Cyclopropane ring formation

[euphaire]

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Hello everyone,

I'm really confused about the ring formation concept and I ran across this reaction from Ohio State University website. It didn't explain what the mechanism is. I couldn't find any answers on google or my study books. Could someone tell me what's the mechanism for this reaction? I've been staring at it for 1 hour now.

The reaction is:
CH2(CH2Br)2 (a propane with two Br's) + 2Na ---> cyclopropane

They didn't write out the by-products. I really can't see how a 1,3-dibromopropane would want to become a highly-strained cyclopropane here.

Thanks in advance!

Cherry

 

[Rabolisk]

It's a complicated coupling reaction between the two terminal carbons. Can't say I have seen it in any typical organic chemistry textbook.

http://en.wikipedia.org/wiki/Wurtz_reaction

 

[orgohacks]

Hello everyone,

I'm really confused about the ring formation concept and I ran across this reaction from Ohio State University website. It didn't explain what the mechanism is. I couldn't find any answers on google or my study books. Could someone tell me what's the mechanism for this reaction? I've been staring at it for 1 hour now.

The reaction is:
CH2(CH2Br)2 (a propane with two Br's) + 2Na ---> cyclopropane

They didn't write out the by-products. I really can't see how a 1,3-dibromopropane would want to become a highly-strained cyclopropane here.

Thanks in advance!
Cherry

The Na is performing a similar role to what you have probably seen with magnesium in the Grignard reaction - it inserts into the carbon-bromine bond to form a carbon-sodium bond (essentially a negatively charged carbon). This carbon is very nucleophilic, and can perform an SN2 reaction on the nearby alkyl bromide to form the 3 membered ring.

3 membered ring formation is much slower than 5 or 6 membered ring formation, but it can happen.

The byproduct is two equivalents of NaBr.

Probably not MCAT material, but since you asked....