The reaction involves the nucleophilic attack of the Grignard reagent ($RMgX$) on the strained epoxide ring of ethylene oxide (oxirane).

1. Mechanism: The alkyl group ($R^-$) of the Grignard reagent acts as a nucleophile and attacks one of the carbon atoms of the ethylene oxide ring, causing the ring to open. This forms a magnesium alkoxide intermediate.
2. Hydrolysis: Subsequent acid hydrolysis of the intermediate yields a primary alcohol.
3. Chain Lengthening: The resulting primary alcohol has a carbon chain that is two carbon atoms longer than the alkyl group of the original Grignard reagent. Reaction: $\underset{\text{Ethylene oxide}}{CH_2-O-CH_2} + RMgX \rightarrow R-CH_2-CH_2-OMgX \xrightarrow{H_3O^+} \underset{\text{Primary Alcohol}}{R-CH_2-CH_2-OH}$