卤代反应-亲核取代和消去反应(大学英文课件)

有机化学

11. Reactions of Alkyl Halides: Nucleophilic Substitutions and EliminationsBased on McMurry’s Organic Chemistry, 6th edition

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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 acid) Nucleophiles that are Brønsted bases produce elimination

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11.1 The Discovery of the Walden Inversion 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 (-) acid3

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Reactions of the Walden Inversion

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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

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11.2 Stereochemistry of Nucleophilic Substitution Isolate step so we

know what occurred (Kenyon and Phillips, 1929) using 1phenyl-2-propanol Only the second and fifth steps are reactions at carbon So inversion certainly occurs in the substitution step

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Hughes’ Proof of Inversion React S-2-iodo-octane

with radioactive iodide Observe, initially that racemization of mixture is twice as fast as incorporation of label so it must be with inversion Racemization in one reaction step would occur at same rate as incorporation

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The Nature of Substitution Substitution, by definition, requires that a "leaving

group", which is also a Lewis base, departs from the reacting molecule. A nucleophile is a reactant that can be expected to participate effectively in a substitution reaction.

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Substitution Mechanisms SN1

Two steps with carbocation intermediate Occurs in 3° allyl, benzyl , Two steps combine - without intermediate Occurs in primary, secondary

SN2

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Reactant and Transition-state Energy LevelsHigher reactant energy level (red curve) = faster reaction (smaller G ).

Higher transitionstate energy level (red curve) = slower reaction (larger G ).10

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Two Stereochemical Modes of Substitution Substitution with inversion:

Substitution with retention:

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11.3 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) A rate law describes relationship between the concentration of reactants and conversi

on 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]12

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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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11.4 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)

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SN2 Process The reaction involves a transition state in which both

reactants are together

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SN2 Transition State The transition state of an SN2 reaction has a planar

arrangement of the carbon atom and the remaining three groups