Cycloalkanes higher, conformations

Higher cycloalkanes have angles at carbon that are close to tetrahedral and are sufficiently flexible to adopt conformations that reduce their tor sional strain They tend to be populated by several different conforma tions of similar stability... 

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes exist in nonplanar conformations. 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

These are not frequently used monomers. There exists a clear connection between the strain of various members in the series of cyclic hydrocarbon molecules and their heats of combustion (see Table 1). The high heats of combustion of the first members are the consequence of the C—C bond angle deviation from 109°28. In cyclohexane, the most stable cycloalkane which can exist in the chair conformation, the C—C bond angle value deviates very little from that observed in unstrained compounds. Cyclopentane exhibits the smallest deviation of the C—C bond angle from the theoretical value. Its higher heat of combustion is due to steric interactions of pairs of neighbouring hydrogen atoms. A similar situation is observed with cycloheptane [12a]. 

The term pseudorotation was first appUed to cyclopentane like inversion, it has an atomic analogue in 5-coordinate compounds (e.g. PF5). ) The name means false rotation , and it is therefore appropriate for any conformational process which results in a conformation superposable on the original, and which differs from the original in being apparenUy rotated about one or more axes. Pseudorotation, in analogy with real molecular and internal rotations, can be free, as in cyclopentane, or more or less hindered, eis in cycloheptane and higher cycloalkanes. In moderately to severely hindered pseudorotation, it is appropriate to consider distinct stable conformations which are pseudorotation partners, and these cases are often amenable to study by dynamic nmr methods. When the barrier to pseudorotation is very low, or in the limit when pseudorotation is free, it is not really justified to talk about separate stable conformations (e.g. the C2 and Cg forms of cyclopentane), because strictly there is only one conformation, and the pseudorotation is simply a molecular vibration. 

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes also exist in nonplanar conformations. The small instabilities of these higher cycloalkanes appear to be caused primarily by torsional strain and repulsive dispersion forces between hydrogen atoms across rings, called transannular strain. The nonplanar conformations of these rings, however, are essentially free of angle strain. 

Cyclopentanes, cyclohexanes, and all larger cycloalkanes exist in dynamic equilibrium between a set of puckered conformations.The lowest energy conformation of cyclopentane is an envelope conformation.The lowest energy conformations of cyclohexane are two interconvertible chair conformations. In a chair conformation, six bonds are axial and six are equatorial. Bonds axial in one chair are equatorial in the alternative chair, and vice versa. A boat conformation is higher in energy than chair conformations. The more stable conformation of a substituted cyclohexane is the one that minimizes axial-axial interactions. 

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