Chapter 6 - Properties and Reactions of Haloalkanes

Chapter 6 - Properties and Reactions of Haloalkanes -...

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Bimolecular Nucleophilic Substitution 6.1. Physical Properties of Haloalkanes The bond strength of C-X decreases as the size of X increases o Longer and weaker bonds as X gets larger because larger size of halogen leads to mismatched sizes and worse overlap for bond The C-X bond is polarized o Halogen has partial negative charge while carbon has partial positive charge o Electrophilic carbon atom is subject to attack by anions and other electron-rich nucleophilic species; cations and other electron-deficient species attack halogen Haloalkanes have higher boiling points than the corresponding alkanes o Coulombic interaction between the partial negative and partial positive charge of C- X bond dipoles (dipole-dipole interaction) o Boiling points increase with size of X because of greater London interactions; more influential if there is greater polarizability of atom (occurs in larger atoms where electrons are held very tightly around the nucleus) o Polarizability is the degree to which its electron cloud is deformed under influence of electrical field; more polarizable = more London interactions = higher boiling point 6.2. Nucleophilic Substitution Electrophilic carbon can react with nucleophile such as hydroxide or ammonia; such a reaction is called nucleophilic substitution where reagent attacks haloalkane and replaces halide Nucleophiles attack electrophilic centers o Negative nucleophile gives neutral products o Neutral nucleophile gives positively charged product (as a salt) o Leaving group is the group displaced o Substrate is the organic starting material, in this case, haloalkane Nucleophilic substitution exhibits considerable diversity o Changes functional group in molecule o Occurs only with primary and secondary halides; primary gives “cleanest” substitutions while secondary can react in other ways o Halides can act as nucleophiles and leaving groups o Nucleophiles are usually bases 6.3. Reaction Mechanisms Involving Polar Functional Groups: Using “Electron- Pushing” Arrows Curved arrows depict the movement of electrons o Show how nucleophilic substitution occurs when a lone pair of electrons on a nucleophile is transformed into a new bond with an electrophilic carbon and pushing
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Chapter 6 - Properties and Reactions of Haloalkanes -...

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