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Magazine Chapter 4: Q49P (page 114)
In light of your answer to Problem 4-48, draw the two chair conformations of 1,1,3-trimethylcyclohexane and estimate the amount of strain energy in each. Which conformation is favored?
Short Answer
Thus, axial methyl groups produce 5.4 kJ/mol more steric strain in conformation II thus, conformation (I) is favored.
Step by step solution
Chair conformation
Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109掳 and all neighboring C-H bonds are staggered.
Confirmation that is favored
The two chair conformations of 1,13-trimethyl cyclohexane is as follows:
In structure II, there is one (CH3-CH3 ) 1,3-diaxial interaction thus it is higher in energy than structure I which has only (CH3-H ) 1,3-diaxial interaction. Thus, the structure I is more stable than structure II.
The structure I have diaxial interactions:
2(CH3-H) = 2 X 3.8 kJ/mol
= 7.6 kJ/mol
Structure II has greater diaxial interaction:
2(CH3-H) = 2 X 3.8 kJ/mol
= 7.6 kJ/mol
(CH3-CH3 ) = 5.4 kJ/mol
The total energy of the second conformer = (7.6+5.4) kJ/mol
The difference in the energy of two conformers = Energy of II-Energy of I
= 13.0-7.6 kJ/mol
= 5.4 kJ/mol
Thus, axial methyl groups produce 54 kJ/mol more steric strain in conformation II thus, conformation (I) is favored.
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