The Substrate: Steric Effects in the SN2 Reaction

The first S_N2 reaction variable to look at is the structure of the substrate. Because the S_N2 transition state involves partial bond formation between the incoming nucleophile and the alkyl halide carbon atom, it seems reasonable that a hindered, bulky substrate should prevent easy approach of the nucleophile, making bond formation difficult. In other words, the transition state for reaction of a sterically hindered substrate, whose carbon atom is “shielded” from the approach of the incoming nucleophile, is higher in energy and forms more slowly than the corresponding transition state for a less hindered substrate (Figure 1.1).

Figure 1.1 Steric hindrance to the S_N2 reaction. As the models indicate, the carbon atom in (a) bromomethane is readily accessible, resulting in a fast S_N2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2 bromopropane (secondary), and (d) 2-bromo- 2-methylpropane (tertiary) are successively more hindered, resulting in successively slower S_N2 reactions.

As Figure 1.1 shows, the difficulty of nucleophile approach increases as the three substituents bonded to the halo-substituted carbon atom increase in size. Methyl halides are by far the most reactive substrates in S_N2 reactions, followed by primary alkyl halides such as ethyl and propyl. Alkyl branching at the reacting center, as in isopropyl halides (2°), slows the reaction greatly, and further branching, as in tert-butyl halides (3°), effectively halts the reaction.  Even branching one carbon away from the reacting center, as in 2,2-dimethylpropyl (neopentyl) halides, greatly hinders nucleophilic displacement.

As a result, S_N2 reactions occur only at relatively unhindered sites and are normally useful only with methyl halides, primary halides, and a few simple secondary halides. Relative reactivities for some different substrates are as follows:

Vinylic halides (R₂C= CRX) and aryl halides are not shown on this reactivity list because they are unreactive toward S_N2 displacement. This lack of reactivity is due to steric factors: the incoming nucleophile would have to approach in the plane of the carbon–carbon double bond and burrow through part of the molecule to carry out a backside displacement.

مواضيع ذات صلة

مركبات الآزو متجانسة الحلقة Homocyclic azo compounds

تصنيف مركبات الآزو Classification of azo compound

أصباغ الآزو Azo Dye

تفاعلات الازدواج Reaction Coupling

أملاح الدايازونيوم الاروماتية      Aromtic Diazonium Salt    

أهمية الكواشف العضوية reagents Importance of organic

الثايوزبينات Thiazepines

الدايازبينات Diazepines

الاوكسازبينات Oxazepines

المركبات الحلقية غير المتجانسة Heterocyclic Compounds

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