Cycloalkenes Cyclopentenes, Cyclopropenes

Among the cycloalkenes, cyclobutene, cyclopropene and cylcohexene are most common. Cyclobutene is about 4 kcal/mol more strained than cyclopentene. The smaller bond angles mean more deviation from 120°, and this makes cyclobutene more reactive than cyclopentene. 

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). 

The bond angle of cycloalkenes can be used to estimate the LUMO and predict reactivity. Smaller bond angles correlate with lower LUMOs and greater reactivity. Thus the bond angles and reactivities for cycloalkenes follow the same order cyclopropene > cyclobutene > cyclopentene = norbomene > cycloheptene = cyclooctene > cyclohexene. Cyclopropene is the most reactive and cyclohexene is the least reactive. 

A student wished to prepare of series of cyclobutanes that can be considered to be dimers of cycloalkenes. Sensitized photolysis of the cycloalkenes proceeded efficiently for cyclopropene, cyclobutene, cyclopentene, cyclohexene, and cyclohcptene. However, cyclooctene produced little or no dimer. Rationalize this result. 

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