Answer :
The reaction between epoxides and Grignard reagents is a type of nucleophilic substitution. In this case, the Grignard reagent acts as the nucleophile and attacks the electrophilic carbon in the epoxide ring.
This results in the formation of a new carbon-carbon bond and the opening of the epoxide ring.
To propose a mechanism for the reaction described in exercise 567b, let's consider the following example:
CH3 CH3 + CH3MgBr -> CH3-CH2-O-MgBr + CH3CH3
Nucleophilic attack
In the first step, the Grignard reagent (CH3MgBr) acts as a nucleophile and attacks the electrophilic carbon in the epoxide ring. This results in the formation of a new carbon-carbon bond and the opening of the ring. The reaction intermediate is an alkoxide ion (CH3-CH2-O-MgBr).
Proton transfer
In the second step, a proton is transferred from the alkoxide ion to the solvent (ether or THF). This step is important because it helps to stabilize the intermediate and facilitate the next step.
Acid-base reaction
In the third step, the alkoxide ion (CH3-CH2-O-MgBr) reacts with water to form the corresponding alcohol (CH3-CH2-OH) and magnesium hydroxide (Mg(OH)2).
Overall, the mechanism for the reaction between epoxides and Grignard reagents can be summarized as follows:
- Nucleophilic attack: Grignard reagent attacks the electrophilic carbon in the epoxide ring, forming an alkoxide ion.
- Proton transfer: A proton is transferred from the alkoxide ion to the solvent, stabilizing the intermediate.
- Acid-base reaction: The alkoxide ion reacts with water to form the corresponding alcohol and magnesium hydroxide.
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Final answer:
The reaction between epoxides and Grignard reagents involves the nucleophilic attack on the less substituted carbon of the epoxide, leading to an alkoxide intermediate that upon acidic work-up yields an alcohol.
Explanation:
The reaction between epoxides and Grignard reagents is an important transformation in organic chemistry that leads to the formation of alcohols. Epoxides, like the 2-ethyl-3-methyl oxirane mentioned, are highly strained three-membered rings bearing oxygen. This strain makes them very reactive towards nucleophiles such as the carbon-atom-centered nucleophile in a Grignard reagent.
The mechanism for the reaction between an epoxide and a Grignard reagent involves the nucleophilic attack by the carbon anion of the Grignard on the less substituted, more accessible carbon atom of the epoxide. This opens the ring, resulting in an alkoxide intermediate. Subsequent work-up with an acid, typically water or a dilute acid, leads to protonation of the alkoxide, yielding the corresponding alcohol. Here, for example, reacting the epoxide with methylmagnesium bromide followed by acidic work-up would generate a secondary alcohol.
