Final answer:
Treating (R)-2-ethyl-2-methyloxirane with MeMgBr and H2O results in the formation of (2S,3R)-3-methyl-2-pentanol through a nucleophilic attack on the oxirane ring, leading to inversion of stereochemistry at the attacked carbon.
Step-by-step explanation:
The question involves the reaction of (R)-2-ethyl-2-methyloxirane with Grignard reagent MeMgBr (methylmagnesium bromide) followed by treatment with water (H2O). The use of MeMgBr opens the oxirane ring through a nucleophilic attack, where the methyl group from the Grignard reagent adds to the less substituted carbon (due to steric hindrance) of the oxirane ring. The (R)-2-ethyl-2-methyloxirane has two carbon centers, with one being more substituted than the other. The methyl group from MeMgBr will attack the less substituted carbon. Upon hydrolysis with water, the resulting molecule will be a secondary alcohol.
The expected product is (2S,3R)-3-methyl-2-pentanol. The ring-opening occurs in such a way that preserves the stereochemistry of the starting material, leading to inversion of configuration at the carbon attacked by the Grignard reagent, which results from the back side attack characteristic of nucleophiles in an Sn2-type reaction.