Q.50 The mechanism of the following reaction involves the formation of a __________ membered
ring.
The reaction shown is a Wittig reaction, and its mechanism involves the formation of a four‑membered ring intermediate called oxaphosphetane.
Explanation of the question
In the question, cyclohexanone reacts with the phosphorus ylide Ph₃P=CH₂ to give methylene cyclohexane (cyclohexylidene methane). This transformation is a classic Wittig reaction, where a carbonyl compound (aldehyde or ketone) is converted to an alkene using a phosphonium ylide. During this process, a transient four‑membered heterocycle, oxaphosphetane, is formed as the key intermediate.
Mechanistic steps:
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Nucleophilic attack of the ylide carbon on the electrophilic carbonyl carbon of cyclohexanone generates a zwitterionic betaine intermediate.
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Intramolecular coordination between the negatively charged oxygen and positively charged phosphorus closes to a four‑membered oxaphosphetane ring (two carbons, one oxygen, one phosphorus).
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Fragmentation of oxaphosphetane breaks the C–O and C–P bonds and forms the C=C bond of the alkene plus triphenylphosphine oxide, driven by strong P=O bond formation.
Therefore, the blank in “The mechanism of the following reaction involves the formation of a ______ membered ring.” is correctly filled as four‑membered.
Discussion of typical options
Such MCQs usually provide options like:
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3‑membered ring
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4‑membered ring
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5‑membered ring
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6‑membered ring
Explanation of each type in this context:
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3‑membered ring – Three‑membered rings (e.g., epoxides, cyclopropanes) arise from reactions such as epoxidation or Simmons–Smith cyclopropanation, not from the Wittig reaction between Ph₃P=CH₂ and cyclohexanone; hence this option would be incorrect here.
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4‑membered ring – The Wittig mechanism uniquely passes through a four‑membered oxaphosphetane intermediate; this matches the reaction in the question and is the correct answer.
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5‑membered ring – Five‑membered cyclic intermediates are typical of pericyclic processes such as Claisen or pericyclic rearrangements, not of the Wittig olefination of a ketone, so this option is also incorrect.
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6‑membered ring – Six‑membered transition states or rings are common in aldol and other condensation reactions but are not involved in this specific Wittig mechanism, making this option incorrect as well.
SEO‑friendly introduction
In organic chemistry, the Wittig reaction mechanism is a powerful method for converting carbonyl compounds into alkenes through a characteristic four‑membered oxaphosphetane intermediate. When cyclohexanone reacts with the phosphorus ylide Ph₃P=CH₂, the key mechanistic feature is the transient formation and breakdown of this four‑membered ring, which ultimately yields methylene cyclohexane and triphenylphosphine oxide.
