B-strain

Figure 5.7, Isotropic yield surfaces in (a) stress space and (b) strain space.

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. ... 

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

Sample (B), strained 3-form, prepared by winding under tension, and then cooled to low temperatures ... 

For some tertiary substrates, the rate of SnI reactions is greatly increased by the relief of B strain in the formation of the carbocation (see p. 366). Except where B strain is involved, P branching has little effect on the SnI mechanism, except that carbocations with P branching undergo rearrangements readily. Of course, isobutyl and neopentyl are primary substrates, and for this reason they react very slowly by the SnI mechanism, but not more slowly than the corresponding ethyl or propyl compounds. 

Walsh S.L., Geter-Douglas B., Strain E.C., Bigelow G.E. Enadoline and butorphanol evaluation of kappa-agonists on cocaine pharmacodynamics and cocaine self-administration in humans. J. Pharmacol. Exp. Ther. 299 147, 2001. 

Fig. 9.12 (A) Time variation of elongational viscosity for PLA-based nanocomposite (MMT = 4wt%) melt at 170°C (B) Strain rate dependence of up-rising Hencky strain. Reprinted from [47], 2003 Wiley-VCH Verlag GmbH Co.

H-MA-MA-Br. This can be attributed to B-strain effect. Furthermore, the rate of activation of a dormant chain end with a MMA-Br terminal unit is approximately 20 times higher than with a MA terminal unit containing the same penultimate unit. This effect is induced by the formation of thermodynamically more stable 3° radical when compared to the less stable 2° radical. Lastly, the combined effects of B-strain and radical stability result in a 100-fold increase of k3 for H-MMA-MMA-Br. 

One possible explanation for the effect of increased branching is the relief of B strain when the bonds connecting the central carbon atom to the substituent groups spread out on passing to a less tetrahedral and more nearly planar arrangement.224... 

Some doubt is thrown on B strain as the sole explanation of the branching effect by the observation that the tri-te -butylboron-ammonia complex is actually less dissociated than the trimethylboron-ammonia complex.227 Since the products of the ionization of these highly branched compounds contain large amounts of rearranged material, another effect may be operating. As will be discussed in the next section, many ionization reactions produce directly an ion of structure different from that of the covalent parent compound. The transition state presumably resembles the new ion or a non-classical... 

A similar explanation has been offered as an alternative to B strain crowding at the backside may restrict a neighboring alkyl group to the position most favorable to interaction of its bonding atomic orbitals with the atomic orbital being vacated by the departing group. 

Figure 3.16 Solvus and spinodal decomposition fields as a function of elastic strain, (a) Strain-free or chemical solvus (b) strain-free spinodal (c) coherent solvus (d) coherent spinodal. From Ganguly and Saxena (1992). Reprinted with permission of Springer-Verlag, New York.

B834 B strain methionine auxotroph used for and selenomethionine labelling... 

Zanamivir (Relenza) is a neuraminidase inhibitor with activity against influenza A and B strains. Like oseltamivir, zanamivir is a reversible competitive antagonist of viral neuraminidase. It inhibits the release of progeny virus, causes viral aggregation at the cell surface, and impairs viral movement through respiratory secretions. Resistant variants with hemagglutinin and/or neuraminidase mutations have been produced in vivo however, clinical resistance to zanamivir is quite rare at present. 

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