Ring strain measurements

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

Stress-Strain Data. Tensile tests were made with an Instron tester at some seven crosshead speeds from 0.02 to 20 inches per minute at five or six temperatures from 30° to —46°C. The tests were made on rings cut with a special rotary cutter from the circular sheets of the elastomers. The dimensions of each ring were determined from the weights of the ring and the disc from its center, the thickness of the ring, accurately measured, and the density of the rubber. Typically, the outside and inside diameters were 1.45 and 1.25 inches, respectively, and the thickness was about 0.085 inch. The test procedure used is described elsewhere (11), and the cubic equation, eq 4 in ref. j 2, was used to compute the average strain in a ring from the crosshead displacement. 

The lines Pr e Pr c indicated in the figures and it is apparent that the inequality is satisfied only for affine behaviour (Figure 4), as expected for the small-strain measurements used. The condition Pr e Pr c need not be met if a significant number of complex ring structures are formed pre-gel which then become elastic-ally active when incorporated in the gel. The present interpretation of the plots in Figures 4 and 5 makes the simplifying assumption that all pre-gel ring structures remain elastically inactive in the final network. 

About 30 years ago, an enthalpy of formation was reported for 3,3,4,4-tetramethyl-l,2-dioxetane . Both by direct microcalorimetric combustion measurements of the neat solid and by reaction calorimetry (of the solid itself, and in acetone solution to form acetone), a consensus value was derived. Now, is the enthalpy of formation plausible , notwithstanding the very large error bars Consider reaction 6 for the dioxetane that produces 2,3-dimethyl-2,3-butanediol . The liquid phase enthalpy of reaction is —329 kJmoU. It is remarkable that this value is compatible with that for the dialkyl peroxides, ca —335 kJmoU, despite the ring strain that might be expected. 

Many of these reactions support a measure of thermodynamic control in nucleophilic capture Conjugated radicals or products formed with release of ring strain are favored. For example, the addition of ethanol to radical cation 110 + is regiospecific, forming the more stable (benzylic) intermediate 111 + the capture of 112 + likewise forms a benzylic radical (113 ). Radical cation 48 + generates a... 

The reaction enthalpies for the acetolysis reaction given in equation 17 are discussed as a measure of ring strain as compared to the strainless and monomeric diethylmagnesium. 

Thermochemical measurements on the oxepin-benzene oxide system are unavailable. However, based upon experimental observations it would appear that the oxepin tautomer is generally more thermally stable than the benzene oxide as a result of the additional ring strain present in the oxirane ring. 

The resulting ring strain is also reflected in an increase in the C—H vibrational frequency and a decrease in the N—H vibrational frequency aa determined by measurement of the infrared and Kaman... 

Polymers that have several values of m in the range 3-13 have been prepared. The estimated ring strain energies of these monomers show a good correlation with measured heats of polymerization. 

Table 14.2 gives the 298 K rate constants for addition to the CH2=CH-, CH2=C<, cis- and trans- -CH=CH-, -CH=C< and >C=C< non-conjugated carbon-carbon double bond units and the various -OC- and conjugated >C=C-C=C< units, based on the measured rate constants for the alkenes with the appropriate alkyl substitutent groups (Atkinson, 1986 Atkinson and Kwok, 1995). There is no evidence for ring-strain effects on OH radical addition to cycloalkenes. 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

Before we discuss the ring strain of different cycloalkanes, we need to consider how ring strain is measured. In theory, we should measure the total amount of energy in the cyclic compound and subtract the amount of energy in a similar, strain-free reference compound. The difference should be the amount of extra energy due to ring strain in the cyclic compound. These measurements are commonly made using heats of combustion. 

The heat of combustion is the amount of heat released when a compound is burned with an excess of oxygen in a sealed container called a bomb calorimeter. If the compound has extra energy as a result of ring strain, that extra energy is released in the combustion. The heat of combustion is usually measured by the temperature rise in the water bath surrounding the bomb. ... 

---