Cambridge AS & A Level9701

Alkanes

Chemistry 9701 Chapter Notes

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AlkanesAlkenes
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In this chapter

  1. 1Introduction to Alkanes
  2. 2Sources and Separation of Alkanes
  3. 3Combustion of Alkanes
  4. 4Free-Radical Substitution
  5. 5Isomerism in Alkanes

1. Introduction to Alkanes

Alkanes are the simplest family of organic compounds, known as hydrocarbons because they contain only hydrogen and carbon atoms. They are described as 'saturated' because all the carbon-carbon bonds are single covalent bonds, meaning each carbon atom is bonded to the maximum possible number of other atoms. This saturation makes them relatively unreactive. The general formula for any alkane is CnH2n+2, where 'n' is the number of carbon atoms. Due to sp³ hybridisation, the carbon atoms in alkanes have a tetrahedral geometry, with bond angles of approximately 109.5°.

General Formula: CnH2n+2

Key term

Saturated Hydrocarbon: A hydrocarbon in which all the bonds between carbon atoms are single bonds, holding the maximum number of hydrogen atoms.

Examiner insight

Examiners frequently ask for definitions of 'saturated' and 'hydrocarbon'. Be precise: saturated refers to single C-C bonds only, and hydrocarbon refers to compounds with ONLY carbon and hydrogen.

Fun fact

Methane (CH4), the simplest alkane, is a potent greenhouse gas produced by cows and is the main component of natural gas used for heating homes.

Worked example 13 marks

An alkane is found to have 4 carbon atoms. State its molecular formula, and draw its displayed formula.

  1. 1

    Step 1: Identify the number of carbon atoms, n. Here, n = 4.

  2. 2

    Step 2: Use the general formula for alkanes, CnH2n+2, to find the molecular formula.

  3. 3

    Step 3: Substitute n=4 into the formula: C4H(2*4)+2 = C4H10.

  4. 4

    Step 4: To draw the displayed formula, draw the 4 carbon atoms in a chain, connected by single bonds.

  5. 5

    Step 5: Add hydrogen atoms to each carbon until every carbon has a total of 4 bonds. The resulting structure is CH3-CH2-CH2-CH3, with all H atoms shown explicitly.

  6. 6

    Displayed Formula: A central drawing showing a 4-carbon chain with each carbon single-bonded to the next, and all remaining bonds filled with H atoms (3 on each end C, 2 on each middle C).

Recap

  • Alkanes are saturated hydrocarbons containing only single C-C and C-H bonds.
  • The general formula for the alkane homologous series is CnH2n+2.
  • The shape around each carbon atom in an alkane is tetrahedral, with bond angles of 109.5°.
  • The term 'saturated' means the molecule contains the maximum possible number of hydrogen atoms.

Quick check

  1. What is the molecular formula for an alkane with 7 carbon atoms?1 mark
  2. Explain the meaning of the term 'hydrocarbon'.1 mark

2. Sources and Separation of Alkanes

The primary source of alkanes is crude oil, a complex mixture of hydrocarbons formed from the remains of ancient marine organisms over millions of years. Because crude oil itself is not very useful, it must be separated into more useful components. This is done by fractional distillation. In a fractionating column, the crude oil is heated to vaporise it. The vapour rises and cools. Different hydrocarbons condense back into liquids at different temperatures according to their boiling points. Longer-chain alkanes have stronger intermolecular forces (van der Waals forces), higher boiling points, and are collected as liquids lower down the column. Shorter-chain alkanes have lower boiling points and are collected as gases higher up.

Key term

Fractional Distillation: A process used to separate a mixture of liquids with different boiling points into individual components (fractions).

Examiner insight

When explaining boiling point differences, you must explicitly mention the type of intermolecular force (van der Waals forces), how its strength relates to molecule size/chain length, and the resulting energy requirement.

Fun fact

The 'gasoline' fraction from crude oil, which we use for petrol, typically contains alkanes with 5 to 12 carbon atoms.

