1 SHARPLESS ASYMMETRIC EPOXIDATION 2 Chapter 6ALKYL HALIDES NUCLEOPHILIC SUBSTITUTION AND ELIMINATION Chapter 6 Alkyl Halides Nucleophilic Substitution and Elimination 3 SOME COMMON PESTICIDES DDT Lindane Kepone Aldrin Chlordane 4 BOILING POINT TRENDS Size of hydrocarbon part Type of halogen of halogen atoms For comparison CH3 CH3 bp 89 oC Chapter 6 Alkyl Halides Nucleophilic Substitution and Elimination 5 RELATIVE SIZE CH3CH2F CH3CH2Cl CH3CH2Br CH3CH2I Chapter 6 Alkyl Halides Nucleophilic Substitution and Elimination 6 COMPARISON OF SN1 AND SN2 REACTIONS
Effect of Nucleophile
SN2 Strong nucleophiles
SN1 Irrelevant since nucleophile does not participate in rate-determining step
Effect of Substrate
SN2 CH3 gt 1o gt 2o (3o is sterically too hindered)
SN1 3o gt 2o (1o or CH3 do not easily form cations)
Effect of Solvent
SN2 Promoted by polar aprotic solvents or non-polar solvents
SN1 Promoted by polar protic solvents
7 COMPARISON OF SN1 AND SN2 REACTIONS
Leaving Group Effect
Both SN2 and SN1 require good leaving groups higher electronegativity and polarizability weak bases
SN2 A bimolecular reaction Rate kRXNuc
SN1 A monomolecular reaction Rate kRX
SN2 Inversion of configuration
SN1 Complete or partial racemization
SN2 Rearrangements not possible (because it is a concerted process)
SN1 Rearrangements are common (because the intermediate cation often rearranges to a more stable cation)
8 COMPARISON OF E1 AND E2 REACTIONS
Effect of the Base
E2 Strong bases are required
E1 Irrelevant since base does not participate in rate-determining step
Effect of Substrate
E2 3o gt 2o gt 1o (CH3 does not undergo E2)
E1 3o gt 2o (1o do not easily form carbocations and usually do not undergo E1 CH3 does not undergo E1)
Effect of Solvent
E2 Solvent polarity is not very important
E1 Promoted by polar protic solvents
Leaving Group Effect
Both E2 and E1 require good leaving groups higher electronegativity and polarizability weak bases
9 COMPARISON OF E1 AND E2 REACTIONS
E2 A bimolecular reaction Rate kRXB-
E1 A monomolecular reaction Rate kRX
E2 Requires a coplanar arrangement of C H and C Leaving Group bonds. Anti-coplanar is preferred but the reaction can also occur through a syn-coplanar if the anti-coplanar is not achievable
E1 Occurs via a flat carbocation. No particular geometry required.
Orientation of Elimination
Both E1 and E2 The predominant product (if more than one is possible) is the alkene with most substituted double bond (the Saytzeff product). This is known as the Saytzeff rule.
E2 Rearrangements not possible (because it is a concerted process)
E1 Rearrangements are common (because the intermediate carbocation often rearranges to a more stable carbocation)
10 SUBSTITUTION VERSUS ELIMINATION
Strong nucleophile and a methyl substrate SN2 reaction.
Strong nucleophile (base) and primary substrate (1o) An SN2 reaction is most likely. Some E2 product might be obtained as well.
Strong nucleophile (base) and secondary substrate (2o) Both SN2 and E2 reactions will occur and a mixture of substitution and elimination products is likely. Difficult to predict whether SN2 or E2 will predominate.
Strong nucleophile (base) and tertiary substrate (3o) An E2 reaction. SN2 does not occur since substrate is too sterically hindered.
11 SUBSTITUTION VERSUS ELIMINATION
Weak nucleophile (base) and methyl substrate No reaction.
Weak nucleophile (base) and primary substrate (1o) No reaction. EXCEPTION If the primary substrate can undergo a concerted process of ionization rearrangement to give a more stable carbocation (as is the case with neopentyl substrates) then the outcome is a mixture of SN1 and E1.
Weak nucleophile (base) and secondary substrate (2o) Both SN1 and E1 will occur and a mixture of substitution and elimination products is likely.
Weak nucleophile (base) and tertiary substrate (3o) Both SN1 and E1.
12 SUBSTITUTION VERSUS ELIMINATION ONE GENERAL CONCLUSION With strong nucleophiles (bases) the reactions occur via bimolecular mechanism SN2 or E2. With weak nucleophiles (bases) the reactions occur via monomolecular mechanism SN1 or E1. 13 SUBSTITUTION VERSUS ELIMINATION
Temperature of the reaction Increase of temperature favors elimination. Reason Elimination starts with two species (base and substrate) but produces three (the conjugate acid of the starting base an alkene molecule and the leaving group). This increased number of molecules (or ions) causes an increase of entropy i.e. a positive entropy change (DS). This in its turn increases the favorable contribution of the TDS-term in the overall Gibbs free energy change (because DG DH TDS) making the Gibbs free energy change more negative.
14 SUBSTITUTION VERSUS ELIMINATION
Steric bulk of the nucleophile (base) In concerted reactions (SN2 or E2) the nucleophile (base) participates in the rate-determining step. It has to approach closely the molecule and attack the electrophilic carbon center (then it is a nucleophile) or the proton at an adjacent carbon (then it is a base). Approaching closely the carbon center is more sensitive to changes of size. That is why the share (percentage) of elimination increases with an increasing size of the nucleophile (base).
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