1.1 Hydrocarbons  (Page 4/6)

Properties of the alkanes

We have already mentioned that the alkanes are relatively unreactive because of their stable C-C and C-H bonds. The boiling point and melting point of these molecules is determined by their molecular structure, and their surface area. The more carbon atoms there are in an alkane, the greater the surface area and therefore the higher the boiling point. The melting point also increases as the number of carbon atoms in the molecule increases. This can be seen in the data in [link] .

You will also notice that, when the molecular mass of the alkanes is low (i.e. there are few carbon atoms), the organic compounds are gases because the intermolecular forces are weak. As the number of carbon atoms and the molecular mass increases, the compounds are more likely to be liquids or solids because the intermolecular forces are stronger.

Reactions of the alkanes

There are three types of reactions that can occur in saturated compounds such as the alkanes.

1. Substitution reactions Substitution reactions involve the removal of a hydrogen atom which is replaced by an atom of another element, such as a halogen (F, Cl, Br or I) ( [link] ). The product is called a halo-alkane . Since alkanes are not very reactive, either heat or light is needed for this reaction to take place. e.g. CH ${}_2$ =CH ${}_2$ + HBr $\to$ CH ${}_3$ -CH ${}_2$ -Br (halo-alkane)

   

   Halo-alkanes (also sometimes called alkyl halides ) that contain methane and chlorine are substances that can be used as anaesthetics during operations. One example is trichloromethane, also known as 'chloroform' ( [link] ).

   

2. Elimination reactions Saturated compounds can also undergo elimination reactions to become unsaturated ( [link] ). In the example below, an atom of hydrogen and chlorine are eliminated from the original compound to form an unsaturated halo-alkene. e.g. $Cl{H}_{2}C-C{H}_{2}Cl\to H_{2}C=CHCl+HCl$

   

3. Oxidation reactions When alkanes are burnt in air, they react with the oxygen in air and heat is produced. This is called an oxidation or combustion reaction. Carbon dioxide and water are given off as products. Heat is also released during the reaction. The burning of alkanes provides most of the energy that is used by man. e.g. $C{H}_4+2O_2\to C{O}_2+2H_{2}O+heat$

The alkanes

1. Give the IUPAC name for each of the following alkanes:
   1. CH ${}_3$ CH ${}_2$ CH ${}_2$ CH ${}_2$ CH ${}_2$ CH ${}_3$

   2. 

   3. CH ${}_3$ CH ${}_3$
2. Give the structural formula for each of the following compounds:
   1. octane
   2. 3-methyl-hexane
3. Methane is one of the simplest alkanes and yet it is an important fuel source. Methane occurs naturally in wetlands, natural gas and permafrost. However, methane can also be produced when organic wastes (e.g. animal manure and decaying material) are broken down by bacteria under conditions that are anaerobic (there is no oxygen). The simplified reaction is shown below: Organic matter $\to$ Simple organic acids $\to$ Biogas The organic matter could be carbohydrates, proteins or fats which are broken down by acid-forming bacteria into simple organic acids such as acetic acid or formic acid. Methane-forming bacteria then convert these acids into biogases such as methane and ammonia.The production of methane in this way is very important because methane can be used as a fuel source. One of the advantages of methane over other fuels like coal, is that it produces more energy but with lower carbon dioxide emissions. The problem however, is that methane itself is a greenhouse gas and has a much higher global warming potential than carbon dioxide. So, producing methane may in fact have an even more dangerous impact on the environment.
   1. What is the structural formula of methane?
   2. Write an equation to show the reaction that takes place when methane is burned as a fuel.
   3. Explain what is meant by the statement that methane 'has a greater global warming potential than carbon dioxide'.
4. Chlorine and ethane react to form chloroethane and hydrogen chloride.
   1. Write a balanced chemical equation for this reaction, using molecular formulae.
   2. Give the structural formula of chloroethane.
   3. What type of reaction has taken place in this example?
5. Petrol (C ${}_8$ H ${}_{18}$ ) is in fact not pure C ${}_8$ H ${}_{18}$ but a mixture of various alkanes . The 'octane rating' of petrol refers to the percentage of the petrol which is C ${}_8$ H ${}_{18}$ . For example, 93 octane fuel contains 93% C ${}_8$ H ${}_{18}$ and 7% other alkanes. The isomer of C ${}_8$ H ${}_{18}$ referred to in the 'octane rating' is in fact not octane but 2,2,4-trimethylpentane.
   1. Write an unbalanced equation for the chemical reaction which takes place when petrol (C ${}_8$ H ${}_{18}$ ) burns in excess oxygen.
   2. Write the general formula of the alkanes .
   3. Define the term structural isomer .
   4. Use the information given in this question and your knowledge of naming organic compounds to deduce and draw the full structural formula for 2,2,4-trimethylpentane. (IEB pg 25)