Worked example 13 marks

Explain why decane (C10H22) is collected at a higher temperature than ethane (C2H6) during the fractional distillation of crude oil.

  1. 1

    Step 1: Identify the key difference between the molecules. Decane is a larger molecule with a longer carbon chain than ethane.

  2. 2

    Step 2: Relate molecular size to intermolecular forces. Larger molecules have more electrons, leading to stronger temporary dipole-dipole interactions (van der Waals forces) between molecules.

  3. 3

    Step 3: Link the strength of intermolecular forces to the energy required to overcome them. More energy is required to overcome the stronger van der Waals forces in decane compared to the weaker forces in ethane.

  4. 4

    Step 4: Connect energy to boiling point. A higher energy requirement means a higher boiling point. Therefore, decane has a higher boiling point and condenses at a higher temperature lower down the fractionating column.

Recap

  • Crude oil is the main source of alkanes.
  • Fractional distillation separates crude oil into fractions based on boiling point.
  • Longer-chain alkanes have stronger van der Waals forces and thus higher boiling points.
  • Fractions with lower boiling points are collected at the top of the column, and those with higher boiling points at the bottom.

Quick check

  1. Name the type of intermolecular force that is overcome when an alkane boils.1 mark
  2. In a fractionating column, where would you find the fraction with the lowest viscosity (runniest)?1 mark

3. Combustion of Alkanes

Although alkanes are generally unreactive, they readily undergo combustion (burning in oxygen), which is a highly exothermic reaction, releasing large amounts of energy. This makes them excellent fuels. There are two types of combustion:

  1. Complete Combustion: Occurs when there is a plentiful supply of oxygen. The only products are carbon dioxide (CO2) and water (H2O).
  2. Incomplete Combustion: Occurs in a limited supply of oxygen. It produces water, but instead of just CO2, it can produce poisonous carbon monoxide (CO) and/or solid carbon (soot).

Complete Combustion: Alkane + O2 -> CO2 + H2O

Incomplete Combustion: Alkane + O2 -> CO + C + H2O

Key term

Complete Combustion: The burning of a substance in a plentiful supply of oxygen to produce the most oxidised products, which for hydrocarbons are carbon dioxide and water.

Common pitfall

When balancing oxygen in combustion equations, students often forget that oxygen exists as O2 and miscalculate the coefficient. Always count the total oxygen atoms on the product side first, then divide by two.

Worked example 12 marks

Write a balanced symbol equation for the complete combustion of propane (C3H8).

  1. 1

    Step 1: Write the unbalanced equation with reactants and products: C3H8 + O2 -> CO2 + H2O.

  2. 2

    Step 2: Balance the carbon atoms. There are 3 carbons on the left, so you need 3 CO2 on the right: C3H8 + O2 -> 3CO2 + H2O.

  3. 3

    Step 3: Balance the hydrogen atoms. There are 8 hydrogens on the left, so you need 4 H2O on the right (since 4 x 2 = 8): C3H8 + O2 -> 3CO2 + 4H2O.

  4. 4

    Step 4: Balance the oxygen atoms. On the right, there are (3 x 2) + 4 = 10 oxygen atoms. Therefore, you need 5 O2 on the left: C3H8 + 5O2 -> 3CO2 + 4H2O.

  5. 5

    Step 5: Check the balancing. Left side: 3 C, 8 H, 10 O. Right side: 3 C, 8 H, 10 O. The equation is balanced.

Worked example 22 marks

State two reasons why the incomplete combustion of alkane fuels in car engines is a problem.

  1. 1

    Reason 1: Incomplete combustion produces carbon monoxide (CO), which is a toxic gas. It is dangerous because it binds irreversibly to haemoglobin in red blood cells, preventing them from carrying oxygen around the body.

  2. 2

    Reason 2: Incomplete combustion can also produce carbon (soot), which can cause respiratory problems and contributes to global dimming.

