chapter8 - CHP 8 Questions 8 8.1-31 33 35 40-42...

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CHP 8 Questions 8.1-31, 33, 35, 40-42. 8. Nucleophilic Substitution Reactions 8.1 The General Reaction In the reaction of chloromethane with hydroxide ion the hydroxide ion is substituted for the chlorine group. The hydroxide ion is donating a pair of electrons to the carbon and is termed a Nucleophile . The nucleophile is electron rich species that seeks an electron deficient site. Nucleophiles are Lewis bases. The carbon of the chloromethane is electron deficient and termed an Electrophile . Electrophiles are Lewis acids. The carbon atom of chloromethane, electrophile, accepts a pair of electrons from the hydroxide ion, nucleophile. To avoid exceeding the proper valence of carbon, the chlorine atom departs with the electron pair of the C-Cl bond. The chloride is termed the Leaving Group . Example The concept of nucleophiles and electrophiles is one of the most important concepts in organic chemistry. This reaction is termed a nucleophilic substitution reaction since a nucleophile replaces the leaving group. The general reaction can be depicted as Nu: - + R-L Nu-R + L: - 8.2 Reaction Mechanisms A reaction mechanism shows the individual steps of a reaction – the order in which the bonds are broken and formed. In a nucleophilic substitution reaction there are three possible mechanisms: 1) the bond to the leaving group is broken first, followed by formation of the bond with the nucleophile, 2) the bond to the nucleophile is formed first, then the bond to the leaving group is broken, 3) the bond breaking and formation occur simultaneously. Pathways 1 and 3 occur while pathway 2 does not since it would require a pentavalent carbon. Examples:
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8.3 Bimolecular Nucleophilic Substitution Considering the reaction of hydroxide ion with chloroethane: OH - + CH 3 CH 2 Cl CH 3 CH 2 OH + Cl - The rate of this reaction is dependent upon the concentration of hydroxide ion and
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chapter8 - CHP 8 Questions 8 8.1-31 33 35 40-42...

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