Cyclobutane, chemical properties

The two smallest cycloalkanes, cyclopropane and cyclobutane, show certain chemical properties that are entirely different from those of the other members of their family. Some of these exceptional properties fit into a pattern and, as we shall see, can be understood in a general way. 

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

Clearly a quantum chemical calculation of the energy surface for this reaction would have to be based on a multiconfigurational wave function, with four active orbitals, the k orbitals of the two ethene molecules, and four active electrons. However, a complication appears cyclobutane is a quadratic molecule with all four carbon-carbon bonds equal. Our wave function does not have this property. The CC bonds between atoms A and B are treated using... 

A unique treatment of cyclopropane has been advanced by Dewar, who introduced the concept of a aromaticity, which explains some of the anomalous chemical and physical properties of cyclopropane. The notion of cr-conjugation implies that three cr-bonds form a cyclic system of six electrons thus cyclopropane is aromatic by the (4n -1- 2) rale. This explanation well accounts for the strain energy of cyclopropane. The actual value (27.5 kcal mol ) is much lower than the predicted value of 104 kcal mol (1 cal = 4.2 J), calculated from the C—C—C bending force constants obtained spectroscopically. - A similar comparison for cyclobutane (antiaromatic by the above notion and the 4n rule) underestimated the strain energy. o -Aromaticity also accounts for such observations as H NMR chemical shifts and the reactivity of cyclopropane toward electrojAiles. 

The monocyclic hydrocarbons (C H2 ) found in nature contain from 3 to 30 carbons, although in principle there is no limit to the ring size. Even so, cyclopentanes and cyclohexanes are the most abundant in nature. The analysis of single compound monocyclic alkanes is in many respects similar to the analysis of the open-chain alkanes. However, for small rings (cyclopropane, cyclobutane) some added methods can be employed because of their higher chemical activity, due to the distortion of the C—C—C angles. The olefmic properties of cyclopropanes were reviewed by Charton. ... 

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