Quick Question about Cyclic Chairs

[ravupadh]

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When you have a methyl substituent and a halogen substituent on the same ring, which one prefers axial and which ones prefers equatorial? Thanks.

 

[MD Odyssey]

Methyl prefers equatorial

The fact that the OP didn't see this means that he / she needs to take a few minutes to build a model of a methylated cyclohexane ring. If you put a methyl group on one of the carbons and look at the axial conformation, you'll see the steric hindrance of the other two axial hydrogens and the methyl. This is usually referred to in the textbooks as a 1,3-diaxial interaction. It becomes really pronounced if you use a tert-butyl group - this fact is commonly exploited to lock the ring in one conformation during certain kinds of reactions.

Also, while beyond the scope of the question, one can use the difference in the Gibbs energy for the conformational change to calculate the ratio of axial to equatorial for a given substituent. Build a model and all should become clear.

 

[MT Headed]

The OP doesn't have a gut feel for atomic sizes yet.

halogen = planet
iodine = jupiter
methyl = solar system
tert-butyl = galaxy

 

[BilalL]

The OP doesn't have a gut feel for atomic sizes yet.

halogen = planet
iodine = jupiter
methyl = solar system
tert-butyl = galaxy

👍😀

I like that!

 

[axo86]

I thought it was helpful for less obvious cases to look at a table of A-values such as this one (it's basically what MD Odyssey was saying about difference in Gibbs energy b/t axial and equatorial positions). Higher A-value means it costs more energy to put that substituent in the axial vs equatorial position. And when you're doing this kind of problem, always watch out for the anomeric effect 😀