Recap

  • Combustion of alkanes is a highly exothermic reaction, making them good fuels.
  • Complete combustion requires excess oxygen and produces only carbon dioxide and water.
  • Incomplete combustion occurs in limited oxygen and produces toxic carbon monoxide and/or soot.
  • Balancing combustion equations is a key skill: balance C, then H, then O.

Quick check

  1. What colour flame is typically associated with incomplete combustion?1 mark
  2. Name the two products of the complete combustion of methane.1 mark

4. Free-Radical Substitution

This is the main reaction of alkanes besides combustion. An alkane reacts with a halogen (like chlorine or bromine) in the presence of ultraviolet (UV) light. In this reaction, a hydrogen atom on the alkane is replaced, or 'substituted', by a halogen atom. The reaction proceeds via a chain reaction mechanism involving highly reactive intermediates called free radicals. A free radical is a species with an unpaired electron. The mechanism has three stages:

  1. Initiation: The UV light provides energy to break the halogen-halogen bond (e.g., Cl-Cl) via homolytic fission, where each atom gets one electron from the bond, forming two halogen free radicals (e.g., 2Cl•).
  2. Propagation: The halogen radical attacks an alkane molecule, creating a new alkyl radical and a hydrogen halide. This alkyl radical then attacks another halogen molecule, regenerating the halogen radical. These two steps form a self-sustaining chain.
  3. Termination: The reaction stops when two free radicals collide and combine to form a stable molecule, removing the radicals from the reaction mixture.

Initiation: X2 --(UV light)--> 2X• (where X = Cl or Br)

Propagation Step 1: X• + R-H -> H-X + R•

Propagation Step 2: R• + X2 -> R-X + X•

Termination: X• + X• -> X2 OR R• + R• -> R-R OR R• + X• -> R-X

Key term

Free Radical: A highly reactive atom or group of atoms with an unpaired electron.

Examiner insight

Marks are consistently awarded for correctly identifying the three stages of the mechanism and writing accurate equations for each. You must state the need for UV light in the initiation step.

Fun fact

The free-radical substitution reaction can continue, replacing more hydrogen atoms. This is why reacting methane and chlorine can produce a mixture of chloromethane, dichloromethane, trichloromethane (chloroform), and tetrachloromethane (carbon tetrachloride).

Worked example 15 marks

Describe the mechanism for the reaction of methane (CH4) with chlorine (Cl2) in the presence of UV light to form chloromethane (CH3Cl). Name the three stages.

  1. 1

    Stage 1: Initiation. The UV light causes homolytic fission of a chlorine molecule to produce two chlorine free radicals. Equation: Cl2 -> 2Cl•

  2. 2

    Stage 2: Propagation. This is a two-step chain reaction. First, a chlorine radical removes a hydrogen atom from a methane molecule, forming a methyl radical and hydrogen chloride. Equation: Cl• + CH4 -> •CH3 + HCl

  3. 3

    Then, the methyl radical reacts with another chlorine molecule, forming the product chloromethane and regenerating a chlorine radical. Equation: •CH3 + Cl2 -> CH3Cl + Cl•

  4. 4

    Stage 3: Termination. The reaction is terminated when any two free radicals collide. There are three possible termination steps. e.g., Cl• + Cl• -> Cl2 OR •CH3 + •CH3 -> C2H6 OR Cl• + •CH3 -> CH3Cl

Recap

  • Alkanes react with halogens in the presence of UV light via free-radical substitution.
  • The reaction requires UV light for the initial homolytic fission of the halogen molecule.
  • The mechanism consists of three stages: initiation, propagation, and termination.
  • Propagation involves a chain reaction that produces the main product while regenerating a radical.
  • Termination occurs when two free radicals combine.

