Conformation, cycloalkanes energy

Conformational free energies (-AG ) for substituent groups Strain energies of cycloalkanes... 

Beginning with cycloheptane which has four conformations of similar energy confer matronal analysis of cycloalkanes becomes more complicated The same fundamental principles apply to medium and large rings as apply to smaller ones—but there are more atoms and more bonds to consider and more conformational possibilities... 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Conformational analysis (Section 4.8) A means of assessing the energy of a substituted cycloalkane by totaling the steric interactions present in the molecule. 

Conformation has a major influence on the chemical reactivity of cycloalkanes. To understand its effect in any one reaction, we first need to know what the conformation is of the transition state, and this requires a knowledge of the reaction mechanism. Next, we have to decide what amount of energy is required for the reactants to achieve transition-state conformations. For example, consider the E2 elimination discussed in Section 8-8D. The preferred transition state requires the leaving groups to be antarafacial and coplanar ... 

Comparison of the heats of combustion of cycloalkanes (Table 9.1) shows that cyclopropane, cyclobutane, and cyclononane yield more energy per methylene group than the other cycloalkanes. This can be attributed to strain resulting from bond-angle distortion (Baeyer strain), eclipsed conformations (Pitzer strain), and trans-annular, repulsive van der Waals interactions. Common (five- and six-membered) rings and large (more than twelve-membered) rings have little or no strain. This... 

By examining the conformations of the cycloalkanes, we are able to determine the origin of these strain energies. Let s begin with the smallest one, cyclopropane, and see what causes the large amount of strain energy that it has (Figure 6.8). 

Draw the conformations of cycloalkanes, compare their energies, and predict the most stable conformations. 

Key Ideas In summary, cycloalkanes adopt their minimum-energy conformations for a combination of three reasons ... 

In cycloalkanes (general formula C H2 ), the carbon atoms at the end of a chain are attached together to form a closed loop. This connectivity reduces the ease with which the molecule changes conformations by rotation about single bonds and introduces both steric and angle strain into the total energy of the molecule. Consequently, cycloalkanes have particular preferred conformations such as boat and chair, and the molecule must absorb energy to move between them. 

Figure 20. (a) Structure of the chain fold in crystalline monoclinic cycloalkane c-(Cff2)34, representative of the 100 fold in polyethylene (after ref 109) (b) calculated minimum energy conformation of a 110 chain fold in orthorhombic polyethylene (after ref 116). 

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