2,7-dimethylnaphtalene can be prepared by the reaction of a Grignard reagent A and an acetal (B). [V — Analytical Chemistry Chemistry Question
Preparation of 2,7-dimethylnaphthalene
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:
1. Grignard addition of 3-methylbenzylmagnesium bromide (A) to the deprotected aldehyde of B to form an alcohol intermediate.
2. Acid-catalyzed deprotection of the acetal to yield an aldehyde.
3. Intramolecular electrophilic aromatic substitution (SE) of the carbonium ion onto the benzene ring to form a tetralin derivative.
4. Dehydration and subsequent aromatization (loss of two water molecules) to yield 2,7-dimethylnaphthalene.
[VISUAL]
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
D: [2.2](2,7)naphthalenophane (a macrocyclic dimer bridged by two ethylene chains)
E: Coronene
[VISUAL]