Predict the major products of the following reactions, including stereochemistry where appropriate.
(c) (2S,3R)-2-ethyl-2,3-dimethyloxirane + CH₃O^– / CH₃OH
(d) (2S,3R)-2-ethyl-2,3-dimethyloxirane + H⁺ / CH₃OH

Verified step by step guidance

1. Step 1: Recognize the type of reaction. Both reactions involve the opening of the epoxide ring in (2S,3R)-2-ethyl-2,3-dimethyloxirane. In part (c), the reaction occurs under basic conditions (CH3O- / CH3OH), while in part (d), the reaction occurs under acidic conditions (H+ / CH3OH). The mechanism and regioselectivity differ depending on the conditions.
2. Step 2: Analyze the mechanism for part (c) (basic conditions). Under basic conditions, the nucleophile (CH3O-) attacks the less sterically hindered carbon of the epoxide ring. This occurs via an SN2 mechanism, leading to inversion of stereochemistry at the attacked carbon. The product will have the methoxy group (-OCH3) added to the less substituted carbon, and the stereochemistry at this carbon will be inverted.
3. Step 3: Analyze the mechanism for part (d) (acidic conditions). Under acidic conditions, the epoxide ring is protonated by H+, making it more electrophilic. The nucleophile (CH3OH) then attacks the more substituted carbon of the epoxide ring due to the stability of the carbocation-like transition state. This occurs via an SN1-like mechanism, leading to retention of stereochemistry at the attacked carbon. The product will have the methoxy group (-OCH3) added to the more substituted carbon.
4. Step 4: Consider stereochemistry. For part (c), the stereochemistry at the less substituted carbon is inverted due to the SN2 mechanism. For part (d), the stereochemistry at the more substituted carbon is retained due to the SN1-like mechanism. Ensure that the stereochemistry of the products is clearly indicated in the final structures.
5. Step 5: Draw the major products for both reactions. For part (c), the product will have the methoxy group added to the less substituted carbon with inversion of stereochemistry. For part (d), the product will have the methoxy group added to the more substituted carbon with retention of stereochemistry. Verify that the stereochemistry matches the expected outcome based on the mechanisms described.

Key Concepts

Epoxide Reactivity

Epoxides are three-membered cyclic ethers that are highly reactive due to the strain in their ring structure. They can undergo nucleophilic ring-opening reactions, where a nucleophile attacks the less hindered carbon atom, leading to the formation of alcohols or other functional groups. Understanding the mechanism of epoxide ring-opening is crucial for predicting the products of reactions involving epoxides.

Stereochemistry

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In reactions involving chiral centers, such as the one in the given question, the stereochemistry of the reactants can influence the stereochemical outcome of the products. Recognizing the stereochemical configuration of reactants is essential for predicting the stereochemical outcomes of the reactions.

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to facilitate the reaction process, often by protonating a functional group to make it more reactive. In the context of the given question, the presence of H+ in the reaction with methanol suggests that the epoxide will be protonated, leading to a more favorable pathway for nucleophilic attack. Understanding how acids influence reaction mechanisms is key to predicting the products formed.