grignard/aldehyde spontaneous rxn

[AlwaysLucky]

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This is from Passage 25 #7 of the Orgo section from the TPRH Workbook.

When 5-bromohexanal reacts with Mg, what is the product?

I thought the answer would be hexanal with the MgBr attached to C5, but it isn't right. Apparently, the 5-MgBr hexanal undergoes "a spontaneous addition rxn" and becomes 2-methylcyclopentane with the OMgBr attached to C1.

Are there other "spontaneous reactions" like this that I should be aware of? I took Orgo 1 about 6 years ago and can't seem to remember exceptions/spontaneous rxns like these

Thanks!

 

[gettheleadout]

Initially the Mg reacts with the Br on the molecule, forming a Grignard reagent at the C5 carbon. Because the molecule contains an aldehyde group (bearing in mind that Grignard reagents are strong nucleophiles and react with carbonyls via attack at the carbonyl carbon) the C5 carbanion intramolecularly attacks the aldehyde at C1. This forms a cyclic structure, and the newly negatively charged oxygen from the original aldehyde carbonyl forms an ionic bond with the positively charged MgBr.

This isn't an exception, per se, but a problem that requires you to remember not only the process of forming Grignard reagents but their reactivity as nucleophiles as well. Grignard reactions with carbonyl groups are quite common.