Predict the product of the substitution reactions, paying attention to the stereochemical outcome.
(b)

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Analyze the structure of the starting material: The molecule contains a bromine atom attached to a chiral carbon, which is bonded to a thiophene ring and two alkyl groups. The bromine is in the wedge position, indicating its stereochemistry.

Identify the reagent and solvent: KSCN (potassium thiocyanate) is a nucleophile, and THF (tetrahydrofuran) is a polar aprotic solvent. Polar aprotic solvents favor SN2 reactions, where the nucleophile attacks the electrophilic carbon directly.

Determine the reaction mechanism: Since the solvent is polar aprotic and the substrate is a secondary alkyl halide, the reaction is likely to proceed via an SN2 mechanism. In an SN2 reaction, the nucleophile (SCN⁻) will attack the carbon bonded to bromine, leading to the inversion of stereochemistry at the chiral center.

Predict the product: The bromine atom will be replaced by the thiocyanate group (SCN). Due to the SN2 mechanism, the stereochemistry of the chiral center will invert, meaning the SCN group will occupy the opposite stereochemical position (dash instead of wedge).

Verify the stereochemical outcome: The final product will have the SCN group in the dash position, opposite to the original bromine atom's wedge position, ensuring stereochemical inversion as expected in an SN2 reaction.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. These reactions can occur via two main mechanisms: SN1 and SN2. The SN1 mechanism is a two-step process where the leaving group departs first, forming a carbocation, while the SN2 mechanism is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs.

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In substitution reactions, the stereochemical outcome is crucial, as it determines whether the product retains or inverts the configuration at the chiral center. Understanding stereochemistry is essential for predicting the specific isomer formed in a reaction.

Leaving Groups

Leaving groups are atoms or groups that can depart from the parent molecule during a chemical reaction, facilitating nucleophilic substitution. A good leaving group is typically stable after departure, such as halides or sulfonate groups. The nature of the leaving group significantly influences the reaction rate and mechanism, making it a key factor in predicting the products of substitution reactions.

Predict the product of the substitution reactions, paying attention to the stereochemical outcome.(b)