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Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations A Summary of Reaction Characteristics Book Title: Organic Chemistry Printed By: Joetta Lloyd (jllo8980@students.vsu.edu) © 2016 Cengage Learning, Cengage Learning
A Summary of Reaction Characteristics
The effects on reactions of the four variables—substrate structure, nucleophile, leaving group, and solvent—are summarized in the following statements and in the energy diagrams of Figure 11-7:
Substrate Steric hindrance raises the energy of the transition state, increasing and decreasing the reaction rate (Figure 11-7a). As a result, reactions are best for methyl and primary substrates. Secondary substrates react slowly, and tertiary substrates do not react by an
mechanism.
Nucleophile Basic, negatively charged nucleophiles are less stable and have a higher ground-state energy than neutral ones, decreasing and increasing the reaction rate (Figure 11-7b).
Leaving group
Good leaving groups (more stable anions) lower the energy of the transition state, decreasing and increasing the reaction rate (Figure 11-7c).
Solvent Protic solvents solvate the nucleophile, thereby lowering its ground-state energy, increasing , and decreasing the reaction rate. Polar aprotic solvents surround the accompanying cation but not the nucleophilic anion, thereby raising the ground-state energy of the nucleophile, decreasing , and increasing the reaction rate (Figure 11-7d).
Figure 11-7
Energy diagrams showing the effects of (a) substrate, (b) nucleophile, (c) leaving group, and (d) solvent on reaction rates. Substrate and leaving group effects are felt primarily in the transition state. Nucleophile and solvent effects are felt primarily in the reactant ground state.
Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations A Summary of Reaction Characteristics Book Title: Organic Chemistry Printed By: Joetta Lloyd (jllo8980@students.vsu.edu) © 2016 Cengage Learning, Cengage Learning
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