The reaction of an alkyl chloride with potassium iodide is generally carried out in acetone to maximize the amount of alkyl iodide that is formed. Why does the solvent increase the yield of alkyl iodide?


(Hint: Potassium iodide is soluble in acetone, but potassium chloride is not.)

Verified step by step guidance

1. Understand the role of the solvent in the reaction: Acetone is used as the solvent in this reaction primarily because it can dissolve potassium iodide (KI) effectively.
2. Recognize the solubility differences: Potassium iodide is soluble in acetone, allowing it to dissociate into K+ and I- ions. This increases the availability of iodide ions to react with the alkyl chloride.
3. Consider the reaction mechanism: The reaction between alkyl chloride and iodide ion is a nucleophilic substitution reaction (SN2), where the iodide ion attacks the alkyl chloride, displacing the chloride ion.
4. Analyze the effect of precipitate formation: As potassium chloride (KCl) is formed, it precipitates out of the solution because it is not soluble in acetone. This removal of KCl drives the reaction forward, shifting the equilibrium towards the formation of more alkyl iodide.
5. Summarize the impact of acetone: By using acetone as a solvent, the reaction is driven towards the formation of alkyl iodide due to the effective solubility of KI and the precipitation of KCl, thus increasing the overall yield of alkyl iodide.

Key Concepts

Solubility and Reaction Dynamics

The solubility of reactants in a solvent significantly influences the reaction dynamics. In this case, potassium iodide is soluble in acetone, allowing it to dissociate into potassium and iodide ions, which can readily participate in the nucleophilic substitution reaction. Conversely, potassium chloride is not soluble in acetone, which limits its presence in the reaction mixture and thus favors the formation of alkyl iodide.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group (like chloride) by a nucleophile (like iodide). The efficiency of this reaction can be enhanced by using a solvent that stabilizes the nucleophile and the transition state. In acetone, the iodide ion is better able to attack the alkyl chloride, leading to a higher yield of alkyl iodide.

Polar Aprotic Solvents

Acetone is classified as a polar aprotic solvent, which means it has a significant dipole moment but does not have hydrogen atoms bonded to electronegative atoms that can participate in hydrogen bonding. This property allows polar aprotic solvents to solvate cations effectively while leaving anions like iodide free to act as nucleophiles. This enhances the nucleophilicity of iodide, promoting the formation of alkyl iodide in the reaction.