Haloalkanes and Haloarenes - Substitution vs Elimination

  • Haloalkanes and haloarenes are organic compounds that contain a halogen atom (such as chlorine, bromine, or iodine) attached to a carbon atom.
  • These compounds undergo reactions involving either substitution or elimination.
  • Substitution reactions replace the halogen atom with another atom or group of atoms.
  • Elimination reactions involve the removal of the halogen atom to form a double bond or a triple bond.
  • The choice between substitution and elimination depends on several factors, such as the substrate, the nucleophile/base, and the reaction conditions.

Factors affecting the choice between substitution and elimination

  1. Nature of the substrate
    • Primary substrates favor substitution reactions.
    • Tertiary substrates favor elimination reactions.
    • Secondary substrates can undergo both substitution and elimination.
  1. Reactivity of the nucleophile/base
    • Strong nucleophiles/bases favor elimination reactions.
    • Weak nucleophiles/bases favor substitution reactions.
  1. Steric hindrance
    • Substitution reactions are favored when the substrate and nucleophile/base are not sterically hindered.
    • Elimination reactions are favored when there is bulky substitution around the carbon-halogen bond.

Substitution reactions

  • Substitution reactions involve the replacement of the halogen atom with another atom or group of atoms.
  • The nucleophile attacks the carbon atom bonded to the halogen atom, leading to the formation of a new bond and the elimination of the halogen atom.
  • The reaction follows either an SN1 or SN2 mechanism.
  • Example: SN2 reaction of bromomethane with hydroxide ion CH3Br + OH- → CH3OH + Br-
  • The rate of SN2 reactions depends on the concentration of both the substrate and the nucleophile.

Elimination reactions

  • Elimination reactions involve the removal of the halogen atom to form a double bond or a triple bond.
  • The base abstracts a proton from an adjacent carbon atom, leading to the elimination of the halogen atom.
  • The reaction follows either an E1 or E2 mechanism.
  • Example: E2 reaction of bromoethane with hydroxide ion CH3CH2Br + OH- → CH2=CH2 + Br- + H2O
  • The rate of E2 reactions depends on the concentration of both the substrate and the base.

Factors influencing SN1 and E1 reactions

  1. Stability of the carbocation
    • More stable carbocations favor SN1 and E1 reactions.
    • Tertiary substrates form more stable carbocations and thus, undergo SN1 and E1 reactions.
  1. Nucleophile/base strength
    • Weak nucleophiles/bases favor SN1 and E1 reactions.
    • Strong nucleophiles/bases favor SN2 and E2 reactions.
  1. Solvent
    • Polar protic solvents (such as water or alcohols) favor SN1 and E1 reactions.
    • Polar aprotic solvents (such as acetone or DMF) favor SN2 and E2 reactions.

Factors influencing SN2 and E2 reactions

  1. Steric hindrance
    • SN2 and E2 reactions are favored when the substrate and the nucleophile/base are not sterically hindered.
  1. Reactivity of the nucleophile/base
    • Strong nucleophiles/bases favor SN2 and E2 reactions.
    • Weak nucleophiles/bases favor SN1 and E1 reactions.
  1. Leaving group ability
    • Good leaving groups favor SN2 and E2 reactions.
    • Weak leaving groups favor SN1 and E1 reactions.

