l19b

l19b - 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: Mechanisms © 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: Mechanisms In a previous lecture we introduced nucleophilic substitution. We stated that the reaction involves the displacement of a leaving group from an sp 3 hybrid- ized carbon (the substrate carbon) by a nucleophile. In this lecture we dis- cuss the mechanisms by which the displacement occurs. Two chemical events take place in nucleophilic substitution: (1) attack by the nucleophile on the substrate carbon and (2) departure of the leaving group from the substrate carbon. When both events happen simultaneously, the reaction is said to fol- low the SN2 mechanism. When departure of the leaving group takes place before the nucleophile attacks, the reaction is said to follow the SN1 mechanism. Let’s look at the more straightforward and useful SN2 mechanism first. Nucleophilic substitution mechanisms: SN2 SN2: Energy diagram In the SN2 mechanism, the nucleophile (:Nuc ) uses an unshared electron pair to push out the leaving group (LG) from the substrate carbon; the leaving group departs with a pair of electrons. The course of events is described us- ing curved-arrows that show the motion of electron pairs: C LG Nuc + C Nuc + LG The energy diagram of the SN2 consists of one “hill” on which the transition state resides:
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Nucleophilic substitution: Mechanisms © Bruno I. Rubio 2 Reaction coordinate G º !" C LG Nuc + C Nuc + LG C LG Nuc !" ..... ..... The rate of the SN2 reaction depends on the concentrations of the species that must encounter each other to achieve the transition state. Thus, rate of SN2 = k [substrate][nucleophile] where k is the rate constant of the reaction. Problem In Experiment # 1 [substrate] = 0.05 M and [nucleophile] = 0.05 M. In Experiment # 2 [substrate] = 0.10 M and [nucleophile] = 0.01 M. Assuming that the nucleophile and the substrate are the same in both experiments and assum- ing that both reactions follow the SN2 mechanism, determine which reaction is faster. Answer Rate of Experiment # 1 = k ! 0.05 ! 0.05 = 0.0025 k . Rate of Experi- ment # 2 = k ! 0.10 ! 0.01 = 0.001 k . Experiment # 1 is faster. SN2: Nucleophiles In an SN2 reaction, the nucleophile must push out the leaving group; thus, SN2 nucleophiles must exhibit powerful nucleophilic ability (i.e., sterically unhindered strong bases or big-atom nucleophiles); most SN2 nucleophiles have a negative charge. Some typical SN2 nucleophiles are OH , CH 3 O , CN , and I .
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Nucleophilic substitution: Mechanisms © Bruno I. Rubio 3 Problem Predict the products of these SN2 reactions and tell which is fast- est. Explain the reasoning for your choice of the fastest reaction. (a) CH 3 CH 2 I + N(CH 2 CH 3 ) 3 " (b) CH 3 CH 2 I + N(CH 2 CH 3 ) 2 " (c) CH 3 CH 2 I + N(CH 3 ) 2 " Answer Reaction (c) is fastest because it employs the most strongly basic, most electron-rich, and least sterically crowded nucleophile.
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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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l19b - CH 203 O R G A N I C C H E M I S T R Y I...

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