2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:
c. A trans reactant is more reactive than a cis reactant.
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Identify the reactant and the product in the reaction. The reactant is 2-acetoxycyclohexyl tosylate, and the product is 1,2-cyclohexanediol diacetate.
Recognize that the reaction involves a nucleophilic substitution where the acetate ion (CH3CH2CO-) attacks the carbon attached to the tosylate group (OTs), leading to the formation of the diacetate product.
Understand that the stereochemistry of the reactant affects the reactivity. The trans isomer has substituents on opposite sides of the cyclohexane ring, while the cis isomer has substituents on the same side.
Consider the steric hindrance in the reaction. The trans isomer has less steric hindrance because the bulky groups are further apart, making it easier for the nucleophile to attack the carbon attached to the tosylate group.
Conclude that the trans reactant is more reactive than the cis reactant due to the reduced steric hindrance, allowing for a more efficient nucleophilic substitution reaction.
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Key Concepts
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Stereochemistry
Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In this reaction, the stereochemistry of the reactants influences the stereochemical outcome of the products. The presence of asymmetric centers in 2-acetoxycyclohexyl tosylate leads to different stereoisomers, which can react differently based on their spatial orientation.
The reactivity of stereoisomers can vary significantly due to steric and electronic factors. In this case, the trans isomer of 2-acetoxycyclohexyl tosylate is more reactive than the cis isomer. This increased reactivity can be attributed to less steric hindrance in the trans configuration, allowing for a more favorable approach of the acetate ion during the nucleophilic attack.
Nucleophilic substitution reactions involve the replacement of a leaving group by a nucleophile. In this reaction, the acetate ion acts as a nucleophile attacking the electrophilic carbon in the tosylate group. The stereospecific nature of the reaction means that the configuration of the starting material directly influences the configuration of the product, highlighting the importance of stereochemistry in nucleophilic substitutions.