Quick check

  1. What is the essential condition for the reaction between an alkane and a halogen?1 mark
  2. What is meant by the term 'homolytic fission'?1 mark

5. Isomerism in Alkanes

Isomers are molecules that have the same molecular formula but a different arrangement of atoms in space. For alkanes, the most common type is structural isomerism, specifically chain isomerism. This occurs when the carbon skeleton can be arranged in different ways, either as a continuous 'straight' chain or as a branched chain. For example, C4H10 can exist as butane (a straight chain of four carbons) or as methylpropane (a three-carbon chain with a one-carbon branch). Branched isomers generally have lower boiling points than their straight-chain counterparts because the branching reduces the surface area for contact between molecules, weakening the van der Waals forces.

Key term

Structural Isomers: Molecules with the same molecular formula but a different structural formula (different arrangement of atoms).

Common pitfall

Students often draw a 'bent' version of a straight chain and mistakenly believe it is a branched isomer. Remember that single bonds can rotate freely; unless the connectivity of atoms changes, it is the same molecule.

Fun fact

While pentane (C5H12) has 3 isomers, decane (C10H22) has 75, and the alkane C40H82 has over 62 trillion possible structural isomers!

Worked example 13 marks

Draw the displayed formulas and state the names of the three structural isomers of pentane, C5H12.

  1. 1

    Step 1: Identify the molecular formula: C5H12.

  2. 2

    Step 2: Draw the straight-chain isomer first. This is a chain of 5 carbon atoms with hydrogens to make up 4 bonds on each carbon. This is named pentane.

  3. 3

    Step 3: Draw a branched isomer. Make the main chain shorter (4 carbons) and add the 5th carbon as a branch. Place the branch on an inner carbon (carbon 2). This gives 2-methylbutane.

  4. 4

    Step 4: Try to make the main chain even shorter (3 carbons). Use the remaining two carbons as branches. Both must be placed on the central carbon atom. This gives 2,2-dimethylpropane.

  5. 5

    Step 5: Draw the full displayed formulas for all three, ensuring every C has 4 bonds and every H has 1 bond. The three isomers are pentane, 2-methylbutane, and 2,2-dimethylpropane.

Recap

  • Structural isomers have the same molecular formula but different structural formulas.
  • Chain isomerism in alkanes involves straight-chain and branched-chain arrangements.
  • The number of possible isomers increases rapidly as the number of carbon atoms increases.
  • Branched isomers have lower boiling points than straight-chain isomers due to weaker van der Waals forces.

Quick check

  1. How many structural isomers does propane (C3H8) have?1 mark
  2. Which has a higher boiling point: pentane or 2-methylbutane? Explain why.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Alkane X has the molecular formula C6H14. Draw the skeletal formula for two structural isomers of X that are not straight-chain alkanes and name them.4 marks
  2. Explain, with reference to structure and bonding, why alkanes are generally unreactive.3 marks
  3. Write a balanced chemical equation for the incomplete combustion of ethane (C2H6) to form carbon monoxide and water.2 marks
  4. The reaction of ethane with bromine requires UV light and proceeds by a free-radical substitution mechanism. Write equations for one initiation step and two propagation steps for this reaction.3 marks
  5. Define the term 'homologous series' and give two characteristics of the alkane homologous series.3 marks
  6. A fraction obtained from crude oil contains hydrocarbons with 10 to 16 carbon atoms. State the name of this fraction and suggest one major use for it.2 marks
  7. Predict and explain the difference in boiling points between 2,2-dimethylpropane and pentane. Both are isomers with the formula C5H12.4 marks
  8. Cracking is an important industrial process. A long-chain alkane, C15H32, is cracked to produce one molecule of ethene (C2H4) and one molecule of a shorter-chain alkane. Deduce the molecular formula of the alkane produced.2 marks
  9. What is meant by 'homolytic fission'? Illustrate your answer with the bond fission in a chlorine molecule, Cl2.2 marks
  10. A student writes the following termination step for the reaction between methane and chlorine: H• + Cl• -> HCl. While this is a valid termination step, explain why very little HCl is formed by this specific step in practice.3 marks

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