TBR Ochem Ch.2 Passage 1.Q4 (cyclohexane, gauche and anti)

[jjunior]

Hi all,

"Cis-1,4-dimethylcyclohexane, in its most stable chair conformation, CANNOT have which of these interactions?"
A. CH3/H gauche
B. CH3/H anti
C. CH3/CH3 gauche
D. H/H anti

- i understand that the answer is C, bc the methyls are on carbons not next to each other.
- i have a question regarding gauche and anti conformation in cyclohexane, is it only refering to 2 Cs right next to each other? for example, it can only be assigned between C1 and C2, or C2 and C3. then where would those anti conformation come into play? since the C right next to each other in a cyclohexane chair conformation is always gauche?

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[loltopsy]

Gauche means that two groups at 60 degrees away from each other on adjacent atoms. The most stable conformation of ethane has gauche interactions out the ass, but it also has anti hydrogens.

Gauche can only be used for substituents attached to adjacent atoms.

 

[BerkReviewTeach]

for example, it can only be assigned between C1 and C2, or C2 and C3. then where would those anti conformation come into play? since the C right next to each other in a cyclohexane chair conformation is always gauche?

It is true that when we talk about gauche and anti, we are talking about substituents on adjacent (bonded) carbons (C₁ and C₂, C₂ and C₃, etc...). So pick any two adjacent carbons and note that one will have an equatorial substituent that is up and an axial substituent that is down while the other has an equatorial substituent that is down and an axial substituent that is up. Figure 1 shows this for the two methyl groups in the upper picture. Notice that in the reactant drawing, the two methyl groups are anti to one another. Basically, the substituents on adjacent carbons in cyclohexane are always gauche except for the axial-up and axial-down realtionship which is anti.