Four-membered ring compounds strain energy

The first mechanism is a pinacol rearrangement and the compound is symmetrical so it doesn t matter which alcohol is protonated. Both three- and four-membered rings are strained and the cf-bonds are more reactive (higher HOMO energy) than normal. This makes ring contraction an easy reaction even though the strain is not relieved. 

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. 

Table I summarizes the calculated strain energies of compound of the type E H , where n is 4, 6, and 8 and E is C, Si, Ge, and Pb, derived from homodesmic reactions (16b). The strain energies of silicon and carbon tetrahedranes are very similar (140 kcal/mol). However, replacement of the three-membered rings by four-membered rings results in a significant decrease in strain energy 140.3 (Si4H4) > 118.2 (SieHe) > 99.1 (SigHg)...

It is not that this cycloaddition is energetically unprofitable—in spite of the ring strain, the cyclobutane is lower in energy than the two alkenes—so there must be ahighkinetic barrier to the cycloaddition of one alkene to another. This is a deeply importantpoint, and it is just as well that it is true—if alkenes and other double-bonded compounds could readily dimerise to form four-membered rings, there would be few stable alkenes, and life would be impossible. 

A recently reported investigation of the gas-phase iodination of cyclobutane between 589 and 662 K provides a value of Afff (298) = 51.14( 1.0)kcal mol for cyclobutyl radical. The C—bond energy of cyclobutane was found to be 1.8 kcal mol" higher than that of a normal secondary C—H bond and this was related to the increase in strain upon the development of an sp hybridized carbon in a four-membered ring. In gas-phase chlorinations, cyclobutane is also known to be less reactive than cyclopentane and cyclohexane. However, an interesting result has been obtained in the gas-phase chlorinations of chlorocyclobutane between 35 and 195 °C, and of methyl-cyclobutane between 74 and 150°C at ca. 58Torr. " Both substituted compounds are more reactive than the cyclopentyl and cyclohexyl homologues and this has been ascribed to release of some steric strain with formation of the substituted cyclobutyl radical. This is particularly the case for reaction at the tertiary position. 

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