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Introduction to Cycloalkanes

Cycloalkanes also known as naphthenes are kinds of alkanes which have in the chemical structure of their molecules, one or more rings of carbon atoms. Alkanes are categories of organic hydrocarbon compounds which in their chemical structure have only single chemical bonds. Cycloalkanes contains only hydrogen (H) and carbon (C) atoms and because there are no multiple C-C bonds to hydrogenate (add more hydrogen to) they are saturated. For cycloalkanes a common chemical formula would be CnH2 (n+1-g) where g = number of rings and n = number of C atoms in the molecule. Cycloalkanes with a single ring are known as analogously to their normal alkane counterpart of similar carbon count: cyclopentane, cyclopropane, cyclohexane, cyclobutane, etc.  Cycloparaffins are the larger cycloalkanes, with greater than the 20 carbon atoms.

Ring Conformations

Even though the customary line drawings of simple cycloalkanes are geometrical polygons, the definite shape of these compounds in most cases is very distinct.

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Cyclopropane is essentially planar or flat, with the carbon atoms at the corners of an equilateral triangle. Than the optimum 109.5º angles of a normal tetrahedral carbon atom the 60º bond angles are much smaller, and the resulting angle strain significantly affects the chemical behaviour of this cycloalkane. Cyclopropane also suffers considerable eclipsing strain, because all the carbon-carbon bonds are completely eclipsed. By folding (the out-of-plane dihedral angle is about 25º) Cyclobutane get reduced some bond-eclipsing strain, but the total angle strain and eclipsing still remains high. Cyclopentane has very slight angle strain (the angles of a pentagon are 108º), but if it remained planar its eclipsing strain would be large (about 10 kcal/mol). Subsequently, the five-membered ring adopts non-planar puckered conformations when possible. Rings that are larger than cyclopentane would have an angle strain if they were planar. Though, this strain, together with the eclipsing strain inherent in a planar structure, can be relieved through puckering the ring. Cyclohexane is a good illustration of a carbocyclic system that virtually eliminates angle strain and eclipsing by adopting non-planar conformations, like those shown below in the diagram. cyclooctane and Cycloheptane and have greater strain than cyclohexane have, in large part because of transannular crowding (steric hindrance by groups on opposite sides of the ring).

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Structure of planar for cyclohexane is clearly improbable. The bond angles would essentially be 120º, 10.5º larger than the perfect tetrahedral angle. As well as, every carbon-carbon bond in this type of structure would be eclipsed. The resulting eclipsing strains and angle would strictly destabilize this structure. Two carbon atoms on opposite sides of the six-membered ring are lifted out of the plane of the ring, much of the angle strain may be removed. Still this boat structure has two eclipsed bonds (colored magenta in the drawing) and severe steric crowding of two hydrogen atoms on the "stern" and "bow" of the boat. This steric crowding is frequently called steric hindrance. The steric hindrance can be partially relieved by twisting the boat conformation, but the twist-boat conformer still keeps some of the strains that characterize the boat conformer. At last, a relatively strain-free chair conformer is formed by lifting one carbon above the ring plane and the other below the plane. This is the predominant structure which is adopted by molecules of cyclohexane.

An energy diagram for these conformational interconversions is displayed below. The activation energy for the chair-chair conversion is suitable chiefly to a high energy twist-chair form (TC), in which eclipsing strain and significant angle are present. A facile twist-boat (TB)-boat (B) equilibrium intervenes as one chair conformer (C) changes to the other.

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Investigations which are concerning with the conformations of cyclohexane were started by H. Sachse (year 1890) and E. Mohr (year 1918), but it was not until 1950 that a full treatment of the manifold consequences of interconverting chair conformers and the dissimilar orientations of pendent bonds was explained by D. H. R. Barton who was awarded by the Nobel Prize 1969 together with O. Hassel).  This discussion presents some of the important characteristics of this conformational analysis.

By the careful examination of a chair conformation of cyclohexane, we discover that the twelve hydrogens are not structurally equal. Six out of them are located about the periphery of the carbon ring and are termed as equatorial. And the other six are oriented below and above the approximate plane of the ring (three in each location), and are termed as axial since they are aligned parallel to the symmetry axis of the ring. In the stick model displayed in the diagram, the equatorial hydrogens are in blue color, and the axial hydrogens are in red color. Because there are two equivalent chair conformations of cyclohexane in rapid equilibrium, all twelve hydrogens have 50% axial and 50% equatorial character.

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Since axial bonds are parallel to one another, substituents larger than hydrogen usually suffer greater steric crowding when they are oriented axial rather than equatorial. Subsequently, substituted cyclohexanes will preferentially adopt conformations in which large substituents assume equatorial orientation. The methyl carbon is colored blue in the two methylcyclohexane conformers that are shown above in the diagram. When the methyl group occupies an axial position it suffers steric crowding by the two axial hydrogens located on the similar side of the ring. This steric hindrance or crowding is related with the red-colored hydrogens in the structure. A careful inspection of the axial conformer displays that this steric hindrance is because of two gauche-such as orientations of the methyl group with ring carbons #3 and #5. The use of models is mainly helpful in evaluating and recognizing these relationships.

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