CHEM 203 Lecture Notes - Lecture 26: Nucleophilic Aromatic Substitution, Relative Permittivity, Racemic Mixture
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Principle: hindered alkyl halides, such as tertiary ones, may undergo substitution reactions, but typically with non-basic nucleophiles (such as h2o, ch3oh, etc. ; i. e. , nucleophiles that are the conjugate bases of acids with pka"s < 0) and in media of high dielectric constant ( > 20). Reminder: basic nucleophiles (pka > 5) promote e2 reaction of tertiary halides. Ionization of the substrate under the above conditions, formation of a planar carbocation, reaction of the latter with the nucleophile; e. g. ch3 ch2. + br then: planar carbocation (c atom is sp2) (or any other tertiary halide) Ch3 if ch3oh (nu:) attacks from the bottom: if ch3oh (nu) attacks from the top: S-configuration there is no reason why ch3oh (in general, the nucleophile) should favor attack from one side or the other (top or bottom) of the carbocation. Therefore, top- and bottom-face attack will occur with equal probability, leading to a 1:1 mixture of (s)- and (r)-products, i. e. , to a racemic mixture.
