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)

    Chemical Reactions with Alkenes

    Alkenes are unsaturated hydrocarbons that can undergo addition reactions to form saturated organic substances. This video discusses various types of addition reactions including

    • hydrohalogenation
    • halogenation
    • hydrogenation
    • hydration

     

    What are Addition Reactions?

    Addition reactions are when extra atoms or molecules are added to an existing organic substance. When alkenes undergo addition reactions, extra atoms are added to the carbon atoms adjacent to the carbon-carbon double bond (C=C). 

    Addition reactions are possible with alkenes because they are unsaturated, which means the carbon atoms adjacent to the C=C bond are only bonded to three other atoms each (a carbon atom can form bonds with up to four other atoms).

    Alkenes are chemical reactive because they 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 (alkenes contain electron-dense C=C 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 atom and a hydrogen atom.

      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.

      Since electrophiles are positively charged, they attract nucleophiles e.g. alkenes.  

      Carbocations, Major and Minor Products

      When alkenes undergo addition reactions, more than one product can be formed. This is because new atoms can be added to either of the two carbon atoms adjacent to the C=C bond. 

      For example, when the electrons in the C=C bond of propene moves to form a new bond with the hydrogen atom in HBr, the hydrogen atom can form a bond with either the first or the second carbon atom.

      When the hydrogen bonds with the second carbon atom, the first carbon atom becomes positively charged due to the presence of three bonds (carbon wants four bonds). This intermediate molecule is known as a carbocation. Since the positively charged carbon is adjacent to only one other carbon, this is a primary carbocation. 

      When the hydrogen bonds with the first carbon atom, the second carbon atom becomes positively charged. Since the second carbon is adjacent to two other carbon atoms, this is a secondary carbocation. 

      It is useful to know that primary carbocations are less stable than secondary carbocations. Secondary carbocations are less stable than tertiary carbocations.  

        The stability of the carbocations determines the proportion of products formed at the end of an addition reaction.

        The major product is the one that is formed in greater quantity and from a more stable carbocation intermediate. 

        The minor product is the one that is formed in lesser quantity and from a less stable carbocation intermediate.

        Major and minor products of addition reactions of alkenes are position isomers.

        For example, the hydrohalogenation of propene with HBr.

        • 2-bromopropane is the major product because it is derived from a secondary carbocation
        • 1-bromopropane is the minor product because it is derived from a primary carbocation
        • Secondary carbocations are formed more often because they are more stable than  primary carbocations

        Markovnikov’s Rule

        Markovnikov’s rule is used to identify major and minor products of an addition reaction. 

        For hydrohalogenation of an alkene:  

         

        • The proton (H) is attached to the carbon atom with greater number of adjacent hydrogen atoms.
        • The halogen (X) is attached to the carbon atom with the greater number of adjacent carbon atoms. 

         

        In the reaction between propene and HBr:

        • 2-bromopropane is the major product because the bromine atom is bonded to the second carbon atom, which has more adjacent carbon atoms
        • 1-bromopropane is the minor product because the bromine atom is bonded to the first carbon atom, which has less adjacent carbon atoms

        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.

           

           

          Markovnikov's rule does not apply for halogenation of alkenes because the two added atoms are identical (both are halogens). In other words, major nor minor products are formed from halogenation of alkenes.

          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:

          • 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. In other words, there are no major nor minor products in hydrogenation.

            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 and acid as a catalyst. The reaction requires a large amount of water which dilutes the acid (catalyst)

            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.

                 

                 

                • The proton (H) is attached to the carbon atom with more adjacent hydrogen atoms.
                • The alcohol (–OH) is attached to the carbon atom with more adjacent carbon atoms.

                For example, the hydration of propene produces two position isomers: propan-1-ol and propan-2-ol.

                • Propan-2-ol is the major product because the –OH is bonded to the second carbon atom which has more adjacent carbon atoms
                • Propan-1-ol is the minor product because the –OH is bonded to the first carbon atom which has more adjacent carbon atoms

                 

                Alternatively, major and minor products of hydration of alkenes are related to relative stability of carbocations.

                • Propan-2-ol is the major product because it is derived from the more stable secondary carbocation
                • Propan-1-ol is the minor product because it is derived from the less stable primary carbocation 

                 

                BACK TO MODULE 7: ORGANIC CHEMISTRY