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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: ijsup on February 18, 2009, 11:16:50 PM

Title: 2-methylcyclohexanol dehydration product NMR
Post by: ijsup on February 18, 2009, 11:16:50 PM

 Hi. I just did a lab where we dehydrated 2-methylcyclohexanol. We were told it would form 1-methylcyclohexene and/or 3-methylcyclohexene and we are asked to determine the major product. I figure the major product is 1-methylcyclohexene because it is the most substituted C=C. However, we are supposed to determine it using C13 NMR. We are given the "hint" to figure out how the spectra of the 1-and 30 methylcyclohexenes would differ. I don't know how to figure this out. I'm not sure what the difference between the two isomer's NMR would be. Any suggestions?

Title: Re: 2-methylcyclohexanol dehydration product NMR
Post by: aldoxime_amine on February 19, 2009, 12:59:54 AM

alpha effect of the methyl on the sp2 hybridized carbon in the major product will cause a downfield shift while in other product, gamma effect of the methyl group will cause an upfield shift on the sp2 hybridized carbon.

Please correct me if i am wrong. I am new to spectroscopy.

Title: Re: 2-methylcyclohexanol dehydration product NMR
Post by: azmanam on February 19, 2009, 07:44:51 AM

If you didn't decouple the ¹³C spectra, you could tell a di-sub olefin from a tri-sub.  You could also do DEPT experiments, but that may be outside the scope of your lab.

aldoxime_amine: I think that's ok, too.  sp² centers are more electronegative than sp³, so if you can identify the methyl signal in the ¹³C, you should be able to see which one is more downfield, and that will tell you which one is tri-sub.