Alkene Reactions

This is part of HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons

HSC Chemistry Syllabus

  • investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to: – hydrogen (H2) – halogens (X2) – hydrogen halides (HX) – water (H2O) (ACSCH136)

  • investigate, write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens

     

    How Can We Conduct Addition Reactions With Alkenes?

    • Alkenes are good nucleophiles
    • Nucleophiles are molecules which can initiate an ‘attack’ on a nearby molecule and hence start a reaction. Nucleophiles are molecules which commonly exhibit one or all of the following properties:
    • Negatively charged (either formal or partial charge)
    • Electron dense centres
    • Reactive bonds e.g. pi-bonds

     

    Hydrohalogenation of Alkene

    • Addition of a hydrogen halide molecule (e.g. HBr) to an alkene forms an alkane with the same number of carbon atoms, but with a newly added halogen.

    Ethene (ethylene) reacts with hydrogen bromide

    Intermediate is an unstable, primary carbocation

    Bromoethane is formed

    Hydrogen halide are considered good electrophiles because the electronegative halogen e.g. Br causes the adjacent hydrogen atom to acquire a partial positive charge (d+).

     

    • Halogens are also good ‘leaving groups’ due to their high electronegativity and stability as anions.

    Markovnikov’s Rule & Major and Minor Products

    • As a result of carbocation rearrangement, isomers may be produced from a single reaction. The product resulted from the more stable carbocation is always the major product. For example, the hydrohalogenation of propene with HBr.

    2-bromopropane is the major product because the bromine atom is added to the middle carbon atom which has two other carbon atoms already bonded to it, as compared to the first carbon (which only has one)

     

    The outcome of carbocation rearrangement is described (and simplified) by Markovnikov’s rule

     

     

    The rule states that in an alkene addition reaction with hydrogen halide (HX), the proton (H) is attached to the carbon atom with more hydrogen substituents while the halogen (X) is attached to the carbon atom with the greatest number of carbon substituents.

     

    Halogenation of Alkene

    • Halogenation of an alkene adds two halogen atoms across the double bond. This creates an alkane molecule with the same number of carbon atoms, but two new halogen atoms.

     

     

    • Since elemental halogen molecules are non-polar, halogenation is carried out in a non-polar solvent, most commonly CCl4which is also inert so it does not interfere with the desired halogenation of an alkene.

     

    Hydrogenation of Alkene

    • Hydrogenation of an alkene adds two hydrogen atoms across the double bond. This creates an alkane molecule with the same number of carbon atoms.

     

    • Since hydrogen gas is diatomic and non-polar, a transition metal catalyst is required to initiate the reaction. The catalyst causes hydrogen to become polarised and a better electrophile. Catalysts include:
      • Nickel (Ni)
      • Palladium on carbon (written as Pd/C)
      • Platinum (Pt)

     

     

    • Markovnikov’s rule does not apply to hydrogenation of alkenes because the two added atoms are both hydrogens.

     

    Hydration of Alkene

    • Hydration of alkene adds an –OH substituent across the double bond. This transforms an alkene into an alcohol

     

    • Reagent: dilute acid (e.g. dilute H2SO4)
      • Hydration uses water as a reactant. The reaction requires a large amount of water which dilutes the acid (catalyst)
      • Hydration requires acid (H+) as a catalyst.

     

    • H+ ions act as electrophiles to initiate the first step of the reaction. Water molecules will then be added onto the positive carbocation. The conjugate base of the acid will then remove a proton from the newly added water to produce an alcohol. 

     

    • Common acid catalysts for hydration include:
      • Sulfuric acid (H2SO4)
      • Phosphoric acid (H3PO4)
      • Nitric acid (HNO3)

     

    Note that dilute HCl will not initiate a hydration reaction but instead hydrohalogenation, so it cannot be used as an acid catalyst to produce alcohol from alkene.

     

    • Markovnikov’s rule applies to hydration reactions.
      • When water attacks a tertiary carbocation, a tertiary alcohol is produced
      • When water attacks a secondary carbocation, a secondary alcohol is produced
      • When water attacks a primary alcohol, a primary alcohol is produced.

     

     

    The rule states that in an alkene addition reaction with water, the proton (H) is attached to the carbon atom with more hydrogen substituents while the alcohol (–OH) is attached to the carbon atom with the greatest number of carbon substituents.

    For example, the hydration of 2-methylpropene produces two position isomers.

    Tert-butyl alcohol (tertiary alcohol) is formed from a tertiary carbocation.

     

    2-methyl-1-propanol (primary carbocation) is formed from a primary carbocation.