PYTR Class

Learning Objectives

1. Outline electrophiles
2. Outline the chemical reactions of electrophilic addition
   • Hydration of ethene (addition of water)
   • Addition of halogens
   • Addition of hydrogen halides
3. [AHL]Outline the mechanisms of electrophilic addition using named examples:
   • Ethene + Br₂
   • Propene + HBr
     • This must use positive induction explanation

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Part 1 What are Electrophiles?

Definition:

• Electrophiles are species that seek electrons and participate in electrophilic reactions by accepting a pair of electrons.
• They are electron-deficient and typically have a partial positive charge (δ⁺) or a full positive charge (+)
• Attracted to nucleophiles (electron-rich species)
• Examples:
  • Cations such as H⁺, NO₂⁺
  • Neutral with an electron-deficient atom such as the B in BF₃ or Al in AlCl₃
  • Instantaneous dipole such as Br₂ (Bromine molecule). One side can become Br⁺ if the molecules gets too close to another electron-rich species
  • Alkyl halide such as HBr with permanent H⁺
• Often involved in reactions like electrophilic substitution and electrophilic addition.

Part 2: Electrophilic Addition Reactions

Introduction to alkenes

• Double bonds are sp2 hybridized
• Geometry: planar triangular shape
  • bond angle: 120°
  • This is a fairly open structure that makes it relatively easy for incoming groups to attack.
• The pi bond = area of electron density above and below the plane of the bond axis.
• Weak because less closely associated with the nuclei
• Attracts electrophiles i.e electrophiles can easily be added in

In general, electrophilic addition requires the electrophile to undergo a heterolytic fission. This type of split is uneven where one side will get both of the shared electrons (an electron pair). This leaves one part to become a cation and the other an anion. The cation is usually added first, followed by the anion. This mechanism is further discussed in the AHL parts.

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Popular Electrophilic Addition Reactions

Hydration of ethene (addition of water)

• Requires concentrated acid such as sulfuric acid
• This reaction produces alcohols
• Example:

Addition of halogens

• Generally spontaneous reaction to produce dihalogenoalkanes
• Coloured halogen will discolour.
  • This can be used to distinguish an alkane with an alkene
• Example:

Addition of hydrogen halides

• Produces halogenoalkanes
• Bond enthalpy of hydrogen halides decreases down the group. Therefore, the reaction proceeds faster down the group i.e
  • HI > HBr > HCl
• Example:

Part 3: [AHL] Mechanisms Ethene + Br₂

Please use the following slide show

Part 4: [AHL] Mechanisms Propene + HBr

Please use the following slide show

Predicting Products using Markovnikov’s rule

• the hydrogen will attach to the carbon that is already bonded to the greater number of hydrogens
• If there is not hydrogen involved, for instance an interhalogen IBr and BrCl, the more electropositive part will first be added into the alkene to form a carbocation. This means what this more electropositive part will behave as H = added to carbon that already has highest number of H atom attached to it

Positive Inductive Effect

• This stabilises the carbocation
• Secondary carbocation is more stable than a primary carbocation because there are two sides that give the positive induction to the C+ in a secondary carbocation

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Exercises