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 saturated hydrocarbons when substituted with halogens

Reactions of Alkanes

Alkanes are saturated hydrocarbons that can undergo substitution reactions.

    What Reactions are Possible with Alkanes? 

    Alkanes are saturated hydrocarbons, meaning every carbon atom in the molecule is bonded to a maximum of four other atoms. As a result, alkanes cannot undergo addition reactions.

    Catalytic Cracking

    Cracking occurs when long-chain alkanes are broken into smaller hydrocarbons including alkenes. 

      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. There are a wide range of catalysts available for cracking, the knowledge of these catalysts is not essential for HSC Chemistry. 

      Cracking is an important step in the refining of crude oil as it produces a variety of smaller organic substances that can be reacted further to produce other organic substances. 

        Substitution Reaction with Alkanes

        When an alkane react with halogens, hydrogen atoms in the alkane molecule are substituted with halogen atoms. This type of reaction is known as substitution. 

        However, substitution reactions generally require UV light to occur. UV light supplies sufficient energy to meet the activation energy of the reaction. It is important to note that UV light is not a catalyst in this setting, but only a source of energy.  

        If a halogen is supplied in excess, alkane molecules will become completely substituted.

        For example, when excess Cl2 is reacted with methane, four stepwise substitution reactions take place until the final product, tetrachloromethane, is formed.

         

         

        Substitution of hydrogen atoms with halogen atoms is selective. Hydrogen atoms bonded to tertiary carbon atoms (those adjacent to three other carbon atoms) are preferred. 

        When a tertiary carbon is not present, hydrogen atoms bonded to secondary carbon atoms (those adjacent to two other carbon atoms) are preferred over hydrogen atoms bonded to primary carbon atoms.

          The bromine water test is used to differentiate between unsaturated and saturated hydrocarbons. In the absence of UV light, alkanes cannot undergo substitution with bromine. In contrast, unsaturated hydrocarbons (alkenes and alkynes) can undergo addition reactions with bromine. You can read more about it here.

          Combustion

          Combustion is the reaction with oxygen. The products of combustion depends on the type of combustion.

          • Complete combustion: reaction with excess oxygen in the presence of heat to produce carbon dioxide and water.

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

          • Incomplete combustion: when oxygen is limited, incomplete combustion occurs, producing additional products such as carbon monoxide (odourless, toxic gas) and soot which is carcinogenic.

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

          Combustion reactions are irreversible and exothermic.

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