Cyclopentane twist

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

The symmetric puckered conformations of cyclopentane are the Cs symmetric envelope (E) (10) with four carbon atoms in a plane and the C2 s)mrmetric twist (T) (11) with three carbon atoms in a plane [88]. Unlike cyclohexane, these conformations are of almost equal energy and are separated by barriers of about RT or less [89]. There are ten envelope conformations, each with one of the five carbon atoms out of the plane in one of the two directions, and ten corresponding twist conformations. The individual conformations freely exchange which atom or atoms are out of the plane, a process termed pseudorotation, and the whole sequence of conformations is called the pseudorotational itinerary (O Fig. 9). 

As has been pointed out by Hart <84CHEC-I(7)185>, thiiranes fused to cyclopentanes adopt a boat conformation with C(3) and the sulfur atom on the same side of the C(1)C(2)C(4)C(5) plane. The angle of deviation from this plane is about 30° for C(3) and 60-70° for the S atom. Cyclohexane rings having fused thiiranes prefer a twisted half-chair conformation. Thiiranes fused to larger cycloalkanes can adopt a wide variety of conformations. 

MM calculations of germacyclopentanes indicate that the symmetric twist form is the most stable in contrast to cyclopentane where the envelope and twist forms have much... 

Oxolane, as flexible as cyclopentane, will be discussed first. It is practical to assign names to certain conformations such as twist 72 (2.38), envelope (2.39), and twist (2.40), whose symbols are copied from those of cyclohexane (equilibrium 2.8). 

Among the eight theoretically possible trishomo homologs of cubane, four are chiral, comprising two of Cj, one of C2, and one of D3 symmetry. Inspection of a molecular model of D3 trishomocubane (138) reveals that this beautiful rigid cage-shaped molecule is composed of six twisted cyclopentane moieties with the same chirality, as the symmetry number 6 inherent in this symmetry demands. 

Pseudorotation Term used by some authors to describe the out of plane motion of the ring atoms in cyclopentane during fast conformational interchange of the many envelope and twist conformers. Usage is discouraged. 

Pseudorotation - the process whereby envelope (E) and twisted (T) conformers (with Cs and C2 symmetry, respectively) of cyclopentane are interconverted - was first proposed on the basis of a study of the dynamic puckering in this molecule [62]. The process may be conveniently mapped by two parameters the puckering ampli-... 

Contrary to what Baeyer predicted, cyclohexane is more stable than cyclopentane. Furthermore, cyclic compounds do not become less and less stable as the number of sides increases. The mistake Baeyer made was to assume that all cyclic molecules are planar. Because three points define a plane, the carbons of cyclopropane must lie in a plane. The other cycloalkanes, however, are not planar. Cyclic compounds twist and bend in order to attain a structure that minimizes Ae three different kinds of strain that can destabilize a cyclic compound ... 

Cycloheptane and cyclooctane data on the thermal properties are also given in Table 3.1 They show little change from the cyclopentane and cyclohexane properties. Again, there is no indication of increasing amounts of conformational entropy in the transition entropies. For cyclooctane in solution H and NMR could prove ring-inversions and pseudorotation among the boat-chair conformations through the twist-boat-chair intermediate to very low temperatures (100 K). Only about 6% of the cydooctane could be found at about 300 K in the other three crown-family... 

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