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11.4.2 Substitution reactions of alkanes

Substitution reactions

Definition

Substitution reaction

A reaction in which an atom or group of atoms in a molecule is swapped for a different atom or group of atoms.

  1. In a substitution reaction, one atom or group of atoms in a molecule is replaced by a different atom or group of atoms.

  2. Alkanes react with chlorine by substitution, so a chlorine atom takes the place of a hydrogen atom in the alkane.

  3. The hydrogen atom that is removed joins the other chlorine atom from the Cl2\text{Cl}_2Cl2​ molecule to form hydrogen chloride, HCl\text{HCl}HCl.

  4. A substitution reaction therefore gives two products.

    1. The organic product is a chloroalkane, which is an alkane with a chlorine atom in place of one hydrogen atom.
    2. The other product is always hydrogen chloride.

  5. The carbon chain itself is not changed, so the product has the same number of carbon atoms as the alkane you started with.

  6. The general word equation is shown below.

    alkane+chlorine→UV lightchloroalkane+hydrogen chloride \text{alkane} + \text{chlorine} \xrightarrow{\text{UV light}} \text{chloroalkane} + \text{hydrogen chloride} alkane+chlorineUV light​chloroalkane+hydrogen chloride

The need for ultraviolet light

Definition

Photochemical reaction

A reaction that only takes place when light, such as ultraviolet light, supplies the energy needed to start it.

Definition

Activation energy

The minimum energy that colliding particles must have to react, given the symbol EaE_aEa​.

  1. A mixture of an alkane and chlorine does not react in the dark, even though both substances are present.
  2. The reaction only starts when the mixture is exposed to ultraviolet (UV) light, such as the UV in sunlight.
  3. UV light provides the activation energy, EaE_aEa​, which is the minimum energy the particles need to react.
  4. A reaction that needs light to supply this energy is called a photochemical reaction.
  5. The UV light is a condition, not a reactant, so you write it above the arrow and leave it out when you balance the equation.
  6. When the reaction happens, the pale yellow-green colour of the chlorine fades as the chlorine is used up.

Methane and chlorine

  1. Methane reacts with chlorine in UV light to form chloromethane and hydrogen chloride.

    methane+chlorine→UV lightchloromethane+hydrogen chloride \text{methane} + \text{chlorine} \xrightarrow{\text{UV light}} \text{chloromethane} + \text{hydrogen chloride} methane+chlorineUV light​chloromethane+hydrogen chloride

  2. The balanced symbol equation uses molecular formulae.

    CH4+Cl2→UV lightCH3Cl+HCl \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl} CH4​+Cl2​UV light​CH3​Cl+HCl

  3. The structural formula of chloromethane is CH3Cl\text{CH}_3\text{Cl}CH3​Cl, showing three hydrogen atoms and one chlorine atom on the single carbon atom.

  4. The displayed formula of chloromethane shows every bond, with the C−Cl\text{C}-\text{Cl}C−Cl bond in place of one C−H\text{C}-\text{H}C−H bond.

    H∣H−C−Cl∣H \begin{array}{ccccc} & & \text{H} & & \\ & & | & & \\ \text{H} & - & \text{C} & - & \text{Cl} \\ & & | & & \\ & & \text{H} & & \end{array} H​−​H∣C∣H​−​Cl​

  5. Hydrogen chloride has the displayed formula H−Cl\text{H}-\text{Cl}H−Cl.

Example

Chlorination of methane

  • Methane, CH4\text{CH}_4CH4​, starts with four hydrogen atoms around one carbon atom.
  • One hydrogen atom is removed and one chlorine atom takes its place, so the carbon now holds three H atoms and one Cl atom.
  • The removed hydrogen atom bonds to the second chlorine atom, forming HCl\text{HCl}HCl.
CH4+Cl2→UV lightCH3Cl+HCl \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl} CH4​+Cl2​UV light​CH3​Cl+HCl
  • Each side has one carbon atom, four hydrogen atoms and two chlorine atoms, so the equation is balanced.

