tube and add 2 drops of 1-chlorobutane into the other test tube. Stopper and shake the tubes. Observe closely. Record your observations. Determination of the Rate Law Measure 1.0 mL of a 15% solution of sodium iodide in acetone into two clean, dry test tubes. At the same time (or do one at a time, marking the time carefully!), add 0.1 mlL (or 2 drops) of 1.0 M 1-bromobutane to one test tube and add 0.1 mL (or 2 drops) of 2.0 M 1-bromobutane to the other. Record your observations and compare the rates of the two reactions Measure 1.0 mL of a 1.0 M solution of 1-bromobutane into two clean, dry test tubes. Into one test tube, add 0.1 mL (or 2 drops) of a 7.5% solution of sodium iodide in acetone. At the same time, add 0.1 mL (or 2 drops) of a 15% solution of sodium iodide in acetone into the other test tube. (Alternately, use one test tube at a time, noting care- fully the amount of time for precipitate formation.) Record your observations and com pare the rates of the two reactions. Part B: Determination of Factors Affecting the Relative Rates of SNl Reactions All test tubes must be dry. Effect of Structure of the Alkyl Halide Measure 2 mL of a 0.1 M solution of silver nitrate in absolute ethanol into each of three clean, dry 10-cm test tubes. Add 1 drop of 1-bromobutane into the first test tube, 1 drop of 2-bromobutane into the second, and 1 drop of 2-bromo-2-methylpropane to the third. Stopper and shake the tubes continually. Watch closely for evidence of cloudiness or the formation of a white precipitate. Record all observations. Relative Rates of Nucleophilic Substitution Reactions 265 Experiment 8.1 Effect of the Leaving Group Measure 2 mL of a 0.1 M solution of silver nitrate in absolute ethanol into two clean, dry test tubes. Add 1 drop of 2-bromo-2-methylpropane into one test tube. At the same time, add 1 drop of 2-chloro-2-methylpropane into the other test tube. Stopper and shake. Record your observations. Effect of Solvent Polarity Measure 2 mL of a 0.1 M solution of silver nitrate in absolute ethanol into a dry test tube. Into a second dry test tube, measure 2 mL of a 0.1 M silver nitrate solution in 5% ethanol/95% acetone. At the very same time (get a partner to help!) add 1 drop of 2- chloro-2-methylpropane to each of the test tubes. Stopper and shake. Observe. Record your observations. Determination of the Rate Law To a clean, dry test tube, add 0.5 mL of 0.1 M 2-chloro-2-methylpropane in ethanol. To another clean, dry test tube, add 0.5 mL of 0.2 M 2-chloro-2-methylpropane in ethanol. At the same time, add 1.0 mL of 0.1 M silver nitrate solution in ethanol to each of the two test tubes. (Alternately, do one at a time and carefully note the time it takes for pre- cipitation.) Record your observations and compare the rates for the two reactions. To a clean, dry test tube, add 1.0 mL of 0.1 M silver nitrate in ethanol. To another clean, dry test tube, add 0.5 mL of 0.1 M silver nitrate solution in absolute ethanol and 0.5 mL absolute ethanol (to ensure that the volumes are the same in the two test tubes). At the same time, add 1.0 mL of 0.1 M 2-chloro-2-methylpropane in ethanol to each of the two test tubes. (Alternately, work with one test tube at a time and carefully note the time it takes for precipitation.) Record your observations and compare the rates for the two reactions.

Answer 1

In case for Sn2 reaction

Structure of alkyl halide: Primary alkyl halides are most likely to undergo Sn2 reaction.Since the formation of intermediate is easier for primary,then comes the secondary alkyl halide and at last comes the tertiary alkyl halide.

Stearic Effects: If the alkyl halide is hindered ,then the rate of formation of the transition state is difficult in case of Sn2 mechanism.Hence if the alkyl halide is stearically hindered then the rate of Sn2 reaction is very low.

Effect of Leaving Group: If the leaving group is a strong leaving group.Then the reaction is faster.Since Iodine is a strong leaving group,the rate is faster in case of iodides,then bromides and then comes chlorides.

Determination of rate law:From the intermediate transition state both substrate and nucleophile are responsible for the formation of intermediate.Hence it is a second order reaction.

For Sn1 Reaction:

Structure of alkyl halide:The rate of Sn1 reaction depends on the stability of carbocation.As we know that tertiary carbocation is more stable than secondary carbocation then at last comes the primary carbocation.Hence tertiary alkyl halide is more reactive and the rate of Sn1 reaction is faster in case of Tertiary alkyl halide.

Effect of leaving group:The carbocation is the intermediate.There is no effect or presence of leaving group in the intermediate carbocation.But from the stability of ions I^- is relatively stable in gaseous state but flouride is stable in aqueous state but exothermic.Hence cloride and Bromide have high rate of reaction.

Effect of solvent polarity:The more polar the solvent the more stable the intermediate carbocation is.Hence a more polar solvent has a faster rate of Sn1 mechanism.In presence of silver nitrate the rate of reaction is faster.

Determination of the rate law:The intermediate is only carbocation.Hence the rate of reaction depends only on concentration of substrate but independent of the concentration of the nucleophile.Hence its a first order reaction.