Planar conformation cyclopropane

The underlying reason could be that the Walsh model of cyclopropane has a quasi-cyclopropenyl ring system which could be more effective at accepting an electron pair than the external p system, which is so adept at electron pair donation. Delocalization into the former one would require the planar conformation observed for the radical anions 127 and 128 . 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

Cyclic alkanes undergo pseudorotation because rotation by 360° is not possible. Pseudorotation in cyclic alkanes leads to many conformations. Cyclopropane is planar, with relatively weak banana bonds. The lowest energy conformation of cyclobutane is a puckered conformation. The lowest energy conformation of cyclopentane is an envelope conformation. The lowest energy conformation of cyclohexane is an equilibrating mixture of two chair conformations. 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

The cyclopropane ring is necessarily planar, and the question of conformation does not arise. The C—C bond lengths are slightly shorter than normal at 1.5 A, and the H—C—H angle of 115° is opened somewhat from the tetrahedral angle. These structural... 

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... 

A further feature of three-membered rings is that they must be planar, and a consequence of this is that, in cyclopropane, all C-H bonds are in the high-energy eclipsed state. There can be no conformational mobility to overcome this. 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered conformation shown in Figure 3.11. 

The prefix cyclo- is used to name cycloalkanes. Cyclopropane is planar, but larger carbon rings are puckered. Cyclohexane exists mainly in a chair conformation with all bonds on adjacent carbons staggered. One bond on each carbon is axial (perpendicular to the mean carbon plane) the other is equatorial (roughly in that plane). The conformations can be interconverted by flipping the ring, which requires only bond rotation and occurs rapidly at room temperature for cyclohexane. Ring substituents usually prefer the less crowded, equatorial position. 

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

For instance, in solutions substituted cyclopropanes with locally planar groups, such as the acetyl (MeC=0) or vinyl (H2C=CH) groups, may exist as equilibria of several conformers. One can find bisected (VI) perpendicular (VII) or intermediate (gauche) conformationsIn these conformations, furthermore, s-trans and s-cis arrangements... 

Most interestingly, reduction of 9-cyclopropylanthracene and 1,4-dicyclopropyl-naphthalene led to the planar cyclopropyl conformations in 127 and 128 as opposed to the normally observed bisected cyclopropyl conformations as, for example, in the cyclopropylsemidione 129 . In all other cyclopropane derivatives previously studied except 127 and 128 , the excess charge density present in the relevant p orbital of the electrophore, is zero or positive. Even in semidione radical anion systems is calculated (HMO) to be positive +0.10). Thus it is reasonable to assume that the change in the conformational preference of cyclopropyl may be related to the change in sign of q. ... 

Cycloalkanes also exist in different conformations. The only exception to this is cyclopropane. Because it has only three carbon atoms, it is always planar. 

Figure 15.6 Depicting cycloalkanes. Cycloalkanes are usually drawn as regular polygons. Each side is a C—C bond, and each corner represents a C atom with its required number of H atoms. The expanded formulas show each bond in the molecule. The ball-and-stick and space-filling models show that, except for cyclopropane, the rings are not planar. These conformations minimize electron repulsions between adjacent H atoms. Cyclohexane (D) is shown in its more stable chair conformation.

The next example is of the Isomers for substituted cycloalkanes [77], assumed to undergo such rapid configurational changes as to average the environment of equatorial/awal substituents, so that only the avers e planar ring conformation need be considered [78]. We shall illustrate the enumeration with substituted cyclopropanes whose cycle indices can be obtained from the symmetries of the graphs S-7,... 

INDO calculation of hfs for planar and twisted (0 a ) [73Edg2] Radical obtained by photolysis of acety nd 90° 45 and 45°) conformations including bond angle deformation, peroxide in cyclopropane solutions of 1,3-butadiene. ... 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

The experimental data in Table 3.1 show that the calculated values of total angle strain are approximately correct only for cyclopropane, cyclobutane, and cyclopentane. Cyclohexane is definitely not the strained compound Baeyer s theory predicts, and the larger ring compounds are also not very strained. Any chemist today can explain the discrepancy between these calculated and experimental values of strain energy cyclohexane is not planar. In either the chair or boat conformations (Figure 3.12), all bond angles can be approximately 109.5°. In the chair conformation of cyclohexane, all bonds are staggered, and there are no apparent van der Waals repulsions in the molecule. ... 

For geometric reasons, cyclopropane is necessarily planar, and questions of conformation do not apply. The most important structural information obtained by electron diffraction measurements relate to the C-C bond lengths, which are slightly shorter than normal at 1.510 A, and to the H-C-H angle of 115°, which is opened up somewhat relative to the angle in cyclohexane. ... 

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