Strain, Back

The cause of this effect has been called B-strain (back-strain), and in this exanqjle only a modest rate enhancement is observed. As the size of the groups is increased, the effect on rate becomes larger. When all three of the groups in the above example are t-butyl, the solvolysis occurs 13,500 times faster than in the case of t-butyl p-nitrobenzoate. ... 

Correct manual lifting and handling of material prevents strains and injury and also reduces the effort required. Persistently incorrect lifting and handling may lead to the person suffering from permanent back strain or other health problems. Points to consider when preparing to lift material are ... 

From Table IV the relative magnitudes of the reactant state "sensitivity factor" (N) are 4>1>2=3= zero. From this analysis the decomposition rates of traiw-phenyl, alkyl diazenes (2) and iert-butyl peresters (3) can be predicted by assuming a dependence only on transition state effects, with no need to incorporate the back strain of the reactants into the equation. 

Back strain effects are most important for the homolysis of hydrocarbons (4), a highly endothermic reaction, which does not produce a stable molecule byproduct, as do diazenes (N2) and peresters (CO2). Destabilization of the reactants in reaction 4 back strain is essential in lowering the energy of activation of reaction. The results of this study suggest that only reaction 4 requires the use of A values to obtain a good correlation between reaction temperatures and calculated product radical stabilities. 

It is somewhat contradictory and not yet fully understood why the back strain effect on the rate of perester decompositions is so large. We had reasoned before from the discussion of conformational effects that the Ca-CO-bond of 25 is only stretched to a small extent at transition state. From an analysis of bond energies5 18 it becomes questionable if the homolysis of C-N-bonds (as in 20 ) and C-C-bonds (as in 25) is likely to be directly comparable5,12a 18 In addition the extent of Ca-CO-cleavage at the transition state of fragmentation of 25 may well be itself dependent on the... 

Uses. Diazepam is one of the oldest medications for treating muscle spasms, and has been used extensively in treating spasms associated with musculoskeletal injuries such as acute low-back strains. Diazepam has also been used to control muscle spasms associated with tetanus toxin the use of valium in this situation can be life-saving as well by inhibiting spasms of the larynx and other muscles.51,65... 

Incorporation of bulkier, more highly fluorinated aryl groups increases the Childs Lewis acidity, but calorimetric measurements on the reactions of III and V with simple Lewis bases or metallocenes reveal that back-strain upon pyramidalization at boron limits the eflFective Lewis acidity of these compounds. This notion is supported computationally.98 As an alternative to bulky, exhaustively fluorinated aryl groups, incorporation of the boron atom into an anti-aromatic 9-borafluorene ring, VI, results in comparable LA strength (Childs acidity = 0.70 0.02) to B(C6F5)3, despite the loss of two fluorine atoms. 

B strain Stands for back strain, and is the compression strain resulting from the steric crowding of non-bonding groups. 

Relief of steric strain also influences the rate of SnI reactions, although the effect is usually relatively small. When R is large there is more steric hindrance in the tetrahedral precursor than in the planar carbocation, accelerating the ionization process. A comparison of the chlorides in Table 2.15 3ig illustrates that as the steric crowding increases, the rate of ionization is accelerated. 31g Such crowding was called back strain by Brownl3ld and it contributes to the increased rate of ionization observed with hindered halides. 32d... 

Table 2.15 Increased Rate of Ionization Due to Back Strain...

---