Steric hindrance bimolecular nucleophilic substitution

Students of reaction mechanism will recognize intuitively that the difference between the narrow and broad borderline regions observed for nucleophilic substitution of azide ion at secondary and tertiary carbon (Fig. 2.2) is due to the greater steric hindrance to bimolecular nucleophilic substitution at the tertiary carbon. This leads to a large difference in the effects of an a-Me group on (s ) for the stepwise solvolysis and s ) for concerted bimolecular nucleophilic... 

There is minimal steric hindrance in the transition state for coupled concerted bimolecular nucleophilic substitution at primary carbon [D 0, Fig. 2.4(11)] to... 

A number of methods for cleaving phosphorus esters proceed by nucleophilic attack on the alkyl group of the ester, rather than attack at the phosphorus. As expected for a bimolecular nucleophilic substitution reaction, these reactions are fastest for benzyl and methyl groups, and the rate drops rapidly as the steric hindrance around the alkyl group increases. Thus,... 

We have learned that primary haloalkanes undergo only bimolecular nucleophilic substitution. In contrast, secondary systems often transform through carbocation intermediates and tertiary systans virtually always do. The reasons for this difference are twofold. First, steric hindrance inCTeases along the series, thereby slowing down Sn2. Second, increasing alkyl substitution stabilizes carbocation centers. Only secondary and tertiary cations are energetically feasible under the conditions of the SnI reaction. 

Chapters 11 and 12 discuss reactions of alkyl halides to give either substitution or elimination products. It is clear from Chapter 12 that elimination occurs when the nucleophile is also a strong base and when substitution is inhibited due to steric hindrance. There are many cases in which substitution and elimination compete, particularly when the substrate is a secondary alkyl halide. The solvent plays an important role in these reactions, and solvent identification is a key parameter for distinguishing bimolecular versus unimolecu-lar (ionization) processes. The nature of the alkyl halide (1°, 2°, or 3°) is important, as is the strength of the nucleophile and whether or not that nucleophile can also react a strong base. This chapter will discuss those factors that influence both substitution and elimination, as well as introduce several assumptions that will help make predictions as to the major product. 

As you wiU have gathered from the preceding discussion, secondary haloalkanes exhibit the most varied substitution behavior. Both Sn2 and SnI reactions are possible Steric hindrance slows but does not preclude bimolecular nucleophilic attack. At the same time, unimolecular dissociation becomes competitive because of the relative stability of secondary... 

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