Substitution Reactions: two reactants exchange parts to give new products A-B + C-D allylic and benzylic alkyl halides can react by either SN1 or SN2 OH 2
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Both the alkyl halide and nucleophile are involved in the rate- limiting step (the reaction NaN3 N3 Cl NaN3 N3 Cl o Suitability of alkyl halides as substrates
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139
Chapter 7 Substitution Reactions
7.1 Introduction to Substitution Reactions
Substitution Reactions
: two reactants exchange parts to give new productsA-B + C-D A-D + B-C
Elimination Reaction
: a single reactant is split into two (or more) products. Opposite of an addition reaction (Chapter 8) A- B A + B C C BrH HH H H C C H HH H + H-OH + Na-BrNaOH H 3 C H 2COH + H-BrH
3 C H 2CBr + H-OH
Nucleophilic Substituion
- A nucleophile may react with an alkyl halide or equivalent (electrophile) such that the nucleophile will displace the halide (leaving group) and give the substitution product. 140Characteristics of a good leaving group
a. Good leaving groups tend to be electronegative, thereby withdrawing electron density from the C-LG bond making C more electrophilic ( b. Leaving group depart with a pair of electrons and often with a negative charge. Good leaving groups can stabilize a negative charge, and are the conjugate bases of strong acid. 7> 141HO , H 2 N , RO F Cl Br I <<1 1 200 10,000 30,000
>15 3.1 -3.0 -5.8 -10.4 LG: Relative Reactivity: Increasing reactivity in the nucleophilic substitution reactions
pKa: Charged Leaving Groups: conversion of a poor leaving group into a good one p K a of H 3 O = -1.7 C OH H H Nu _ CNuH H H H H OH 2 C OH H H H H 141142
7.2 Alkyl Halide
Naming Halogenated Organic Compounds - Use the systematic nomenclature of alkanes; treat the halogen as a substituent of the alkane.F - fluoro Cl - chloro Br - bromo I - iodo
Structure of Alkyl Halides
Reactivity of alkyl halide is dictated by the substitution of the carbon bearing the halogen primary (1°) : one alkyl substituent secondary (2°) : two alkyl substituents tertiary (3°) : three alkyl substituents CX H H R1° carbonC
X H H H CX H R R2° carbonC
X R R R3° carbon71
1437.3 Possible Mechanisms for Substitution Reactions
Concerted
- bond making and bond breaking processes occur in the same mechanistic step with no intermediate.Stepwise
(non-concerted) - reaction goes through distinct steps with a discrete reaction intermediate(s). 1447.4 The S
N2 Mechanism
Kinetics
CBr H H H + HO COH H H H + Br rate = k [CH 3Br] [OH
[CH 3Br] = CH
3Br concentration
[OH ] = OH concentration k = rate constant Second-order reaction (bimolecular) - the rate is dependent on the concentration of both reactants (nucleophile and electrophile)If [OH
] is doubled, then the reaction rate is doubledIf [CH
3 -Br] is doubled, then the reaction rate is doubled S N2 - Substitution, Nucleophilic, bimolecular (2
nd order) 72 145O OH OH HO O (S)-(-) Malic acid D = -2.3 ° PCl 5 O Cl OH HO O Ag 2 O, H 2 O O OH OH HO O (R)-(+) Malic acid D = +2.3 ° PCl 5 Ag 2 O, H 2 O O Cl OH HO O (+)-2-Chlorosuccinic acid (-)-2-Chlorosuccinic acid