Cyclobutane, strain energy

The modification of molecular conformation from the highly strained non-isolable dimer molecule to the V-shaped dimer molecule (6 OPr-dimer) is explained in terms of relaxation of the strain energy due to the bond angle in the non-isolable dimer, which accumulated during the cyclobutane formation. Therefore, strictly speaking, the process going from the non-isolable dimer into the V-shaped dimer (6 OPr-dimer) is not a... 

The values of these Arrhenius parameters contrast dramatically with those obtained for the bicyclo[2,2,0]hexane isomerization. In this compound there is no weak bridgehead bond, and hence the reaction path is more closely akin to that for cyclobutane itself. The similarity of the A factors for this reaction and that for other simple cyclobutanes supports this contention. If this is so, then the lowering of the energy of activation in this bicyclic compound by some 7 kcal mole from that observed in the alkylcyclobutanes is to be attributed to extra strain energy in this molecule. 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below ... 

Stability. The four-membered ring in the diazabiphenylene series of compounds is strained due to bond angle deformation . The conventional ring strain energy for cyclobutane is 111 kj moP alkyl substituents stabilize the ring by only a few kilojoules per mole. 

The strain energies of cyclopropane (26.5 kcaFmol) and cyclobutane (26 kcal/ mol) are nearly the same despite the apparent much greater bond angle deformation with the former. One reason is that the weak C—C bonds in cyclopropane are compensated in part by the stronger C—H bonds. The increased strength results from the greater s character, and it is known that C—H bond strengths increase with... 

Although cyclopropane and cyclobutane have similar strain energies, they differ markedly in their reactivity toward electrophiles. Thus, whereas cyclopropane reacts readily with bromine to give 1,3-dibromopropane, cyclobutane does not react with bromine. 

The increased reactivity of cyclopropanes results from the presence of bent bonds which can interact with electrophiles, and can be more easily cleaved thermally than ordinary C C bonds. One indication of the consequences of the distortion is found in the strain energies (SE)37 that are calculated as the difference between the observed heat of formation and that estimated for a strain-free model. One might, for example, consider cyclohexane as strain-free, and then a model for cyclopropane would be half the heat of formation for cyclohexane. The available data for heats of formation of cyclopropane and cyclobutane derivatives are given in Table 2,38 The heat of formation of cyclohexane is — 29.4 kcal/mol, and the strain energy of cyclopropane is 12.7-0.5( — 29.4) or 27.5 kcal/mol. 

In many cases, the strain energies for bicyclic compounds are approximately the sum of the strain energies of the component rings. This is seen with bicyclo[2.1.0]pentane, bi-cyclo[3.1.0]hexane, and many other compounds. It applies even to cubane, in which the strain energy is equal to six times the strain energy of a cyclobutane ring. 

The calculated energies again confirm that cyclopropane is much more easily attacked by electrophiles than is cyclobutane, and this accounts for the common observation that cyclobutanes are much less reactive toward electrophiles than are cyclopropanes, despite the similar strain energy relief for these compounds.55 The reactions of cyclopropane with other electrophiles, such as mercuric ion,65 and metal radical cations,67 have also been studied. 

Many of the unique properties of cyclopropanes, and to a lesser extent, cyclobutanes, are derived from the formation of bent bonds. They may act in a fashion similar to 7r-bonds in interacting with electron-deficient centers, and are more easily cleaved thermally via electrophilic attack than are ordinary C-C bonds. The strain energy associated with bond angle deformation is also an important quantity, especially when considering thermal reactions. 

Several authors have pointed out that the conventional strain energy (CSE) of 1 (27.5 kcal mol1) is about of the same magnitude as that of cyclobutane (26.5 kcal mol1) and there-... 

TABLE 8. Bond energies and strain energies of cyclopropane, cyclobutane and propane as calculated by the virial partitioning method"... 

TABLE 10. Conventional strain energies (CSE), hybridizations, s-character, overlap values, overlap repulsions and geminal delocalizations of propane, cyclobutane, cyclopropane and their heterologues with X = NH, O, SiH2, PH, S from Reference 47 ... 

It is well-recognized that the hydrocarbons cyclopropane and cyclobutane have nearly identical strain energies, and so these microcycles have been quite naturally paired in numerous treatments of molecular strain. How similar are cyclobutylamine (12, X = NH2, 13, n = 4, X = NH2) and cyclopropylamine (2, X = NH2,13, n - 3, X = NH2) and other correspondingly monosubstituted cyclobutanes and cyclopropanes18 What about... 

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