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Topic: why is the % equatorial of trimethylsilylcyclohexane not 100%?  (Read 2985 times)

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jennyjo

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Please help us!! I have a question about monosubstituted cyclohexane.  We calculated the % equatorial VS % axial for trimethylsilylcyclohexane and tert-butylcyclohexane.  We found that  trimethylsilylcyclohexane  had a 97% equatorial (which we would have expected to be 100%) and tert-butylcyclohexane was 100% equatorial(which we expected to have a % equatorial less then that of trimethylsilylcyclohexane). We know that Silicon is 150% larger then Carbon ???, so why isn't Silicon 100% equatorial and Carbon is?  If you can help it would be much appreciated! Thanks,
jenny

PS we made the calulations with Spartan

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Re:why is the % equatorial of trimethylsilylcyclohexane not 100%?
« Reply #1 on: March 09, 2005, 03:43:47 PM »
Think about the effect of introducing a silicon in that position: the bond from the carbon in the ring to the silicon will be a lot longer than the bond to the quaternary carbon in the t-butyl case!  Think about what the high energy interactions in the axial case are and how they would be affected by the longer C-Si bond.

For reference, C-C bonds are ~154 pm long, which C-Si bonds are ~185 pm.

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