Strain in cycloalkanes

The amount of strain in cycloalkanes is shown in Table 4.6, which lists heats of combustion per CH2 group. As can be seen, cycloalkanes larger than 13 membered are as strain-free as cyclohexane. 

We expect that the total strain in cycloalkanes of the type (CH2) should decrease rapidly in the order = 2>ra = 3> = 4. However, the data of Table 12-3 show that the order actually is 3 = 4 > 2. This difference in order often is disguised by dividing the heats of combustion by the numbers of CH2 groups and showing that the heats of combustion per CH2 are at least in the order expected from bond-angle strain. This stratagem does not really solve the problem. 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

As determined by heats of combustion, strain in cycloalkanes varies with ring size. 

What kinds of effects contribute to the ring strain in cycloalkanes We answer this question by exploring the detailed structures of several of these compounds. 

C-C bond Fluonnation increases the bond strengths in cycloalkanes, including cyclobutanes [75, 94], but by contrast, it decreases C-C bond strengths and increases nng strain in cyclopropanes and other three-membered nng compounds [75 94. 9S]... 

Conformational analysis is far- simpler in cyclopropane than in any other cycloalkane. Cyclopropane s three carbon atoms are, of geometric necessity, coplanar-, and rotation about its carbon-carbon bonds is impossible. You saw in Section 3.4 how angle strain in cyclopropane leads to an abnormally large heat of combustion. Let s now look at cyclopropane in more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry. 

In this chapter we explored the three-dimensional shapes of alkanes and cycloalkanes. The most important point to be taken from the chapter is that a molecule adopts the shape that minimizes its total strain. The sources of strain in alkanes and cycloalkanes are ... 

What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

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 ... 

While the conformation of cycloalkanes has been discussed in detail later, it will be worth while to see how the ring strain in such compounds is calculated as this will give us a broad picture about their relative stability. 

We know that all cycloalkanes do not have the same relative stability. Cyclohexane is most stable while cyclopropane and cyclobutane are much less stable, because they have a ring strain in their molecules. 

Products of ethylation and methylation of enolates of cycloalkane-1,3-diones with ring sizes 7-10 have been studied under a variety of alkylating reagent-solvent systems. Decrease in the 0 C alkylation ratios with increase in ring size is believed to be a consequence of greater steric strain in the conjugated enolate resonance contributor and consequent diminution in the proportion of D-attack. 

Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds. For cycloalkanes (see Table 1.2), the smaller the ring, the larger the overall strain becomes. What may appear at first to be surprising is that medium-sized rings containing 8-11 atoms... 

Unlike most homologous series, the different members of the cycloalkane family exhibit different chemical reactivities. We already have seen that chemical reactivity is related to the strain in the carbon-carbon bonds and that ideally the carbon bonds should have bond angles of 109.5°. 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

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