3.3.2 Alkanes

3.3.2.1 Fractional distillation of crude oil Alkanes are saturated hydrocarbons Definition: Hydrocarbons contain hydrogen and carbon atoms only. Definition: Saturated means the compound contains only single bonds.

Boiling point of hydrocarbons As the number of carbon atoms increases the boiling point increases. Short hydrocarbons are gases and long hydrocarbons are viscous liquids.

Boiling point of hydrocarbons As the hydrocarbons get longer there are more/ stronger Van der Waals forces between the molecules. This means more energy is needed to separate the hydrocarbon molecules, so the boiling point increases.

Fractional distillation Petroleum (a.k.a. crude oil) is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.

Fractional distillation Petroleum is heated and turned into a vapour (gas). The column is kept hot at the bottom and cooler at the top (temperature gradient) The hydrocarbon gases condense at different heights depending on their boiling points.

Fractional distillation When the gases condense, they turn into liquids which can be tapped off (removed). Each different liquid formed is called a fraction.

Fractional distillation Hydrocarbons which are short have low boiling points and are collected at the top of the column. Hydrocarbons which are long have high boiling points and are collected at the bottom of the column.

Fuel for lorries and buses Fractional distillation Fraction Use Refinery gases Bottled gas Gasoline Fuel for cars Naptha Making chemicals Kerosene Fuel for aeroplanes Diesel oil Fuel for lorries and buses Fuel oil Fuel for ships Bitumen Road surfaces

3.3.2.2 Modification of alkanes by cracking The demand for the shorter hydrocarbons (refinery gases and petrol) is greater than the longer hydrocarbons.

There is a large supply of long hydrocarbons but a low demand There is a large supply of long hydrocarbons but a low demand. There is a short supply of short hydrocarbons but a high demand. To solve this problem we break the long hydrocarbons into shorter hydrocarbons by a process called cracking.

Cracking involves breaking C–C bonds in alkanes. There are two different types of cracking: Thermal cracking – uses high temperatures and high pressures Catalytic cracking – uses a catalyst

Thermal cracking It involves breaking the C-C bond so that long alkanes are broken down to produce shorter alkanes and a high percentage of alkenes. Thermal cracking happens at high temperatures (between 450°C - 900°C) and high pressures (7000 kPa).

Thermal cracking Example: Write an equation to illustrate the thermal cracking of one molecule of tetradecane, C14H30, in which the products are ethene and propene, in the ratio of 2:1, and one other product Answer: C14H30 → 2C2H4 + C3H6 + C7H16

Thermal cracking Ethene, C2H4 is a valuable product of cracking. It can be used to make polymers such as poly(ethene). Poly(ethene) is used to make plastics.

Catalytic cracking Catalytic cracking takes place at a slight pressure (100 kPa), high temperature (450 °C) and in the presence of a zeolite catalyst. It is used mainly to produce motor fuels and aromatic hydrocarbons Catalytic cracking produces branched, cyclic and aromatic hydrocarbons.

Catalytic cracking Branched hydrocarbons are used as motor car fuels as they burn more smoothly than unbranched hydrocarbons.

Catalytic cracking Branched hydrocarbons are used as motor car fuels as they burn more smoothly than unbranched hydrocarbons.

3.3.2.3 Combustion of alkanes Alkanes are used as fuels. Combustion of alkanes and other organic compounds can be complete or incomplete.

When hydrocarbons burn in plenty of air (oxygen) they produce carbon dioxide and water. This is called complete combustion. hydrocarbon + oxygen → carbon dioxide + water Example: Complete combustion of propane: C3H8 + 5O2 → 3CO2 + 4H2O

hydrocarbon + oxygen → carbon monoxide + water When we burn hydrocarbons and there is not enough oxygen they produce carbon monoxide (CO) and water. This is called incomplete combustion. hydrocarbon + oxygen → carbon monoxide + water Example: Incomplete combustion of propane: C3H8 + 3½O2 → 3CO + 4H2O

Carbon monoxide (CO) is a poisonous gas Carbon monoxide (CO) is a poisonous gas. Carbon monoxide attaches to the red blood cells instead of oxygen. Therefore reducing the amount of oxygen the blood can carry to the cells. The body is starved of oxygen and this can lead to death.

Carbon is the solid product of incomplete combustion Example: Incomplete combustion of methane: CH4 + O2 → C + 2H2O

Sulfur impurities Combustion of hydrocarbons containing sulfur leads to sulfur dioxide (SO2) that causes air pollution. Sulfur dioxide leads to acid rain and damages the environment (plants, animals and buildings).

Sulfur impurities Sulfur dioxide dissolves in water vapour to form sulfurous acid (H2SO3) SO2 + H2O → H2SO3 The sulfurous acid (H2SO3) is then oxidised by a series of reactions to form sulfuric acid (H2SO4). The sulfuric acid causes rain to become acidic.

Sulfur impurities In power stations, the acidic sulfur dioxide is removed from the chimney (flue) gases by reacting it with an alkaline substance such as quicklime (CaO).

