2,7-dimethylnaphtalene can be prepared by the reaction of a Grignard reagent A and an acetal (B). [V — Analytical Chemistry Chemistry Question
Synthesis of 2,7-dimethylnaphthalene and Coronene
2,7-dimethylnaphtalene can be prepared by the reaction of a Grignard reagent A and an acetal (B).
[VISUAL]
Suggest reaction conditions for the preparation of A and B.
Model Answer
i. Radical bromination: Br2/UV or N-bromosuccinimide, then Mg + diethyl ether (iodine as catalyst)
ii. ethylene glycol, acid catalysis
Explain with a mechanism the formation of 2,7-dimethylnaphtalene.
Model Answer
The mechanism involves the nucleophilic attack of the Grignard reagent A onto the acetal B, followed by acid workup which hydrolyzes the acetal to form an aldehyde intermediate. Under acidic conditions, this intermediate undergoes an intramolecular electrophilic aromatic substitution (cyclization) followed by dehydration (loss of two water molecules) to yield 2,7-dimethylnaphthalene.
2,7-dimethylnaphtalene is converted to E via the indicated reactions. (E is a fancy compound with molecular formula C24H12.) Identify C, D and E.
Model Answer
C: 2,7-bis(chloromethyl)naphthalene (formed by radical chlorination of the two methyl groups of 2,7-dimethylnaphthalene with 2 equivalents of N-chlorosuccinimide)
D: [2.2](2,7)naphthalenophane (a dimer formed by the zinc-mediated coupling of 2,7-bis(chloromethyl)naphthalene)
E: coronene (C24H12, formed by intramolecular cyclization and dehydrogenative aromatization/oxidation of D under palladium and heat)