7 - 7.6 General Features of Nucleophilic Substitution...

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7.6 General Features of Nucleophilic Substitution (Lewis acid-base rxns) Three components: R – an alkyl group R containing an sp 3 hybridized C bonded to X. X – an atom X called a leaving group , which is able to accept the e - density in the C-X bond. Most common LG’s are halogen atoms (-X), but –OH 2 + and –N 2 are also encountered. :Nu - - a nucleophile ; contain a lone pair or a π bond. Steps: 1. Find the sp 3 hybridized C with the leaving group. 2. Identify the nucleophile, the species with a lone pair or pi bond. 3. Substitute the nucleophile for the LG and assign charges to any atom that is involved in bond breaking/bond formation. 7.7 The Leaving Group What makes a good leaving group? The more stable the leaving group X: - , the better able it is to accept an e pair. In comparing 2 LG’s, the better LG is the weaker base. What are the periodic trends in LG ability? Left-to-right across a row, basicity decreases so LG ability increases . Down a column, basicity decreases so LG ability increases. (I- is weakest base, best LG) All good LG’s are weak bases w/ strong conjugate acids having low pK a values. Good LGs: R-Cl, R-Br, R-I, R-OH 2 + Poor LGs: R-F, R-OH, R-NH 2 , R-H, R-R Equilibrium favors the products of nucleophilic substitution when the LG is a weaker base than the Nu.
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7.8 The Nucleophile Nucleophiles attack other electron-deficient atoms (usually C). Nucleophilicity vs. Basicity The stronger base is the stronger Nu. A negatively charged Nu is always stronger than its conjugate acid. Right-to-left across a row of the periodic table, nucleophilicity increases as basicity increases.
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This note was uploaded on 02/13/2010 for the course CH 221 taught by Professor Staff during the Spring '08 term at John Carroll.

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7 - 7.6 General Features of Nucleophilic Substitution...

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