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SHARPLESS ASYMMETRIC EPOXIDATION. Chapter 6 ALKYL HALIDES: NUCLEOPHILIC SUBSTITUTION AND ELIMINATION. Chapter 6: Alkyl Halides: Nucleophilic Substitution and Elimination. SOME COMMON PESTICIDES. DDT. Lindane. Kepone. Aldrin. Chlordane. BOILING POINT TRENDS. Size of hydrocarbon part.

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Chapter 6ALKYL HALIDES: NUCLEOPHILIC SUBSTITUTION AND ELIMINATION

Chapter 6: Alkyl Halides: Nucleophilic Substitution and Elimination

some common pesticides
SOME COMMON PESTICIDES

DDT

Lindane

Kepone

Aldrin

Chlordane

boiling point trends
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

relative size
RELATIVE SIZE

CH3CH2F

CH3CH2Cl

CH3CH2Br

CH3CH2I

Chapter 6: Alkyl Halides: Nucleophilic Substitution and Elimination

comparison of s n 1 and s n 2 reactions
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 > 1o > 2o (3o is sterically too hindered)
    • SN1: 3o > 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
comparison of s n 1 and s n 2 reactions7
COMPARISON OF SN1 AND SN2 REACTIONS
  • Leaving Group Effect:
    • Both SN2 and SN1 require good leaving groups – higher electronegativity and polarizability, weak bases
  • Kinetics:
    • SN2: A bimolecular reaction: Rate = k[RX][Nuc]
    • SN1: A monomolecular reaction: Rate = k[RX]
  • Stereochemistry:
    • SN2: Inversion of configuration
    • SN1: Complete or partial racemization
  • Rearrangements:
    • 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)
comparison of e1 and e2 reactions
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 > 2o > 1o (CH3does not undergo E2)
    • E1: 3o > 2o (1o do not easily form carbocations and usually do not undergo E1; CH3does 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
comparison of e1 and e2 reactions9
COMPARISON OF E1 AND E2 REACTIONS
  • Kinetics:
    • E2: A bimolecular reaction: Rate = k[RX][B-]
    • E1: A monomolecular reaction: Rate = k[RX]
  • Stereochemistry:
    • 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.
  • Rearrangements:
    • 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)
substitution versus elimination
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.
substitution versus elimination11
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.
substitution versus elimination12
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.

substitution versus elimination13
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.
substitution versus elimination14
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).