Chapter 3: Conf. Anal. questions

Chapter 3: Conformations of Alkanes and Cycloalkanes

Conformational Analysis Questions

Qu 1:
The energy diagram shows the relative energies of the conformations of 2,2,3-trimethylpentane produced during the rotation about the C3-C4 bond.
Match the structural representations provided to the appropriate points on the energy profile.

Qu 2:

There are four possible stereoisomers of 1-tert-butyl-3-methylcyclohexanes in which the cyclohexane is in a chair conformation. The calculated heats of formation, ΔH_f, of these four structures are listed to the right.
Draw the four possible structures.

Match the heats of formation, ΔH_f, values to the appropriate structures

Justify your choice.

Using the available data, calculate :

i) the heat of combustion, ΔH_c, of the most stable structure

ii) the equilibrium constant, K, for the interconversion of the two cis forms at 25^oC.

kcal/mol ΔH_f (1) = -38.16
ΔH_f (2) = -40.53
ΔH_f (3) = -44.41
ΔH_f (4) = -46.84
ΔH_c (H₂, gas) = -68.0
ΔH_c (C, graphite) = -94.0
R = 1.987 cal/molK
kJ/mol
ΔH_f (1) =   -159.66
ΔH_f(2) =    -169.58
ΔH_f (3) =   -185.81
ΔH_f (4) =   -195.98
ΔH_c (H₂, gas) = -284.51
ΔH_c (C, graphite) = -393.3
R = 8.314 J/molK

Qu 3:
(tough)

Cyclopropane is unique among the cycloalkanes in that it can be hydrogenated with H₂ and Pt metal, giving propane. Similarly, propene can be hydrogenated to give propane.
The heats of formation (ΔH_f) for the hydrocarbons are:
propane = - 24.8 kcal/mol,    cyclopropane = + 12.8 kcal/mol , and   propene = + 4.9 kacl/mol
You should recall that the ΔH_f for the elements ( e.g. H₂) is defined to be zero.

(1.) From this data, calculate the heats of hydrogenation (ΔH_h) of cyclopropane and propene.

(2.) Show this data on a typical energy diagram in which the enthalpies of propene + H₂, cyclopropane + H₂ and propane are given in rough relative positions.

(3.) The bond energy of a C-C single bond is about 83 kcal/mol, a C=C double bond is about 146 kcal/mol. Using this data, together with the data from part (1), calculate a "ring strain" for cyclopropane, and compare this with the "ring-strain" value of 28 kcal/mol found experimentally from combustion data.

Answers ?

Qu 1:	The energy diagram shows the relative energies of the conformations of 2,2,3-trimethylpentane produced during the rotation about the C3-C4 bond. Match the structural representations provided to the appropriate points on the energy profile.