(A true story.) Chemistry lab students added an excess of ethylmagnesium bromide to methyl furoate, expecting the Grignard reagent to add twice and form the tertiary alcohol. After water workup, they found that the product was a mixture of two compounds. One was the expected product having two ethyl groups, but the unexpected product had added three ethyl groups. Propose a mechanism to explain the formation of the unexpected product.

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он он CH-CH2 O CH3CH MgBr ether CH,CH3+ CH2CH CH2CH CH2CHj OCH3 methyl furoate expected unexpected