Which of the following cycloalkanes has the smallest heat of combustion per CH_2

Question

Which of the following cycloalkanes has the smallest heat of combustion per CH_2 group? cyclopropane cyclobutene cyclopentane cyclohexane cycloheptane Which of the following statements regarding cyclobutene is correct? The lowest energy conformation of cyclobutene is a planar one in which all of the bond angles is 90degree. The lowest energy conformation of cyclobutene is known as the chair conformation. The lowest energy conformation is one in which the bond angles are slightly less than 90degree even though this increases angle strain. The lowest energy conformation is one in which the bond angles are greater than 90degree so that angle strain is significantly reduced. None of the above statements is correct. In the first propagation step of the free radical chlorination of methane, which of the following occurs? Cl_2 dissociates A chlorine radical abstracts a hydrogen A carbon radical reacts with Cl_2 A carbon radical reacts with a chlorine radical Two chlorine radicals combine In the reaction of Cl_2 with ethane and UV light, which of the following reactions would be a chain termination event(s)? reaction V reactions I and IV reactions III and IV reactions I and II reaction III In the reaction of CI_2 with ethane and UV light, which of the following reactions would be a propagation event(s)?

Explanation / Answer

12) D) cyclohexane

with decrease in the number of CH2 groups in the cycloalkane the increase in steric crowding increases 1)ring angle strain and 2) strain due to eclipsing .So, less energy is released on combustion.However optimal ring size and less eclipsing due to chair conformation of cyclohexane with H-atoms oriented axially and equatorially and in opposite directions in consecutive CH2 makes it the least strained ring .

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