Alkanes cyclic, ring strains

Ring strain in small cyclic alkanes and silanes... 

Cyclic (CH2)T Same as in acyclic alkanes ring strain increases the wavenumbers up to 3100 cm-1 >CH2 (scissoring) lower than in acyclic alkanes (10-15 cm-1) ... 

The cycloalkane with three or four carbon atom rings behave like alkanes (i.e., they are reactive). Their reactivity is due to the fact that such cyclic structures are highly strained. 

The estimates of the RSE for small cyclic alkanes, cyclo-olefins, and cyclic ethers using all three reactions are listed in Table 3.21. The failure of the isodesmotic reaction is readily apparent by examining the RSE of the cyclic alkanes. For the rings having five or more carbon atoms, the isodesmic method predicts that there is no strain energy. Rather, it predicts that these cyclic species are more stable than their acyclic counterparts. The homodesmotic and GE reactions, which are identical for the cyclic alkanes, provide RSEs in excellent agreement with CRSE estimates. 

It is worthwhile to visit early views about naturally occurring organic ring compounds. The first cyclic molecule to have its structure established was benzene in 1865 and, for a brief period, six-membered rings were thought to be the only possibility. During the 1880s William H. Perkin, Jr. (1860-1929), the son of William H. Perkin (1838-1907) who discovered the dye mauve, and a student of Baeyer, established the existence of three-, four-, and five-membered carbocycles. Baeyer strain in cyclopropane causes it to react with HCl as if it were an alkene rather than an alkane. 

Pell and Pilcher have tabulated the strain energies of cyclic alkanes, ethers, imines, and sulfides (Table 12), which all decrease with ring size in the order 3 > 4 > 5 > 6. Thus if the same fraction of the strain energy is always developed in the transition state, it would be expected that if this were the only controlling factor the rates of reactions that form rings of this type would be in the order 6 > 5 > 4 > 3. 

The above arguments aside, it is clear (Problem 3.14) that,per -C//2- unit, small ring carbocycles have greater heats of combustion than linear alkanes. The difference is attributed to the strain associated with confining the system, and the original concept, which supposed (incorrectly) that planarity was required for all cyclic systems, is attributed to the German chemist A. Baeyer ([1835-1917] Nobel Prize, 1905) and is called Baeyer strain. 

Except for cyclohexane and the very large cyclic alkanes, all cyclic alkanes are strained to some extent. Any strain that primarily arises from non-standard bond angles is called angle strain (or Baeyer strain). The strain is often further broken down into small angle and large angle strain. However, as discussed, the strain in cyclic systems also arises in part from torsional strain. In the context of rings, this is often called Pitzer strain. 

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