Solvation secondary steric effects

Secondary steric effects on chemical reactivity can result from the shielding of an active site from the attack of a reagent, from solvation, or both. They may also be due to a steric effect on the reacting conformation of a chemical species that determines its concentration. 

The secondary steric effect occurs when there is hindrance to solvation and it is base weakening. The primary steric effect is credited with the enhanced basicity of aniline, which is more basic than 2,6-dimethyl-aniline due to greater steric inhibition for reaction with a proton. The secondary effect is probably responsible for the difference in basicity between A,A-diethylaniline (pA"a, = 6.65) and A,iV-dimethylaniline... 

N methylation changes secondary amines into tertiary amines. While inductive effects might mean an increase in the amine gas phase basicity, the low solvation of tertiary amines generally means that methylation reduces solution basicity (compare entries 1 and 7). In addition to solvation effects, it is thought that steric effects may favour the out con former. In the case of tetramethylcyclam (entry 8)... 

The author s theory which has been used here was developed in detail to explain the polymerisations by ionising radiations of some alkyl vinyl ethers, the polymerisations of which proceed by secondary ions. Although it was shown that the theory is also perfectly serviceable for the tertiary carbenium ions considered here, it must be realised that there is a fundamental difference between these two types of carbenium ions. When one of the bonds of the carbenium ion is a C—H bond, the solvators, especially of course an ion, can get much closer to the positive centre, and they are therefore correspondingly more firmly held to which effect is added that of a smaller steric hindrance. The most researched monomer propagating by secondary cations, apart from the alkyl vinyl ethers, is, of course, styrene. Thus, Mayr s many studies with diaryl methylium cations are directly relevant to the polymerisation of styrene. 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

The differences in basicity between ammonia, and primary, secondary, and tertiary alkylamines result from the interplay between steric and electronic effects on the molecules themselves and on the solvation of their conjugate acids. In total, the effects are small, and most alkylamines are very similar in basicity. 

The success of the reaction depends on the preferential extraction of tetrabutylammonium alkoxide rather than the corresponding hydroxide salt. The preference of the quaternary ammonium ion for alkoxide over hydroxide can be attributed to two factors. First, the soft (larger, more polarizable) quat prefers to pair with the soft (larger, more polarizable) alkoxide rather than the hard hydroxide ion. Second, hydroxide is more effectively solvated by the aqueous medium than is the alkoxide and prefers to remain in that phase. Under phase transfer conditions, secondary alcohols react more slowly than do primary alcohols, probably due to the lower acidity of secondary alcohols or to steric factors. 

X The data Table 5.10) for the aminopyridines and the N-methyl derivatives of their imino tautomers illustrate the general point that of two tautomers in equilibrium the weaker base predominates. A study of the temperature coefficient of the ionization constants for 4-dimethylamino-, 4-methylamino- and 4-aminopyridines suggests that the changes in basic strength caused by 3-substituents are due in the tertiary bases to steric inhibition of resonance, in the primary to steric hindrance to solvation, and in the secondary [cf. 155] to both of these effects . 4-Aminopyridine is useful... 

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