Chapter 3: COMMON REACTION MECHANISMS

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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.

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Nội dung Text: Chapter 3: COMMON REACTION MECHANISMS

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 1
Chapter 3: COMMON REACTION MECHANISMS Reaction mechanism: the description of the step-by-step process by which reactants are changed / converted into products Nucleophilic substitution Nucleophilic addition Electrophilic substitution Electrophilic addition Elimination 2
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 3
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BIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN2) 5
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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 7
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UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTION (SN1) 9
Note: slow step is rate-determining step 10
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Stereochemistry of SN1 reactions 12
However, few SN1 reactions occur with complete racemization 13
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 14
Affects of substrate structure Steric hindrance N 15 Steric effect in the S 2 reaction
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Affects of nucleophile concentration & strength 1) Neither the concentration nor the structure of N the nucleophile affects the rates of S 1 reactions since the nucleophile does not participate in the rate-determining step N 2) The rates of S 2 reactions depend on both the concentration and the structure of the nucleophile 19
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