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l19a - CH 203 O R G A N I C C H E M I S T R Y I...

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Nucleophilic substitution: Introduction © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Nucleophilic substitution: Introduction Nucleophilic substitution is a useful reaction that transforms one functional group into another. Nucleophilic substitution requires the participation of a substrate and a nucleophile. The substrate must have an sp 3 -hybridized carbon bonded to a leaving group. The nucleophile must have an electron-rich atom that has an unshared electron pair available for bond formation. The overall reaction depends on the state of protonation of the electron-rich atom of the nucleophile. If the nucleophile is anionic, the overall reaction is represented by the following scheme: C LG + :Nuc C Nuc + :LG substrate (LG = leaving group) nucleophile sp 3 If the electron-rich atom of the nucleophile bears a proton, the overall re- action is C LG + C Nuc + substrate (LG = leaving group) nucleophile sp 3 :LG H :Nuc H In each case a substrate, which has an sp 3 -hybridized carbon bonded to a leaving group (LG), reacts with a nucleophile, which ends up where the leav- ing group used to be, and the leaving group breaks off from the carbon to which it is originally bonded and departs with an unshared pair of electrons; the leaving group picks up the proton, if any, that the nucleophile origi- nally bears. The reaction wheel below presents some examples of nucleophilic substitution in which the nucleophile bears a negative charge. In each case, 1-chloro-3-
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Nucleophilic substitution: Introduction © Bruno I. Rubio 2 methyl-2-butene functions as the substrate, Cl is the leaving group, and the species over each arrow are nucleophiles. Note that the electron-rich atom of the nucleophile ends up where the leaving group used to be. H 3 C CH 3 Cl H 3 C CH 3 OH H 3 C CH 3 CN H 3 C CH 3 SCH 3 H 3 C CH 3 O CH 3 O H 3 C CH 3 OCH 3 H 3 C CH 3 Br H 3 C CH 3 O O OH CN CH 3 S H 3 C O - O CH 3 O Br O O The reaction wheel below presents some examples of nucleophilic substitution in which the nucleophile is uncharged. In each case, 1-chloro-1- methylcyclohexane functions as the substrate, Cl is the leaving group, and the species over each arrow are nucleophiles. Note that the electron-rich atom of the nucleophile ends up where the leaving group used to be and that the electron-rich atom of the nucleophile loses the proton it originally bears.
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Nucleophilic substitution: Introduction © Bruno I. Rubio 3 H 2 O CH 3 SH H 3 C OH O CH 3 OH CH 3 Cl CH 3 OH CH 3 SCH 3 CH 3 O O CH 3 CH 3 OCH 3 The substrate The substrate in nucleophilic substitution must have an sp 3 -hybridized carbon bonded to a leaving group. We will call the carbon that bears the leaving group the substrate carbon, but sometimes it is called the ! carbon. There are many kinds of leaving groups, but we will focus mainly on halides (F, Cl, Br, I) and sulfonates. All of the compounds below have an sp 3 -hybridized car- bon bonded to a halide; all can function as substrates in nucleophilic sub- stitution.
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l19a - CH 203 O R G A N I C C H E M I S T R Y I...

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