Cyclohexane ring strain

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

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 ... 

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C—H bonds are staggered. Chair cyclohexane has two kinds of positions axial and equatorial. Axial positions are oriented up and down, parallel to the ring axis, whereas equatorial positions lie in a belt around the equator of the ring. Each carbon atom has one axial and one equatorial position. 

Amantadine is an antiviral agent that is active against influenza A infection and against some strains of H5NX avian flu. Draw a three-dimensional representation of amantadine showing the chair cyclohexane rings. 

This monomer may be considered as a compound composed of two cyclohexene rings in the boat configuration. Presumably, the difference in energy between the cyclohexene rings of the monomer and the cyclohexane ring in the polymer, which is free to assume the less strained chair configuration, is responsible for the reactivity. 

O-M-O bond angles required. One can relate bite sizes (distances between the donor atoms) of strain-free chelate rings of different sizes to the dimensions of the chair form of the cyclohexane ring (39), which has the minimum steric strain possible for a cycloalkane. The bite size of strain-free four- and six-membered chelate rings both correspond to... 

Cyclohexane has the lowest heat of combustion per CH2 group (658.7 kJ mol-1).=> the same as unbranched alkanes (having no ring strain) => cyclohexane has no ring strain. 

Cyclopentane has a smaller amount of strain than does cyclohexane when forced to have all its carbons in a plane (flat). Cyclopentane is nearly flat since the bond angles in a regular pentagon (108°) are near the tetrahedral angle (109.5°). The bond angles in a flat cyclohexane are 120°, which are not near the 109.5° bond angle. To avoid this strain, the cyclohexane ring buckles and becomes nonplanar. 

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

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. 

Since cycloalkanes do not have isomers, the heat evolved has been calculated per CH2 group. First we find the heat of combustion of cyclohexane which is most stable and then the heat of combustion per CH2 group. It comes to 157.4 K cals/mole. Now to calculate the ring strain for other cycloalkanes, we multiply 157.4 K cal/mole by n and then subtract it from the heat of combustion of cycloalkane. 

As an illustration, the ring strain in cyclopropane, cyclobutane and cyclohexane and cyclodecane has been calculated ... 

Problem 9.13 Among the cycloalkanes with up to 13 ring C s, cyclohexane has the least ring strain. Would one expect this if cyclohexane had a flat hexagonal ring ... 

Cyclohexane minimizes its ring strain by being puckered rather than flat. The two extreme conformations are the more stable chair and the less stable boat. The twist-boat conformer is less stable than the chair by about 23 kJ/mol, but is more stable than the boat. It is formed from the boat by moving one flagpole" to the left and the other to the right. See Fig. 9-5. 

Indeed, application of ring strain using Siegel s bicyclic annelation strategy32-36 199-200 has proven to be a very fruitful way of achieving the cyclo-hexatriene motif. However, in a recent work, Jemmis and Kiran201 compared the effect of annelation on cyclohexane and benzene. Their results showed that while the annealated cyclohexanes and benzenes... 

Die main things to know about nng structures are in terms of energy. For instance, for small rings, ring strain is lowest in cyclohexane, so it is the most stable ring structure. Also, large substituents in the axial position require more energy and create less stability. 

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