Ethane and chlorine

  1. Ethane reacts with chlorine in the same way, with one hydrogen atom replaced by one chlorine atom.

    C2H6+Cl2→UV lightC2H5Cl+HCl \text{C}_2\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{C}_2\text{H}_5\text{Cl} + \text{HCl} C2​H6​+Cl2​UV light​C2​H5​Cl+HCl

  2. The organic product is chloroethane, which has the structural formula CH3CH2Cl\text{CH}_3\text{CH}_2\text{Cl}CH3​CH2​Cl.

  3. In the displayed formula, the chlorine atom is bonded to one of the two carbon atoms and every carbon atom still has four bonds.

    HH∣∣H−C−C−Cl∣∣HH \begin{array}{ccccccc} & & \text{H} & & \text{H} & & \\ & & | & & | & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{Cl} \\ & & | & & | & & \\ & & \text{H} & & \text{H} & & \end{array} H​−​H∣C∣H​−​H∣C∣H​−​Cl​

  4. All six hydrogen atoms in ethane are in equivalent positions, so replacing any one of them gives the same product.

  5. With excess chlorine, more hydrogen atoms can be replaced, but here you only need monosubstitution, where exactly one hydrogen atom is replaced.

Drawing the products

  1. Copy the carbon chain of the alkane exactly as it is.
  2. Remove one hydrogen atom and draw a chlorine atom bonded to that carbon instead.
  3. Check that every carbon atom still has four bonds, every hydrogen atom one bond and the chlorine atom one bond.
  4. Add HCl\text{HCl}HCl as the second product, and write UV light above the arrow.
Example

Monosubstitution of propane

  • Propane, CH3CH2CH3\text{CH}_3\text{CH}_2\text{CH}_3CH3​CH2​CH3​, has hydrogen atoms on its end carbons and on its middle carbon.
  • Replacing a hydrogen atom on an end carbon gives 1-chloropropane.
CH3CH2CH3+Cl2→UV lightCH3CH2CH2Cl+HCl \text{CH}_3\text{CH}_2\text{CH}_3 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{CH}_2\text{CH}_2\text{Cl} + \text{HCl} CH3​CH2​CH3​+Cl2​UV light​CH3​CH2​CH2​Cl+HCl
  • Replacing a hydrogen atom on the middle carbon gives 2-chloropropane.
CH3CH2CH3+Cl2→UV lightCH3CHClCH3+HCl \text{CH}_3\text{CH}_2\text{CH}_3 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{CHClCH}_3 + \text{HCl} CH3​CH2​CH3​+Cl2​UV light​CH3​CHClCH3​+HCl
  • Both products have the molecular formula C3H7Cl\text{C}_3\text{H}_7\text{Cl}C3​H7​Cl, and either one is a correct monosubstituted product.
Common Mistake
  • Do not leave out HCl\text{HCl}HCl, because a substitution reaction always gives a second product.
  • Do not write UV light as a reactant in the equation, because it supplies energy and contains no atoms.
  • Do not replace more than one hydrogen atom when a question asks for the monosubstituted product.
  • Do not confuse this with the reaction of an alkene, because an alkane has no double bond for chlorine to add across.
Exam technique

Writing about substitution

  • Give the condition as ultraviolet light or UV light, because "light", "heat" or "a catalyst" on its own is not credited.
  • Explain the role of UV light by saying that it provides the activation energy for the reaction.
  • Describe the change as "a hydrogen atom is replaced by a chlorine atom", naming both atoms.
  • In a displayed formula, draw every bond, including the C−Cl\text{C}-\text{Cl}C−Cl bond, and count four bonds around each carbon.
Self review
  • What happens to an atom in a substitution reaction?
  • Which condition is needed for an alkane to react with chlorine?
  • Why is this reaction described as photochemical?
  • Name the two products formed when methane reacts with chlorine.
  • Give the structural formula of the organic product of ethane and chlorine.
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11.4.2 Substitution reactions of alkanes Revision Guide

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Revision notes for CIE IGCSE Chemistry 11.4.2 Substitution reactions of alkanes: explanations and worked examples.

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