Alkane 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 Alkanes

    • Alkanes are saturated hydrocarbons, so they do not undergo addition reactions.

     

    Catalytic Cracking

    • Cracking is the process when long-chain alkanes are broken into smaller hydrocarbons including alkenes which can be used for a diverse range of reactions.

     

    • C–C bonds are broken during cracking which requires a large amount of energy. Therefore, metal-based catalysts are required to lower the activation energy of cracking.

    Combustion

    • Alkanes are volatile and flammable
    • Combustion reactions are irreversible and exothermic
    • Complete combustion: reaction with excess oxygen in the presence of heat to produce carbon dioxide and water.

    $$2C_8H_{18}(l) + 25O_2(g) \rightarrow 16CO_2(g) + 18H_2O(g)$$

     

    • When oxygen is limited, incomplete combustion occurs, producing carbon monoxide (odourless, toxic gas) and soot which is carcinogenic.

                                             

    $$2C_8H_{18}(l) +17O_2(g) \rightarrow 8CO_2(g) + 8C(s) + 18H_2O(g)$$

     

    Substitution Reaction with Alkanes

    • Alkanes cannot undergo addition reactions because they are saturated; all carbon atoms are already connected to four atoms each.
    • However, alkanes can undergo substitution reactions with hydrogen halides or halogens in the presence of UV light. UV light provides the necessary energy to break covalent bonds and allow the reaction to proceed via a radical-based mechanism.
    • Note: UV radiation is not a catalyst as it does not reduce the activation energy but simply provides the energy to meet the activation energy

     

     

    If a halogen is supplied in excess, alkane molecules will become completely substituted. Hydrogen atoms will be replaced by halogens.

     

    For example, when Cl2 is in excess, methane will undergo four stepwise substitution reactions until it is transformed into tetrachloromethane

     

    • Halogenation of alkanes are selective in that:

    Bromine Water Test (click link to check bromine test)