Predict the major products of the following reactions, including stereochemistry where appropriate.
a. 2,2-dimethylxirane + H⁺/H₂¹⁸O (oxygen labeled water)
b. 2,2-dimethylxirane + H¹⁸O^–/H₂¹⁸O (oxygen labeled water)

Verified step by step guidance

1. Step 1: Analyze the structure of 2,2-dimethyloxirane, which is an epoxide. Epoxides are three-membered cyclic ethers that are highly strained and reactive. They undergo ring-opening reactions under acidic or basic conditions.
2. Step 2: For reaction (a), the acidic conditions (H⁺/H₂¹⁸O) will protonate the oxygen atom in the epoxide, increasing its electrophilicity. This will make the carbon atoms in the ring more susceptible to nucleophilic attack by the labeled water (H₂¹⁸O). The nucleophile will attack the more substituted carbon due to the stability of the transition state.
3. Step 3: For reaction (b), under basic conditions (H¹⁸O⁻/H₂¹⁸O), the nucleophile (¹⁸O⁻) will directly attack the less substituted carbon of the epoxide due to steric hindrance and electronic effects. This results in a ring-opening reaction with inversion of stereochemistry at the attacked carbon.
4. Step 4: In both reactions, the ring-opening will produce a diol (two hydroxyl groups) with one of the hydroxyl groups containing the labeled oxygen (¹⁸O). The stereochemistry of the product will depend on the mechanism (acidic or basic conditions).
5. Step 5: Draw the major products for both reactions, ensuring to include the labeled oxygen (¹⁸O) in the appropriate hydroxyl group and indicate stereochemistry where applicable. The acidic reaction will favor attack at the more substituted carbon, while the basic reaction will favor attack at the less substituted carbon.

Key Concepts

Epoxide Chemistry

Epoxides, or oxiranes, are three-membered cyclic ethers that are highly reactive due to the strain in their ring structure. In reactions involving epoxides, nucleophiles can attack the less hindered carbon atom, leading to ring-opening and the formation of various products. Understanding the mechanism of epoxide ring-opening is crucial for predicting the products of the given reactions.

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the context of the reactions provided, it is important to consider how the stereochemistry of the starting materials influences the stereochemical outcome of the products, especially when nucleophiles attack the epoxide. This includes recognizing chiral centers and potential stereoisomers.

Isotope Labeling

Isotope labeling involves using isotopes, such as H¹⁸O, to trace the movement of atoms through a chemical reaction. In the reactions described, the presence of labeled water can help identify which oxygen atom is incorporated into the product. This concept is essential for understanding the mechanism and predicting the major products, as it provides insight into the reaction pathways and the fate of specific atoms.