Nucleophilic Substitution

Introduction:

Alkyl halides undergo many reactions in which a nucleophile displaces the halogen atom bonded to the central carbon of the molecule. The displaced halogen atom becomes a halide ion.

Some typical nucleophiles are the hydroxy group (? OH), the alkoxy group (RO? ), and the cyanide ion (C N). Reaction of these nucleophiles with an alkyl halide (R—X) gives the following reactions and products:

The halogen ion that is displaced from the carbon atom is called the leaving group, and the overall reaction is called a nucleophilic substitution reaction.

Procedure:

Sodium Iodide in Acetone. Acetone, with a dielectric constant of 21, is a relatively nonpolar solvent that will readily dissolve sodium iodide. The iodide ion is an excellent nucleophile, and the nonpolar solvent, acetone, favors the Sn2 reactions; it does not favor ionization of the alkyl halide. The extent of reaction can be observed because sodium bromide and sodium chloride are not soluble in acetone and precipitate from solution if reaction occurs.

Ethanolic Silver Nitrate Solution. Label eleven small containers and place 0. mL or 100 mg of each of the following halides in the tubes: 1-chlorobutane, 2-chlorobutane, 2-chloro-2-methylpropane, 1-bromobutan e, 2-bromobutane, 2-chloro-2butene, Iodoethane, 1-Chloroadamantane, Bromobenzene, 1-chloro-2-butene, 1-chloro-2-methylpropane. To each tube rapidly add 1mL of an 18% solution of sodium iodide in acetone. Repeat the procedure with 1% ethanolic silver nitrate solution.

Conclusion

NaI in acetone favors an SN 2 reaction as the iodide nucleophile is strong and acetone is an aprotic solvent which will not help stablize a carbocation intermediate. -chloro-2-butene forms a very stable carbocation because the allylic resonance structures are able to delocalize the positive charge very effectively. 2-bromobutane is a secondary halide and can go either way, but the conditions will favor SN2. 1-chloro-2-methylpropane, and iodoethane are all primary halides and cannot go SN1 and all the conditions very much favor SN2. 1- chloroadamantane is slowed down that backside attack is near impossible so SN1 is the only option, and although acetone does not favor SN1, SN2 is not going to happen.

When ethanoic silver nitrate is used, SN1 is favored because thermodynamics forces any slight ionization of the halides strongly to the right by removing the silver ion as an insoluble silver halide. All secondary and tertiary carbons will form carbocations and react as SN1. Even primary carbons will form carbocations and later rearrange to a more stable ones via hydride/methyl shifts. All of these compoounds should react as SN1. Also as ethanol is a polar protic solvent it will help stablize the carbocation formed throught he formation of hydrogen bonding structures with the solvent.

Temperature will increase the rate of both SN1 and SN2. Certainly, inceased temperature will always favor elimination over substituion. SN1 is more favored by increase in temperature. For all the cases where the slower rate of cation formation is helped by additional thermal energy, there are other cases where difficult SN2 reactions are equally assisted by higher temperatures, the higher temperatures may be offset by the shift to E2 rather than SN2 if the nucleophile is also a strong base.