Halogenation of Hydrocarbons Chemistry Tutorial

Key Concepts

• Hydrocarbons are compounds containing only carbon and hydrogen.
• Hydrocarbons can be classified as either:

  ⚛ saturated

  ⚛ unsaturated

• Saturated hydrocarbons contain ONLY single bonds between carbon atoms (C-C) and are known as alkanes.
• Unsaturated hydrocarbons contain either:⁽¹⁾

  ⚛ double bond between carbon atoms (C=C), known as alkenes

  ⚛ triple bond between carbon atoms (C≡) known as alkynes

• The double bond (C=C) is the active site in alkenes.
      The triple bond (C≡C) is the active site in alkynes.
      Alkanes are not very reactive.
• The elements of group 17 are known as halogens.

  Halogens are fluorine, chlorine, bromine, iodine

• Saturated hydrocarbons (alkanes) can react with halogens only under UV light.
• Saturated hydrocarbons (alkanes) undergo substitution reactions with halogens under UV light: a halogen atom (X) substitutes for a hydrogen atom (H) in the structure of the alkane:

  H |   H |   R- C - C -R'   | H   | H

  +

  X-X

  UV →

  X |   H |   R- C - C -R'   | H   | H

  +

  H-X

• Unsaturated hydrocarbons (alkenes and alkynes) readily react with halogens.
      (special conditions like UV light not required)
• Unsaturated hydrocarbons undergo addition reactions with halogens: halogen atoms (X) are added across the double (C=C) or triple (C≡C) bond:

  R- C = C -R'   | H   | H

  +

  X-X

  →

  X |   X |   R- C - C -R'   | H   | H

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Halogenation of Hydrocarbons

The most commonly demonstrated halogenation reactions are brominations using bromine water, Br₂, which is a red-brown colour.

Alkanes and alkenes tend to be colourless.

So, when bromine water is added to an alkane or alkene and mixed well, initially the mixture turns a red-brown colour due to the bromine.

As the bromination reaction proceeds, the reaction mixture de-colourises (loses its red-brown colour).

If the bromine water does not de-colourise, a reaction has not occurred.

Alkenes readily react with bromine water in an addition reaction, so the bromine water readily de-colourises.

Alkanes are far less reactive than alkenes and will only react with bromine water in the presence of UV light.

Under these conditions, alkanes undergo substitution reactions with halogens, and will slowly de-colourise bromine water.

Example of a Substitution Reaction: Halogenation of Alkanes

Under standard laboratory conditions, alkanes do not react with halogens.

Alkanes can react with halogens in the presence of UV light.

Ethane, CH₃-CH₃, is an example of an alkane.

Bromine, in the form of bromine water (Br_2(aq)), is an example of a halogen.

Under standard laboratory conditions, ethane will not react with bromine water.

In the presence of UV light, ethane will react with bromine in a substitution reaction.
UV light is the condition under which the reaction will occur so it is written above the arrow in the chemical equation.

During this reaction, bromine atoms substitute for (replace) hydrogen atoms, one at a time, initially producing bromoethane (ethyl bromide) and hydrogen bromide as shown in the chemical equation below:

As the reaction proceeds, the intensity of the re-brown colour of the bromine water decreases.

Further substitutions are possible, as shown in the chemical equation below:

Substitutions can continue until the molecule is fully brominated (hexabromoethane).

The halogenation of ethane is not usually demonstrated in the school laboratory because ethane is a gas at room temperature and pressure.
Cyclohexane, however, is a liquid alkane at room temperature and pressure, and this is the alkane most often used to demonstrate:
    (a) lack of reaction with bromine water under standard laboratory conditions
    (b) slow reaction with bromine water under UV light

Example of an Addition Reaction: Halogenation of Alkenes

Alkenes contain a double bond between carbon atoms (C=C).

This double bond is the active site on the molecule.

Alkenes readily react with halogens under standard laboratory conditions.

In this addition reaction, halogens atoms are added across the double bond of the alkene.

In an alkene containing only one double bond, the double bond is broken, the halogen atoms are added, and, the only product of the reaction will be a dihaloalkane.
In a bromination reaction for example, if excess alkene is present, then the reaction mixture will change from a red-brown colour to colourless.

Ethene (ethylene) is an example of an alkene, CH₂=CH₂.

The active site on the ethene molecule is the double bond (C=C).

Bromine, in the form of bromine water (Br_2(aq)) is an example of a halogen.

Ethene will readily react with bromine, so the colour of the bromine water changes from red-brown to colourless.

Bromine atoms will add across the double bond in ethene to produce just one product; 1,2-dibromoethane.

This synthesis of 1,2-dibromoethane from ethene is shown in the chemical equation below:

The addition of bromine across the double bond in ethene is not usually demonstrated in school laboratories because ethene is a gas at room temperature and pressure.

It is far more common to use cyclohexene because this is a liquid alkene at room temperature and pressure.
Cyclohexene will react with bromine in an addition reaction to produce 1,2-dibromocyclohexane.
The red-brown bromine water will be de-colourised as the reaction proceeds.

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