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introduction to nucleophlic reaction

all about nucleophilic sub reactions

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introduction to nucleophlic reaction

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  1. 11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Based on McMurry’s Organic Chemistry, 7th edition

  2. Alkyl Halides React with Nucleophiles and Bases • Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic • Nucleophiles will replace the halide in C-X bonds of many alkyl halides(reaction as Lewis base) • Nucleophiles that are Brønsted bases produce elimination

  3. Why this Chapter? • Nucleophilic substitution, base induced elimination are among most widely occurring and versatile reaction types in organic chemistry • Reactions will be examined closely to see: • How they occur • What their characteristics are • How they can be used

  4. 11.1 The Discovery of Nucleophilic Substitution Reactions • In 1896, Walden showed that (-)-malic acid could be converted to (+)-malic acid by a series of chemical steps with achiral reagents • This established that optical rotation was directly related to chirality and that it changes with chemical alteration • Reaction of (-)-malic acid with PCl5 gives (+)-chlorosuccinic acid • Further reaction with wet silver oxide gives (+)-malic acid • The reaction series starting with (+) malic acid gives (-) acid

  5. Reactions of the Walden Inversion

  6. Significance of the Walden Inversion • The reactions alter the array at the chirality center • The reactions involve substitution at that center • Therefore, nucleophilic substitution can invert the configuration at a chirality center • The presence of carboxyl groups in malic acid led to some dispute as to the nature of the reactions in Walden’s cycle

  7. 11.2 The SN2 Reaction • Reaction is with inversion at reacting center • Follows second order reaction kinetics • Ingold nomenclature to describe characteristic step: • S=substitution • N (subscript) = nucleophilic • 2 = both nucleophile and substrate in characteristic step (bimolecular)

  8. The SN2 reaction (also known as bimolecular nucleophilic substitution) is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. • The reaction most often occurs at an aliphaticsp3 carbon center with an electronegative, stable leaving group attached to it - 'X' - frequently a halide atom.

  9. Kinetics of Nucleophilic Substitution • Rate (V) is change in concentration with time • Depends on concentration(s), temperature, inherent nature of reaction (barrier on energy surface) • Arate law describes relationship between the concentration of reactants and conversion to products • A rate constant (k) is the proportionality factor between concentration and rate Example: for S converting to P V = d[S]/dt = k [S]

  10. Reaction Kinetics • The study of rates of reactions is called kinetics • Rates decrease as concentrations decrease but the rate constant does not • Rate units: [concentration]/time such as L/(mol x s) • The rate law is a result of the mechanism • The order of a reaction is sum of the exponents of the concentrations in the rate law – the example is second order

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