Strain, intramolecular

Table 3.3. Correlation between Intramolecular Strain from Molecular Mechanics (MM) Calculations and Activation Energies for Dissociation of C—C Bonds in Substituted Ethanes"...

What was Ingold s preparation under Thorpe for the research program he set up at Leeds in the mid-1920s In his postgraduate work with Thorpe, Ingold already demonstrated considerable mathematical ability, in combination with a classical, geometrical way of thinking about intramolecular "strain." In 1920, he appended to a paper by R. 

Unfortunately, the situation is even more complicated In bi- and tricyclic compounds the a-SCS may experience further alterations apparently produced by intramolecular strain. The usually large a-SCS(CH3) in 1-methyl-bicyclo[2.2.1]heptane (9) (93) has already been mentioned. The a-effects of other substituents (OH, OCH3, Cl, I, and COOH) in the same molecular system do not correlate with those in bridgehead-substituted bicyclo[2.2.2]octanes or adamantanes again this was attributed to internal strain (125). The a-SCS(F)... 

An interesting observation should be made concerning the dependence of the physical properties on molecular cyclicity, since it will have a significant effect on the formulation of electrolytes for lithium ion cells. While all of the ethers, cyclic or acyclic, demonstrate similar moderate dielectric constants (2—7) and low viscosities (0.3—0.6 cP), cyclic and acyclic esters behave like two entirely different kinds of compounds in terms of dielectric constant and viscosity that is, all cyclic esters are uniformly polar (c = 40—90) and rather viscous rj = 1.7—2.0 cP), and all acyclic esters are weakly polar ( = 3—6) and fluid (77 = 0.4—0.7 cP). The origin for the effect of molecular cyclicity on the dielectric constant has been attributed to the intramolecular strain of the cyclic structures that favors the conformation of better alignment of molecular dipoles, while the more flexible and open structure of linear carbonates results in the mutual cancellation of these dipoles. 

A single bond/ shown by a short stroke, or often by a dot, indicates the mutual ci gcmcnt, or satisfying, of one of the four valencies of each of the two carbon atoms concerned. In a double bond, two, and in a triple bond, three, of the valencies of each carbon atom enter into a similar alliance. Double and triple bonds are evidence of intramolecular strain, and hence of enhanced reactivity as compared with single bonds and molecules containing them are said to be unsaturated. ... 

The paradigm is clear in molecules subject to the phenyl factor, the crystal field energy is large and dominant. The classic organic example where the crystal field dominates molecular structure is biphenyl, where the isolated molecule has its rings twisted by 42 but the crystal lattice imposes coplanarity it is estimated that the intramolecular strain in the coplanar molecule is 8.3kJmol", while the crystal field is 19.4kJ mol" [52c]. 

The presence of configurational disorder in the crystalline state of atactic PAN necessarily implies the presence of conformational disorder to alleviate intramolecular strains. The conformational disorder, in turn, yields small deviations from the perfect straightness of crystalline portions of the chains, and thus some waviness, which comply more easily with the local packing of - CN groups. 

Gould, E. S. Mechanism and Structure in Organic Chemistry Holt, Rinehart and Winston New York, 1959 p. 485 noted that this result could occur because of steric effects between the base and the substrate or because of intramolecular strain in the substrate in conformations required for E2 elimination to occur. 

Correlation between intramolecular strain and activation energies for dissociation of C—C bonds in substituted ethanes... 

In these representations, the configuration about each double bond is as. Because of angle strain, it is not possible to have a trans configuration in cycloalkenes of seven or fewer carbons. To date, frara -cyclooctene is the smallest frawscycloalkene that has been prepared in pure form and is stable at room temperature. Yet, even in this frawscycloalkene, there is considerable intramolecular strain. as-Cyclooctene is more stable than its trans isomer by 38 kj/mol (9.1 kcal/mol). 

It is also clear that intramolecular strain (sometimes called back strain or B-strain ), which results from steric repulsion between groups attached to the carbon bearing the leaving substituent, also plays an important role in accelerating the reaction of haloalkenes. Such ground-state destabilization is, for example, used to account for the observations that, under the same conditions, 2-chloro-2,4,4-trimethylpentane undergoes solvolysis about 20 times faster than 2-chloro-2-methylpropane and 4-chloro-2,2,4,6,6-pentamethylheptane is nearly 500 times faster (Table 7.3) ... 

Figure 11.17 Strategy to report intramolecular strain with a fragmented YFP.

Smaller coordination numbers should lead to shorter Ca-I, Ca-C, and Ca-O/N bonds due to reduced intramolecular strain. However, small coordination numbers are achieved by introduction of bulky groups and ligands, partly counteracting this effect. 

---