Substitution Rxns

Topics: Nucleophilic substitution, Solvent, Protic solvent Pages: 4 (843 words) Published: November 6, 2013

Substitution Reactions*
In class, we have been learning about substitution reactions and the various mechanisms by which they can occur. In a substitution reaction, a leaving group, usually a halide, is replaced by a nucleophile. Two mechanisms have been described for this reaction, a unimolecular (SN1) mechanism, and a bimolecular (SN2) mechanism.

The stepwise unimolecular mechanism proceeds through a carbocation intermediate. These reactions are favored by the use of a substrate that can form a stable carbocation, either through hyperconjugation or resonance. They are promoted by the use of polar protic solvents that can help stabilize the carbocation and leaving group, and often the solvent also serves as the nucleophile. For this experiment, you will be performing solvolysis reaction using ethanol.

The concerted bimolecular reaction has no intermediate. The reaction is promoted by the use of a polar aprotic solvent and requires the presence of a strong nucleophile. For this experiment, you will use sodium iodide (NaI) in acetone.

You will test nine substrates (Figure 1) under each of these two reaction conditions to see which compounds will do SN1, SN2, neither reaction or both. You will observe the formation of a precipitate upon reaction. For the SN2 reaction, NaI is soluble in acetone but the byproducts of reaction, NaCl or NaBr (depending on the leaving group of each compound) are not. For the SN1 reaction, silver nitrate (AgNO3) is added to the ethanolic reaction. This compound is soluble in ethanol, but the silver ion will form insoluble salts with the leaving group (AgCl or AgBr). Thus, observation of a precipitate in your test tube will indicate that a reaction has taken place, and the absence of a precipitate indicates that no reaction has occurred. You should note not just the presence of the precipitate, but how long it took to form and whether heat was required.

The goal of this experiment is to study these nine substrates...
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