Q4-17P - Organic Chemistry (Mcmurry)

Question

Look at Figure 4-12 on page 105, and estimate the percentages of axial andequatorial conformations present at equilibrium in bromocyclohexane.

Step-by-Step Solution

Answer

$70\%$ Equatorial bromocyclohexane and $30\%$  axial bromocyclohexane are obtained.

Step-by-Step Solution Step 1: Chair confirmation

Chair conformation structure for a six-membered ring is when atoms 2, 3, 5, and 6 lie in the same plane, atom one lies above the plane, and atom 4 lies below the plane.

Step 2: Axial and equatorial positions:

Axial bonds are the bonds that are perpendicular or vertical to the ring plane. Equatorial bonds are approximately horizontal in the plane of the ring.

Step 3: Energy difference cause and calculation

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial hydroxyl group on C1 is too close to the axial hydrogens three carbons away on C3 and C5, resulting in two times the steric strain.

One 1,3-diaxial interaction results in a one kJ/mol increase in energy of axial bromocyclohexane.

Hence, two such interactions give 2 kJ/mol.

Step 4: Figure 4-12 on page 105

Figure 4-12 of page 105

From this graph, it can be seen that at value two, for energy difference: $70\%$ equatorial bromocyclohexane and  $30\%$ axial bromocyclohexane are obtained.