Stabilities of Cycloalkanes Ring Strain

Chemists in the late 1800s knew that cychc molecules existed, but the limitations on ring size were unclear. Although mrmerous compounds containing hve-membered and six-membered rings were known, smaller and larger ring sizes had not been prepared, despite many efforts. 

What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest experimental way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

Each H H eclipsing interaction in ethane costs about 4.0 kj/mol. How many such interactions are present in cyclopropane What fraction of the overall 115 kJ/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain  

How many H — H eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kj/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have Since the measured total strain of eyelopentane is 26 kJ/mol, how much of the torsional strain is relieved by puckering  

Two conformations of c/s-13-dimethylcyclobutane are shown. What is the difference between them, and which do you think is likely to be more stable  

FIGURE 4.3 Cycloalkane strain energies, calculated from thermodynamic heats of formation. Small and medium rings are strained, but cyclohexane rings are strain-free. 

Baeyer s theory is wrong for the simple reason that he assumed all cycloalkanes to be flat. In fact, as we ll see in the next section, most cycloalkanes are not flat they adopt puckered three-dimensional conformations that allow 

Adolf von Baeyer (1835-1917) was born in Berlin, Germany, and received his Ph.D. at the University of Berlin in 1858, working with Robert Bunsen and August Kekule. After holding positions at Berlin and Strasbourg, he was a professor at Munich from 1875 to 1917. He was the first to synthesize the blue dye indigo and was also discoverer of the first barbiturate sedative, which he named after his friend Barbara. Baeyer was awarded the Nobel Prize in chemistry in 1905. 

Problem 4.9 ds-1,2-Dimethylcyclopropane has more strain than f/w .s-l,2-dimethvTcyclo-propane. How can you account for this difference Which of the two compounds is more stable  

Problem 4.7 Name the following substances, including the cis- or irons- prefix (red-brown = Br)  

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Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

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