Summary

  • Substitution and elimination reactions are two types of reactions that haloalkanes and haloarenes can undergo.
  • Factors such as the nature of the substrate, reactivity of the nucleophile/base, and steric hindrance influence the choice between substitution and elimination reactions.
  • Substitution reactions involve the replacement of the halogen atom with another atom or group of atoms, while elimination reactions involve the removal of the halogen atom to form a double or triple bond.
  • The reaction mechanism, solvent, stability of the carbocation, nucleophile/base strength, solvent polarity, and leaving group ability all play significant roles in determining the outcome of these reactions.
  1. Substitution Reactions: SN1 Mechanism
  • SN1 (substitution nucleophilic unimolecular) reactions occur in two steps:
    1. Formation of a carbocation intermediate
    2. Attack of the nucleophile on the carbocation
  • The rate of SN1 reactions depends only on the concentration of the substrate, as the nucleophile attack occurs after the rate-determining step.
  • Nucleophiles prefer to attack the carbocation from the side that leads to the most stable product.
  • SN1 reactions are favored for tertiary substrates and substrates with stable carbocations.
  • Example: SN1 reaction of tert-butyl chloride with hydroxide ion (CH3)3CCl → (CH3)3C+ + Cl- (CH3)3C+ + OH- → (CH3)3COH
  1. Substitution Reactions: SN2 Mechanism
  • SN2 (substitution nucleophilic bimolecular) reactions occur in a single step, where the nucleophile attacks the substrate at the same time as the leaving group leaves.
  • The rate of SN2 reactions depends on the concentrations of both the substrate and the nucleophile.
  • Nucleophiles attack the substrate from the side opposite to the leaving group, resulting in an inversion of stereochemistry (Walden inversion).
  • SN2 reactions are favored for primary and methyl substrates with weakly basic nucleophiles.
  • Example: SN2 reaction of methyl chloride with hydroxide ion CH3Cl + OH- → CH3OH + Cl-
  1. Elimination Reactions: E1 Mechanism
  • E1 (elimination unimolecular) reactions occur in two steps:
    1. Formation of a carbocation intermediate
    2. The base abstracts a proton to form a double bond or a triple bond
  • The rate of E1 reactions depends only on the concentration of the substrate, as the proton abstraction occurs after the rate-determining step.
  • E1 reactions are favored for tertiary substrates and substrates with stable carbocations.
  • Example: E1 reaction of tert-butyl bromide with hydroxide ion (CH3)3CBr → (CH3)3C+ + Br- (CH3)3C+ + OH- → (CH3)3C=CH2 + H2O
  1. Elimination Reactions: E2 Mechanism
  • E2 (elimination bimolecular) reactions occur in a single step, where the base abstracts a proton at the same time as the leaving group leaves.
  • The rate of E2 reactions depends on the concentrations of both the substrate and the base.
  • E2 reactions are favored for primary and secondary substrates with strong bases.
  • E2 reactions can result in a stereoselectivity, with anti-periplanar elimination favored.
  • Example: E2 reaction of ethyl bromide with hydroxide ion CH3CH2Br + OH- → CH2=CH2 + Br- + H2O
  1. Comparison: SN1 vs SN2 Reactions SN1 Reactions:
  • Unimolecular
  • Two steps
  • Rate depends on substrate concentration
  • Preferred for tertiary substrates
  • Nucleophile attacks carbocation
  • Stereochemistry not always conserved SN2 Reactions:
  • Bimolecular
  • One step
  • Rate depends on substrate and nucleophile concentration
  • Preferred for primary or methyl substrates
  • Nucleophile attacks opposite to leaving group
  • Inversion of stereochemistry (Walden inversion)
  1. Comparison: E1 vs E2 Reactions E1 Reactions:
  • Unimolecular
  • Two steps
  • Rate depends on substrate concentration
  • Preferred for tertiary substrates
  • Base abstracts proton after carbocation formation E2 Reactions:
  • Bimolecular
  • One step
  • Rate depends on substrate and base concentration
  • Preferred for primary or secondary substrates
  • Base abstracts proton simultaneously with leaving group
  1. Factors Influencing SN1 Reactions
  • Stability of the carbocation: More stable carbocations favor SN1 reactions.
  • Nucleophile strength: Weak nucleophiles favor SN1 reactions.
  • Solvent: Polar protic solvents favor SN1 reactions.
  • Leaving group ability: Good leaving groups favor SN1 reactions.
  1. Factors Influencing SN2 Reactions
  • Steric hindrance: SN2 reactions are favored when there is no substituent hindrance on the carbon-halogen bond.
  • Nucleophile strength: Strong nucleophiles favor SN2 reactions.
  • Solvent: Polar aprotic solvents favor SN2 reactions.
  • Leaving group ability: Good leaving groups favor SN2 reactions.
  1. Factors Influencing E1 Reactions
  • Stability of the carbocation: More stable carbocations favor E1 reactions.
  • Base strength: Weak bases favor E1 reactions.
  • Solvent: Polar protic solvents favor E1 reactions.
  • Leaving group ability: Good leaving groups favor E1 reactions.
  1. Factors Influencing E2 Reactions
  • Steric hindrance: E2 reactions are favored when there is no substituent hindrance on the carbon-halogen bond.
  • Base strength: Strong bases favor E2 reactions.
  • Solvent: Polar aprotic solvents favor E2 reactions.
  • Leaving group ability: Good leaving groups favor E2 reactions.

Factors Influencing SN1 Reactions

  • Stability of the carbocation:
    • More stable carbocations favor SN1 reactions.
    • Tertiary substrates form more stable carbocations and thus, undergo SN1 reactions.
  • Nucleophile strength:
    • Weak nucleophiles favor SN1 reactions.
    • Strong nucleophiles prefer SN2 reactions.
  • Solvent:
    • Polar protic solvents favor SN1 reactions.
    • These solvents stabilize the carbocation intermediate.
  • Leaving group ability:
    • Good leaving groups favor SN1 reactions.
    • Weak leaving groups tend to favor SN2 reactions.