The product of the reaction between quicklime (CaO) and sulfur dioxide is calcium sulfite (CaSO3) which can be oxidised to make calcium sulfate (CaSO4), which is used to make plasterboard for lining interior walls. CaO + SO2 → CaSO3

Internal Combustion engines Nitrogen and oxygen can also react under high temperatures to form nitrogen oxides (NOx). Nitrogen oxides are produced when the fuel-air mixture is sparked and ignites in a car engine. This provides sufficient energy for nitrogen to react with oxygen to form nitrogen monoxide (NO) N2 + O2 → 2NO

Internal Combustion engines Nitrogen monoxide (NO) reacts further with more oxygen to form nitrogen dioxide (NO2). 2NO + O2 → 2NO2 Nitrogen dioxide (NO2) reacts with water vapour (H2O) and more oxygen. This forms nitric acid (HNO3), which leads to acid rain 2NO2 + H2O + ½O2 → 2HNO3

Internal Combustion engines Some of the hydrocarbons pass through the engine without being burnt. These are called unburnt hydrocarbons They can react with nitrogen oxides in the presence of sunlight to produce an irritating photochemical smog (haze in the air).

Internal Combustion engines Exhaust systems of cars have been fitted with catalytic converters. These devices help to remove carbon monoxide, nitrogen oxides and unburnt hydrocarbons from car exhausts.

Carbon monoxide + nitrogen oxides → carbon dioxide + nitrogen Internal Combustion engines The catalytic converter contains transition metals such as platinum, palladium and rhodium (they are the catalysts) which are spread in a thin layer. Carbon monoxide + nitrogen oxides → carbon dioxide + nitrogen 2CO + 2NO → 2CO2 + N2

3.3.2.4 Chlorination of alkanes Definition: A covalent bond is a shared pair of electrons When the covalent bond between two atoms breaks and one electron is transferred to each atom producing two free radicals. This is called homolytic fission. In general: X-Y → X∙ and Y∙ A radical is a species with an unpaired electron. ∙ = unpaired electron

There are three steps to this reaction: Initiation Propagation Methane and chlorine  Methane and chlorine do not react together in the dark or at room temperature. But in the presence of ultra-violet light they react forming chloromethane and hydrogen chloride. CH4 + Cl2 → CH3Cl + HCl Name of the mechanism: Free radical substitution There are three steps to this reaction: Initiation Propagation Termination

Cl2 → Cl∙ + Cl∙ (or Cl2 → 2Cl∙) Methane and chlorine  Step 1: Initiation - Making two free radicals Cl2 → Cl∙ + Cl∙ (or Cl2 → 2Cl∙) Condition: UV light (breaks the Cl-Cl bond forming chlorine radicals, Cl•)

Methane and chlorine  Step 2: Propagation - A free radical reacts with a molecule to make a free radical and a new molecule CH4 + Cl• → •CH3 + HCl •CH3 + Cl2 → CH3Cl + Cl• Overall: CH4 + Cl2 → CH3Cl + HCl

Methane and chlorine  Step 2: Propagation - A free radical reacts with a molecule to make a free radical and a new molecule

Methane and chlorine  Step 2: Propagation – The initial product formed is chloromethane (CH3Cl). However, the free radical substitution reaction can happen again and again producing: dichloromethane (CH2Cl2); trichloromethane (CHCl3) and tetrachloromethane (CCl4)

Methane and chlorine  Step 2: Propagation – Further substitution

Step 3: Termination - Two free radicals react to make a molecule. Methane and chlorine  Step 3: Termination - Two free radicals react to make a molecule. Unpaired electrons pair up to form a covalent bond. No radicals are made and this ‘kills’ the radicals and the reaction. •CH3 + •CH3 → C2H6 Makes an alkane: ethane Cl• + Cl• → Cl2 Makes chlorine Cl• + •CH3 → CH3Cl Makes chloromethane  

 Reaction conditions Formation of chloromethane (CH3Cl) is favoured by an excess of methane. More chance of chlorine radical (Cl•) reacting with a methane molecule. The chlorine radicals (Cl•) are used up before further substitution Formation of tetrachloromethane (CCl4) is favoured by an excess of chlorine. More chance of chlorine radical (Cl•) reacting with a product molecule as there is a large amount of Cl• 

 Question: Write all the equations for the formation of 1-chloropropane (CH3CH2CH2Cl) from chlorine and propane. Answer: Mechanism: Free radical substitution Initiation: Cl2 → Cl• + Cl• (Need UV light) Propagation: CH3CH2CH3 + Cl• → •CH2CH2CH3 + HCl •CH2CH2CH3 + Cl2 → ClCH2CH2CH3 + •Cl Termination: •CH2CH2CH3 + •Cl → ClCH2CH2CH3 (Other termination reactions do not produce 1-chloropropane such as Cl• + Cl• → Cl2 and •CH2CH2CH3 + •CH2CH2CH3 → CH3CH2CH2CH2CH2CH3)