Chapter 3:
Conformations of Alkanes and Cycloalkanes 		 Ch 3 contents

Conformational analyzis 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, DHf, of these four structures are listed to the right.

Draw the four possible structures.

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

Justify your choice.

Using the available data, calculate :

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

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

 
kcal/mol
DHf (1) =  -38.16
DHf (2) =  -40.53
DHf (3) =  -44.41
DHf (4) =  -46.84
DHc (H2, gas) =  -68.0
DHc (C, graphite) =  -94.0
R = 1.987 cal/molK

kJ/mol

DHf (1) =   -159.66
DHf(2) =    -169.58
DHf (3) =   -185.81
DHf (4) =   -195.98
DHc (H2, gas) =  -284.51
DHc (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 (DHf) 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 DHf for the elements ( e.g. H2) is defined to be zero.

(1.)  From this data, calculate the heats of hydrogenation (DHh) 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 ?