Q49P - Organic Chemistry (Mcmurry)

Question

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?

Step-by-Step Solution

Answer

Thus, axial methyl groups produce 5.4 kJ/mol more steric strain in conformation II thus, conformation (I) is favored.

Step 1: Chair conformation

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C-H bonds are staggered.

Step 2: 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.