Organic Chemistry 2 · Ethers and epoxide opening
4-Chlorobutyl acetate from THF and acetyl chloride
Conditions: ZnCl2 (cat.)
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The mechanism, step by step
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THF oxygen adds to the acyl carbon
The ether oxygen of tetrahydrofuran (THF) is a weak nucleophile, but acetyl chloride is a very reactive electrophile, and the ZnCl2 catalyst (not drawn) coordinates its chlorine. The oxygen lone pair adds to the carbonyl carbon and the C=O π electrons move onto oxygen, giving a tetrahedral intermediate.
The curved arrows
- A THF oxygen lone pair forms the new O–C bond to the acyl carbon.
- The C=O π pair moves onto the carbonyl oxygen, which becomes an alkoxide.
Chloride leaves to give the acyloxonium ion
The alkoxide re-forms the C=O bond and pushes out chloride, the best leaving group on the tetrahedral carbon. The product is an acyloxonium ion: THF carrying an acetyl group on a positively charged oxygen, which makes the ring carbons next to oxygen strongly electrophilic.
The curved arrows
- An oxygen lone pair moves back in to re-form the C=O π bond.
- The C–Cl pair leaves with chlorine as chloride ion.
Chloride opens the ring by SN2
Chloride attacks a CH2 next to the positive oxygen from the backside, and the C–O bond breaks in one SN2 step. The ring opens, and the acetylated oxygen becomes the neutral ester oxygen, the leaving group of this substitution.
The curved arrows
- A chloride lone pair forms the new C–Cl bond at a ring carbon next to oxygen.
- The ring C–O pair moves onto oxygen, opening the ring to the neutral ester.
4-Chlorobutyl acetate
4-Chlorobutyl acetate contains all four carbons of THF as a chain with the ester at one end and chlorine at the other. The acyl chloride has effectively added across the C–O bond of the cyclic ether.
What you'll practice
Draw the oxygen of tetrahydrofuran adding to acetyl chloride, loss of chloride to give an acyloxonium ion, then chloride opening the ring by SN2 to give 4-chlorobutyl acetate.
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