Cycloalkane rings Common

The last point we ll consider about cycloalkane stereochemistry is to see what happens when two or more cycloalkane rings are fused together along a common bond to construct a polycyclic molecule—for example, decalin. 

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. 

Other aliphatic rings are common, including cyclopentane (with five carbons) and cycloheptane (with seven carbons). Forming a cycloalkane ring from... 

The napthanes (C H2n), or cycloalkanes, are ring or cyclic saturated structures, such as cyclo-hexane (CgH 2) though rings of other sizes are also possible. An important series of cyclic structures is the arenes (or aromatics, so called because of their commonly fragrant odours), which contain carbon-carbon double bonds and are based on the benzene molecule. 

In addition to the ahphatic (chain) molecules, the saturates contain cycloalkanes, called naphthenes, having mainly five or six carbons in the ring (Fig. 5). Methyl derivatives of cyclopentane and cyclohexane ate commonly found in greater quantity than the parent unsubstituted stmctures and can be present at levels above 2% (2). Fused-ting dicycloalkanes such as decahydronaphthalenes (decalins) and hexahydroindans are also common, but nonfused bicyhc naphthenes, eg, cyclohexyl cyclohexane, are not. 

Alicyclic Hydrocarbons. These refer to cyclic analogues of aliphatic hydrocarbons and are named accordingly, using the piefix cyclo-." Their properties are similar to their open-chain aliphatic counterparts. Alicyclic hydrocarbons are subdivided into monocyclic (cycloalkanes, cycloalkenes, cycloalkynes, cycloalkadienes, etc.) and polycyclic aliphatic compounds. Monocyclic aliphatic structures having more than 30 carbon atoms in the ring are known, but those containing 5 or 6 carbon atoms are more commonly found in nature [47, p. 28]. 

Larger cycloalkanes have increasing rotational freedom, and the very large rings (C25 and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C3-C7L however, are severely restricted in their molecular motions. 

Substituted cyclohexanes are the most common cycloalkanes and occur widely in nature. A large number of compounds, including steroids and many pharmaceutical agents, have cyclohexane rings. The flavoring agent menthol, for instance, has three substituents on a six-membered ring. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

Cycloalkanes of ring sizes ranging from three to 30 are found in nature. Compounds containing five-membered rings (cyclopentane) and six-membered rings (cyclohexane) are especially common. 

Adsorption modes and hydrogenolysis were also correlated with other metal-catalyzed reactions. Gault noticed striking similarities in product distributions of isomerization and ring opening of cycloalkanes. Kinetic and tracer studies provided useful data252 268 to arrive at the conclusion that a common surface intermediate is... 

Comparison of the heats of combustion of cycloalkanes (Table 9.1) shows that cyclopropane, cyclobutane, and cyclononane yield more energy per methylene group than the other cycloalkanes. This can be attributed to strain resulting from bond-angle distortion (Baeyer strain), eclipsed conformations (Pitzer strain), and trans-annular, repulsive van der Waals interactions. Common (five- and six-membered) rings and large (more than twelve-membered) rings have little or no strain. This... 

These are cyclic alkanes (hydrocarbons) and are also called alicyclic compoimds. Their general formula in CH2n. In these the carbon atoms are linked to form a rings of various sizes. The six membered carbon ring being most commonly found. Most of there cycloalkanes are unreactive to chemical reagents. 

From heat of combustion data, cyclopropane has 26.7 kcal/mol (111.6 kJ/mol) of strain energy. Most of this strain is due to angle strain, but the contribution due to torsional strain is also significant. As we will see later, this strain energy causes cyclopropane to be more reactive than a normal alkane or cycloalkane. However, even though cyclopropane rings are reactive, they are fairly common in organic chemistry. 

Although all the simple cycloalkanes (up to about C2o) have been synthesized, the most common rings contain five or six carbon atoms. We will study the stabilities and conformations of these rings in detail because they help to determine the properties of many important organic compounds. 

Before we discuss the ring strain of different cycloalkanes, we need to consider how ring strain is measured. In theory, we should measure the total amount of energy in the cyclic compound and subtract the amount of energy in a similar, strain-free reference compound. The difference should be the amount of extra energy due to ring strain in the cyclic compound. These measurements are commonly made using heats of combustion. 

We will cover the conformations of cyclohexane in more detail than other cycloalkanes because cyclohexane ring systems are particularly common. Carbohydrates, steroids, plant products, pesticides, and many other important compounds contain cyclohexanelike rings whose conformations and stereochemistry are critically important to their reactivity. The abundance of cyclohexane rings in nature is probably due to both their stability and the selectivity offered by their predictable conformations. Nature probably forms more six-membered rings than all other ring sizes combined. 

Suhsiittltcd cydohoxanea arc the mnat common cycloalkanes because of their wide occurrence in nature. A vast number of compountU. includine many important pharmaceutical ag-enta, contain cyclohexane rings. 

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