Open-chain alkanes

Substituted cycloalkanes are named by rules similar to those we saw in the previous chapter for open-chain alkanes (Section 3.4). For most compounds, there are only two steps. 

In many respects, the chemistry of cycloalkanes is like that of open-chain alkanes both are nonpolar and fairly inert. There are, however, some important differences. One difference is that cycloalkanes are less flexible than open-chain alkanes. Jn contrast with the relatively free rotation around single bonds in open-chain alkanes (Sections 3.6 and 3.7), there is much less freedom in cycloalkanes. 

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. 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Simple carboxylic acids derived from open-chain alkanes are systematically named by replacing the terminal -e of the corresponding alkane name with -oic acid. The —C02H carbon atom is numbered Cl. 

Chapter 3, Organic Coi ipounds Alkanes and Their Stereochemistiy—The chapter has been revised to focus exclusively on open-chain alkanes. 

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. 

This general picture is similar to that observed in the ring-closing propensity of open chain alkanes, where a gem-dimethyl group greatly improves the yield of cycles as opposed to polymers (see 87, 88). This is known as the Thorpe-Ingold effect and has been examined in depth 1451. The steric bulk of the substituents diminishes the conformational space available for the open pre-... 

Open-chain alkanes, alkyl halide reduction, 29-31 Organosilicon hydrides bond strengths, 5-6 hypervalent silicon species, 9-11 ionic hydrogenation, 5 trivalent silicon species, 7-9 Orthoesters, reduction of, 97-99 Oxime reduction, 102... 

The simplest of the saturated cyclic hydrocarbons, or cycloalkanes, is cyclopropane, C3Hg, the molecules of which are made up of three carbon atoms to each of which two hydrogen atoms are attached. Cyclopropane is somewhat more reactive than the corresponding open-chain alkane, propane, C3Hg. Other cycloalkanes make up a part of ordinary gasoline. 

Bond angle deformation is one of the more common sources of strain. A C—C—C bond in an open-chain alkane has an angle of 111°. For relatively small angular deviations, the increase in energy would approximately be given by... 

Butane (C4H10) can exist in two different isomeric forms, e.g. n-butane and isobutane (2-methylpropane). Open chain alkanes have free rotation about their C—C bonds, hut cycloalkanes cannot undergo free rotation, so substituted cycloalkanes can give rise to cis and trans isomers (see Section 3.2.2). 

This results in a molecule whose energy is comparable to a completely staggered, open-chain alkane. This is easy to see by viewing the molecule in a Newman projection. Viewing the molecule through the ring so that the two side carbon-carbon bonds are seen head on, as in a Newman projection, generates the view in (6.5). 

Most cycloalkanes resemble the acyclic (noncyclic), open-chain alkanes in their physical properties and in their chemistry. They are nonpolar, relatively inert compounds with boiling points and melting points that depend on their molecular weights. The cycloalkanes are held in a more compact cyclic shape, so their physical properties are similar to those of the compact, branched alkanes. The physical properties of some common cycloalkanes are listed in Table 3-4. 

Unlike open-chain alkanes, cycloalkanes have much less rotational freedom. 

Strategy The steps for naming a cycloalkane are very similar to the steps used for naming an open-chain alkane. 

The isomerization of open-chain alkanes with more than six carbon atoms gives isobutane as the main product, together with disproportionated materials, even though the reaction proceeds by the monomolecular pathway [144]. On the other hand, for cyclic alkanes the monomolecular process with preservation of the cyclic structure seems to be the most probable, judging from the results for cyclohexane. The absence of isobutane in the products indicates that the reaction path does not involve open-chain intermediate species. Therefore, it is of interest to try cycloalkanes larger than cyclohexane for clarification of the reaction mechanism along with the catalytic action of S04/Zr02. 

In this chapter we shall take up the alicycUc hydrocarbons (a//phadc cyclic hydrocarbons). Much of the chemistry of cycloalkanes and cycloalkenes we already know, since it is essentially the chemistry of open-chain alkanes and alkenes. But the cyclic nature of some of these compounds confers very special properties on them. It is because of these special properties that, during the past fifteen years, alicyclic chemistry has become what Professor Lloyd Ferguson, of the California State College at Los Angeles, has called the playground for organic chemists. It is on some of these special properties that we shall focus our attention. 

Examination of the data for a great many compounds has shown that the heat of combustion of an aliphatic hydrocarbon agrees rather closely with that calculated by assuming a certain characteristic contribution from each structural unit. For open-chain alkanes each methylene group, —CH2—, contributes very close to 157.4 kcal/mole to the heat of combustion. Table 9.2 lists the heats of combustion that have been measured for some of the cycloalkanes. 

WeVe discussed only open-chain alkanes up to this point, but chemists have known for over a century that compounds with rings of carbon atoms also exist. Such compounds are called cycloalkanes, or alicyclic compounds (aliphatic cyclic). Since cycloalkanes consist of rings of -CH2- units, they hav e the general formula or C R-in, and are represented by polygons... 

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). 

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