Sizes of groups

If, on the other hand, it is assumed that the initial electrophilic attack is reversible and that addition can occur above or below the plane defined by the alkenes, two diastereomeric cations woidd be formed, perhaps in approximately equal concentration. Due to the proximity of the chiral acyl group, the probability of achieving the transition states (approximately represented by 146 and 147) would be unequal and a function of the difference in sizes of groups R and R (146 and 147). 

From an Arrhenius type plot like that of Fig. 2.82 the activation energies for these processes are obtained an the values are 14.6 and 57.0 kcal mol-1. these results give some idea about the differences in sizes of groups involved in the relaxation. 

Size-group (microns) Number of particles in each size Number less than maximum size of groups Percent of particles in each size-group Percent less than maximum size of group... 

May be limited in size of group Potential limitations in study design... 

The critical size of groups on the amino atom suggests the importance of the amino group (here protonated) fsx receptor attachment. The effect of the added chain length, once the critical size requirement is met, could be increased affinity. It appears to have been reasonably proved that die protonated. species of the phenothiazincs can bind to DA receptors.- ... 

Cleavage of thioethers. von Braun found that a dialkyl thioether is cleaved by cyanogen bromide at a somewhat elevated temperature (60-70°, sealed tube) to an alkyl thiocyanate and an alkyl bromide. Results for the first three mixed ethers disclose a clear relationship of Haftfestigkeit to size of group ... 

When the size of groups is responsible for reducing the reactivity at a site within a molecule, this is attributed to steric hindrance. When the size of groups is responsible for increasing the reactivity at a site within a molecule, this is attributed to steric acceleration. 

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