Greg Nard added an equivalent of 3,4-epoxy-4-methylcyclohexanol to an ether solution of methyl magnesium bromide and then added dilute hydrochloric acid. He expected that the product would be a diol. He did not get any of the expected product. What product did he get?
The correct answer and explanation is :
The product that Greg Nard likely obtained is 4-methylcyclohex-3-en-1-ol, a conjugated alkene, instead of the expected diol.
Explanation:
In this reaction, Greg Nard added 3,4-epoxy-4-methylcyclohexanol to an ether solution of methyl magnesium bromide (a Grignard reagent), followed by dilute hydrochloric acid. Let’s break down the chemistry step by step:
- Epoxide Opening with Grignard Reagent: The epoxide ring in 3,4-epoxy-4-methylcyclohexanol is a highly reactive electrophile. Grignard reagents, such as methyl magnesium bromide, are nucleophilic and can attack the epoxide. The methyl group from the Grignard reagent would open the epoxide ring, adding a methyl group to the less substituted carbon of the epoxide ring (this is because Grignard reagents typically attack the less hindered carbon in epoxides due to steric factors). In this case, methyl magnesium bromide would add a methyl group to carbon 3, leading to the formation of a tert-butyl alcohol intermediate (a magnesium alkoxide).
- Protonation: Upon addition of dilute hydrochloric acid, the alkoxide formed by the attack of the Grignard reagent is protonated, resulting in the formation of a hydroxyl group at carbon 3, yielding 3-methyl-4-hydroxy-4-methylcyclohexanol. This intermediate contains a hydroxyl group at position 3 and the epoxide oxygen was eliminated.
- Elimination of Water: The reaction conditions (acidic) can encourage the dehydration of the intermediate product, particularly with the hydroxyl group and the adjacent methyl group, leading to the formation of a double bond between positions 3 and 4. The proton from the hydroxyl group at position 4 is eliminated, resulting in the formation of a conjugated alkene, 4-methylcyclohex-3-en-1-ol.
The key point here is that instead of forming a diol (two hydroxyl groups), the reaction conditions favored elimination over hydration, which led to the formation of an alkene. Thus, 4-methylcyclohex-3-en-1-ol is the unexpected product.