Cycloalkanes Cyclopentanes, Cyclopropanes

At one time all cycloalkanes were believed to be planar It was expected that cyclopentane would be the least strained cycloalkane because the angles of a regular pentagon (108°) are closest to the tetrahedral angle of 109 5° Heats of combustion established that this is not so With the exception of cyclopropane the rings of all cycloalkanes are nonplanar... 

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

The data in Figure 4.3 show that Baeyer s theory is only partially correct. Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted, and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of 14 carbons or more are strain-free. Why is Baeyer s theory wrong ... 

Cycloparaffins (cycloalkanes single bonds) / C. /C C H2 - no double bonds Cyclopropane, cyclobutane, cyclopentane Compounds are unsaturated since ring can be broken C H2 + H, —> C H2 +2... 

Cycloalkanes. Cycloalkanes are conformationally restricted alkanes. Three rings are employed in drug design cyclopropane, cyclopentane, cyclohexane (the latter two are... 

Problem 9.5 (a) Calculate AH of combustion per CHj unit for the first four cycloalkanes, given the following AH s of combustion, in kJ/mol cyclopropane, -2091 cyclobutane, -2744 cyclopentane, -3320 cyclohexane, -3952. (b) Write (i) the thermochemical equation for the combustion of cyclopropane and (ii) the theoretical equation for the combustion of a CH unit of any given ring, (c) How do ring stability and ring size correlate for the first four cycloalkanes ... 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

The compounds we ve been dealing with thus far have all been open-chain, or acyclic, alkanes. Cycloalkanes, which contain rings of carbon atoms, are also well known and are widespread throughout nature. Compounds of all ring sizes from 3 through 30 carbons and beyond have been prepared. The four simplest cycloalkanes having three carbons (cyclopropane), four carbons (cyclobutane), five carbons (cyclopentane), and six carbons (cyclohexane) are shown at the top of the next page. 

Holroyd studied the type of the radicals formed in irradiated cyclopropane with the radical sampling technique and found several intermediates besides the cyclopropyl radical. Not all the hydrocarbons formed were identified but it was found that cyclopropyl and allyl radicals were both present in approximately a 1 1 ratio, indicating that ring-opening is a major process in cyclopropane. The extent of ring-opening decreases for cyclobutane and decreases further for cyclopentane. Similar results that the probability of C-C bond rupture falls from cyclopropane to cyclohexane were observed in the radiolysis of liquid cycloalkanes in an ESR study. ... 

Cycloalkanes are alkanes that contain rings of-CH2- units. The first three members are cyclopropane (C3H6), cyclobutane (C4H8), and cyclopentane (C5H10), all shown on the right. 

Cycloalkanes can adopt different conformations (or shapes) cyclopropane is flat, cyclobutane can form a butterfly shape, while cyclopentane can form an open-envelope shape. 

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