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Organic Chemistry 4e Carey | |||||
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Organic Chemistry 4e Carey | |||||
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Self Assessment |
Chapter 3: Conformations of Alkanes and Cycloalkanes |
| Qu 1: | 
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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
isomeric 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.
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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 |
| 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:
(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. |
