Planar carbon cycloalkane

VenepalliBR,AgostaWC (1987) Chem Rev 87 399 Agosta WC (1992) Inverted and Planar Carbon. In Patai S, Rappoport Z (eds) The Chemistry of Alkanes and Cycloalkanes. Wiley, New York, p 927... 

Rg. 1.4 The conceptual progression toward a promising planar-carbon cycloalkane... 

In a review of pyramidal carbon species [18], Rasmussen and Radom approach these from the viewpoint of their potentially planar carbon molecirles [35,36] from which one cap has been removed (Fig. 2.11). The resulting molecides are called hemialkaplanes and hemispiroalkaplanes. The former have a neopentane moiety capped by a cycloalkane, the latter a spiropentane moiety capped by a cycloalkarre. Molecules representing decapped alkaplanes do not have truly pyrarrridal carbon, as pointed out above for bowlane. However, decapping spiroalkaplanes can lead to genuine pyramidal-carbon stractures in particular, the iconic pyranridane. 

An approach which may yield a half-planar carbon like that in a polyprismane is to bend backwards the C-H bonds of a cycloalkane applying this to cyclobutane by using two C2 clamps gives us 55, tricyclo[2.2.1.l " ]octane. 

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. 

If a cycloalkane requires bond angles other than 109.5°, the orbitals of its carbon-carbon bonds cannot achieve optimum overlap, and the cycloalkane must have some angle strain (sometimes called Baeyer strain) associated with it. Figure 3-14 shows that a planar cyclobutane, with 90° bond angles, is expected to have significant angle strain. 

In contrast to the rotational freedom around single bonds in open-chain alkanes, there is much less freedom in cycloalkanes. Cyclopropane, for example, must be a rigid, planar molecule (three points define a plane). No bond rotation can take place around a cyclopropane carbon-carbon bond without breaking open the ring (Figure 3.8, p. 100). 

Cyclopropane is the only cycloalkane in which aU the ring carbons lie in the same plane. In all other cycloalkanes, the ring is nonplanar. A planar cycloalkane is destabilized by torsional strain and, in most cases, angle strain. 

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

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 ... 

To examine the structural a.spects of the.se molecules, you will find your set of models to be indispensable. Cyclopropane is the only flat cycloalkane ring. All larger cycloalkanes are nonplanar. Ring distortion away from a planar structure reduces eclipsing interactions between neighboring carbon-hydrogen bonds. 

The lack of free rotation around C—C bonds in disubstituted cycloalkanes leads to an extremely important kind of isomerism called stereoisomerism. Two different compounds that have the same molecular formula and the same structural formula but different spatial arrangements of atoms are called stereoisomers. For example, consider a molecule of 1,2-dimethylcyclopentane. The cyclopentane ring is drawn in I Figure 1.17 as a planar pentagon with the heavy lines indicating that two of the carbons are in front as one views... 

The carbon atom rings of cycloalkanes are usually shown as planar, although only cyclopropane is planar. Because rotation about the single bonds in the ring is restricted, certain disubstituted cycloalkanes can exist as geometric (cis-trans) isomers. 

The cycloalkanes are commonly represented by polygons in which each corner represents a carbon atom with two attached hydrogen atoms and the lines represent C—C bonds. The C—H bonds are not shown, but are understood. Other common cycloalkanes include cyclobutane (C Hg), cyclopentane (C Hjq), and cyclohexane (CgHj2). These cychc compounds are represented as planar projections below even though chemists know that the rings are not planar in reality. 

The term planar in discussions of cycloalkane conformations refers only to the carbon atoms. Glendening, E. D. Halpern, A. M. /. Phys. Chem. A. 2005,109,635 concluded that hyperconju-gative stabilization has an additional stabilizing effect on the puckered conformation. 

What are the conformations of cycloalkanes Cyclopropane, with only three carbon atoms, is necessarily planar (because three points determine a plane). The C—C—C angle is only 60° (the carbons form an equilateral triangle), much less than the usual sp tetrahedral angle of 109.5°. The hydrogens lie above and below the carbon plane, and hydrogens on adjacent carbons are eclipsed. 

Cycloalkanes with more than three carbon atoms are nonplanar and have puckered conformations. In cyclobutane and cyclopentane, puckering allows the molecule to adopt the most stable conformation (with the least strain energy). Puckering introduces strain by making the C—C—C angles a little smaller than they would be if the molecules were planar however, less eclipsing of the adjacent hydrogens compensates for this. 

Cyclic Hydrocarbons A cyclic hydrocarbon contains one or more rings in its strac-ture. When a straight-chain alkane (C H2 +2) forms a ting, two H atoms are lost as the C—C bond forms to join the two ends of the chain. Thus, cycloalkanes have the general formula C H2 . Cyclic hydrocarbons are often drawn with carbon-skeleton formulas (Figure 15.6, top row). Except for three-carbon rings, cycloalkanes are non-planar, as the models show. This stmctural feature arises from the tetrahedral shape around each C atom and the need to minimize electron repulsions between adjacent H atoms. As a result, orbital overlap of adjacent C atoms is maximized. The most stable form of cyclohexane is called the chair conformation (Figure 15.6D). 

---