Factors Influencing SN2 Reactions

  • Steric hindrance:
    • SN2 reactions are favored when there is no steric hindrance around the carbon-halogen bond.
    • Methyl and primary substrates have less steric hindrance.
  • Nucleophile strength:
    • Strong nucleophiles favor SN2 reactions.
    • Weak nucleophiles prefer SN1 reactions.
  • Solvent:
    • Polar aprotic solvents favor SN2 reactions.
    • These solvents do not stabilize the carbocation intermediate.
  • Leaving group ability:
    • Good leaving groups favor SN2 reactions.
    • Weak leaving groups tend to favor SN1 reactions.

Factors Influencing E1 Reactions

  • Stability of the carbocation:
    • More stable carbocations favor E1 reactions.
    • Tertiary substrates form more stable carbocations and thus, undergo E1 reactions.
  • Base strength:
    • Weak bases favor E1 reactions.
    • Strong bases prefer E2 reactions.
  • Solvent:
    • Polar protic solvents favor E1 reactions.
    • These solvents stabilize the carbocation intermediate.
  • Leaving group ability:
    • Good leaving groups favor E1 reactions.
    • Weak leaving groups tend to favor E2 reactions.

Factors Influencing E2 Reactions

  • Steric hindrance:
    • E2 reactions are favored when there is no steric hindrance around the carbon-halogen bond.
    • Methyl and primary substrates have less steric hindrance.
  • Base strength:
    • Strong bases favor E2 reactions.
    • Weak bases tend to favor E1 reactions.
  • Solvent:
    • Polar aprotic solvents favor E2 reactions.
    • These solvents do not stabilize the carbocation intermediate.
  • Leaving group ability:
    • Good leaving groups favor E2 reactions.
    • Weak leaving groups tend to favor E1 reactions.

Summary: SN1 vs SN2 Reactions

SN1 Reactions:

  • Unimolecular
  • Two steps
  • Rate depends on substrate concentration
  • Preferred for tertiary substrates
  • Nucleophile attacks carbocation
  • Stereochemistry not always conserved SN2 Reactions:
  • Bimolecular
  • One step
  • Rate depends on substrate and nucleophile concentration
  • Preferred for primary or methyl substrates
  • Nucleophile attacks opposite to leaving group
  • Inversion of stereochemistry (Walden inversion)

Summary: E1 vs E2 Reactions

E1 Reactions:

  • Unimolecular
  • Two steps
  • Rate depends on substrate concentration
  • Preferred for tertiary substrates
  • Base abstracts a proton after carbocation formation E2 Reactions:
  • Bimolecular
  • One step
  • Rate depends on substrate and base concentration
  • Preferred for primary or secondary substrates
  • Base abstracts a proton simultaneously with leaving group

Haloalkanes and Haloarenes - Summary

  • Haloalkanes and haloarenes are organic compounds that contain a halogen atom attached to a carbon atom.
  • These compounds can undergo substitution or elimination reactions.
  • The choice between substitution and elimination depends on factors such as the nature of the substrate, reactivity of the nucleophile/base, steric hindrance, solvent, and leaving group ability.
  • Substitution reactions replace the halogen atom with another atom or group of atoms, while elimination reactions involve the removal of the halogen atom to form a double or triple bond.

Substitution Example: SN2 Reaction

  • Example: SN2 reaction of bromomethane (CH3Br) with hydroxide ion (OH-).
  • Reaction:
    • CH3Br + OH- → CH3OH + Br-
    • The hydroxide ion acts as the nucleophile, attacking the carbon atom and displacing the bromine atom.
    • The result is the formation of methanol (CH3OH) and bromide ion (Br-).
  • This reaction follows an SN2 mechanism and occurs in a single step.

Elimination Example: E2 Reaction

  • Example: E2 reaction of 2-bromopropane (CH3CHBrCH3) with ethoxide ion (C2H5O-).
  • Reaction:
    • CH3CHBrCH3 + C2H5O- → CH2=CHCH3 + Br- + C2H6O
    • The ethoxide ion acts as the base, abstracting a proton from the adjacent carbon atom.
    • The result is the formation of propene (CH2=CHCH3), bromide ion (Br-), and ethanol (C2H6O).
  • This reaction follows an E2 mechanism and occurs in a single step.

Recap: Key Points

  • Haloalkanes and haloarenes can undergo substitution or elimination reactions.
  • The choice between substitution and elimination depends on several factors, including the nature of the substrate, reactivity of the nucleophile/base, steric hindrance, solvent, and leaving group ability.
  • Substitution reactions involve the replacement of the halogen atom with another atom or group of atoms, while elimination reactions involve the removal of the halogen atom to form a double or triple bond.
  • The reaction mechanisms for substitution are SN1 and SN2, while elimination reactions follow E1 and E2 mechanisms.
  • Substitution and elimination reactions have different rate-determining steps and depend on different factors.