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Chapter 3: COMMON REACTION MECHANISMS

Reaction mechanism: the description of the step-by-step process by which reactants are changed / converted into products. A nucleophile: an electron-rich species that can form a covalent bond by donating 2 electrons to a positive center. | ORGANIC CHEMISTRY Dr Nam T. S. Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 8647256 ext. 5681 Email: ptsnam@hcmut.edu.vn Chapter 3: COMMON REACTION MECHANISMS Elimination Electrophilic substitution Nucleophilic substitution Nucleophilic addition Electrophilic addition Reaction mechanism: the description of the step-by-step process by which reactants are changed / converted into products NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) A nucleophile: an electron-rich species that can form a covalent bond by donating 2 electrons to a positive center A nucleophile is any negative / neutral molecule that has 1 unshared electron pair Substitution reaction: chemical reaction in which 1 atom / group replaces another atom / group in the structure of a molecule In a nucleophilic substitution reaction, a nucleophile attacks / bonds with the positive center BIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN2) . | ORGANIC CHEMISTRY Dr Nam T. S. Phan Faculty of Chemical Engineering HCMC University of Technology Office: room 211, B2 Building Phone: 8647256 ext. 5681 Email: ptsnam@hcmut.edu.vn Chapter 3: COMMON REACTION MECHANISMS Elimination Electrophilic substitution Nucleophilic substitution Nucleophilic addition Electrophilic addition Reaction mechanism: the description of the step-by-step process by which reactants are changed / converted into products NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) A nucleophile: an electron-rich species that can form a covalent bond by donating 2 electrons to a positive center A nucleophile is any negative / neutral molecule that has 1 unshared electron pair Substitution reaction: chemical reaction in which 1 atom / group replaces another atom / group in the structure of a molecule In a nucleophilic substitution reaction, a nucleophile attacks / bonds with the positive center BIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN2) Stereochemistry of SN2 reactions The nucleophile attacks from the back side / the side directly opposite the leaving group This attacks causes an inversion of configuration UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN1) Note: slow step is rate-determining step Stereochemistry of SN1 reactions However, few SN1 reactions occur with complete racemization Factors affecting the rates of SN1 & SN2 1. The structure of the substrate 2. The concentration & reactivity of the nucleophile 3. The reaction solvent 4. The nature of the leaving group Affects of substrate structure Steric effect in the SN2 reaction Steric hindrance Affects of nucleophile concentration & strength Neither the concentration nor the structure of the nucleophile affects the rates of SN1 reactions since the nucleophile does not participate in the rate-determining step 2) The rates of SN2 reactions depend on both the concentration and the structure of the nucleophile .