Question

Predict the outcomes of the two sets of experiments to be carried out with the 11 halides used in this chapter. Group 1: 1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane, and bromobenzene Group 2: 1-chlorobutane, 2-chloro-2-methylpropane, 1-chloro-2-butene

Will be experimented with sodium iodide in acetone and ethanolic silver nitrate solution.

Answer 1

Group 1: We use a solution of sodium iodide (NaI) in acetone as the nucleophile (the I^- is the actual nucleophile). Acetone is a polar aprotic solvent and cannot stabilize carbocations. Hence, acetone favors S_N2 reactions.

NaI is soluble in acetone. One of the products of the reaction is sodium bromide (NaBr). NaBr is insoluble in acetone and precipitates out.

Primary alkyl bromides undergo S_N2 reactions the fastest followed by secondary alkyl bromides. Tertiary alkyl bromides do not undergo S_N2 reactions at all. Therefore, the order of reactivity with NaI in acetone is

1-bromobutane >> 2-bromobutane >>> 2-bromo-2-methylpropane, bromobenzene

                                                                (do not react via S_N2 mechanism)

Consequently, the solid precipitate of NaBr is obtained instantly with 1-bromobutane while it takes a few minutes for the precipitate to appear with 2-bromobutane. 2-bromo-2-methylpropane, being a tertiary alkyl bromide, does not react at all. Bromobenzene doesn’t undergo S_N2 reaction; bromobenzene undergoes electrophilic aromatic substitution and not nucleophilic substitution.

Group 2: As before, we are using NaI as the nucleophile (again, I^-), but the solvent is ethanolic silver nitrate. Ethanol is a polar protic solvent and can stabilize carbocations. Consequently, S_N1 mechanism is favoured.

One of the side products of the reaction is silver chloride (AgCl), which is insoluble in ethanolic silver nitrate.

Tertiary alkyl chlorides can easily form carbocations, since the carbocations are stabilized by electron-donating inductive effect. The solvent further helps to stabilize the carbocation via dipolar interactions. Primary alkyl chlorides do not form carbocations under any circumstance. Thus, the order of reactivity with NaI in ethanolic silver nitrate is

2-chloro-2-methylpropane >>> 1-chlorobutane, 1-chloro-2-butene

                                                   (do not react via S_N1 pathway)

1-chloro-2-butene doesn’t react with NaI to give a substitution product; rather addition of ethanol may take place to give a hydroxyl derivative. Consequently, the precipitate of silver chloride is observed only with 2-chloro-2-methylpropane.