Chapter 15: Haloalkanes

Question 1

1. What are haloalkanes?
2. Describe the rules for naming haloalkanes.
3. Name the following alkanes.

Answer 1

1. Haloalkanes are compounds containing the elements carbon, hydrogen and at least one halogen.
2. When naming haloalkanes, a prefix is added to the name of the longest chain to indicate the identity of the halogen. When two or mode halogens are present in a structure they are listed in alphabetical order.
3. From left to right:
   chloroethane (primary)
   2-bromopropane (secondary)
   2-iodo-2-methylpropane (tertiary)

Question 2

1. Describe the polarity of a carbon-halogen bond.
2. Why does carbon attract some species?
3. In terms of haloalkanes, what is a nucleophile?
4. Give three examples of nucelophiles.

Answer 2

1. Halogen atoms are more electronegative than carbon atoms. The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom. The carbon-halogen bond is polar.
2. In haloalkanes the carbon atom has a slightly positive charge so it can attract species containing a lone pair of electrons: nucleophiles.
3. A nucleophile is at atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond.
4. See below.

Question 3

Describe the process of nucleophilic substitution briefly.

Answer 3

When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction. A new compound is produced containing a different functional group. The reaction mechanism is nucleophilic substitution.

Question 4

1. Generally, what is hydrolysis?
2. What is the result of the hydrolysis of a haloalkane?
3. What type of reaction is the hydrolysis of a haloalkane?

Answer 4

1. Hydrolysis is a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. This results in the molecule being split into two products.
2. In the hydrolysis of a haloalkane, the halogen atom is replaced by an –OH group.
3. This is an example of a nucleophilic substitution reaction.

Question 5

Describe and explain the process of the hydrolysis of a haloalkane.

Answer 5

1. The nucleophile [OH^–] approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
2. This direction of attack by the OH^– ion minimises repulsion between the nucleophile and the 𝛿– halogen atom.
3. A lone pair of electrons on the hydroxide ion is attracted and donated to the 𝛿+ carbon atom.
4. A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
5. The carbon-halogen bond breaks by heterolytic fission.
6. The new organic product is an alcohol. A halide ion is also formed.

Question 6

Describe the nucleophilic substitution mechanism for the hydrolysis of chloroethane.

Answer 6

Question 7

What substance is used to convert haloalkanes to alcohols? How is a good yield obtained?

Answer 7

• Haloalkanes can be converted to alcohols using aqueous sodium hydroxide.
• The reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good tield of product.

Question 8

Give the equation, including structural formulae, for the hydrolysis of 1-bromobutane.

Answer 8

Question 9

Describe the rate of hydrolysis. Refer to its relation with carbon-halogen bond strength.

Answer 9

• In hydrolysis, the carbon-halogen bond is broken and the –OH group replaces the halogen in the haloalkane.
• The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane.
• The C–F bond is the strongest carbon-halogen bond (highest bond enthalpy) and the C–I bond is the weakest. It can thus be predicted that:
  
  • iodoalkanes react faster than bromoalkanes
  • bromoalkanes react faster than chloroalkanes
  • fluoroalkanes are unreactive as a large quantity of energy is required to break the C–F bond.

Question 10

1. Using structural formulae, give the generalised equation for the hydrolysis of these three haloalkanes: 1-chlorobutane, 1-bromobutane, 1-iodobutane.
2. Describe and explain how the rate of reaction of these reactions can be followed.

Answer 10

1. CH₃CH₂CH₂CH₂X + H₂O → CH₃CH₂CH₂CH₂OH + H⁺ + X^–
2. The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate. As the reaction takes place halide ions [X^–(aq)] are produced which reat with Ag⁺(aq) ions to form a precipitate of the silver halide.
   The nucleophile in the reaction is water, which is present in the aqueous silver nitrate. Haloalkanes are insoluble in water, and the reaction is carried out in the presence of an ethanol solvent. Ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers.

Question 11

Describe the method and observations that one would obtain from an experiment that follows the rate of reaction for the hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane.

Answer 11

Question 12

How can bond enthalpies be used to explain the order of the rate of reaction for 1-chlorobutane, 1-bromobutane and 1-iodobutane?

Answer 12

The compound with the slowest rate of reaction is the one that has the strongest carbon-halogen bond:

• 1-Chlorobutane reacts slowest and the C–Cl bond is the strongest
• 1-Iodobutane reacst fastest and the C–I bond is the weakest.

Question 13

Describe the difference in the rate of reaction of primary, secondary and tertiary haloalkanes.

Answer 13

• The strength of the carbon-halogen bond is not the only factor that influences the rate of hydrolysis. Tertiary haloalkanes are hydrolysed the fastest, while hydrolysis of the primary haloalkane is the slowest.
• The main reason lies with the reaction mechanism. A primary haloalkane will react by a one-step mechanism, whereas a tertiary haloalkane reacts by a two step mechanism.
  
  • In the first step, the carbon-halogen bond of the tertiary haloalkane breaks by heterolytic fission, forming a tertiary carbocation and a halide ion.
  • In the second step, a hydroxide ion attacks the the carbocation to form the organic product.
  • The increased rate can be explained by the increased stability of the tertiary carbocation compared to that of the primary carbocation.

Question 14

1. What are organohalogen compounds?
2. Give some common uses of organohalogen compounds.

Answer 14

1. Organohalogen compounds are molecules that contain at least one halogen atom joined to a carbon chain.
2. Organohalogen compounds are used in pesticides, general solvents, making polymers, flame retardants and refrigerants.

Question 15

1. What is the ozone layer?
2. Why is the continued depletion of the ozone layer bad for us?

Answer 15

1. The ozone layer is found at the outer adge of the stratosphere of Earth. Only a tiny fraction of the gases making up the ozone layer is ozone, but this is enough to absorb most of the biologically damaging ultraviolet radiation (UV-B) from the Sun’s rays, allowing only a small amount to reach the Earth’s surface.
2. Increased UV-B radiation is commonly linked to sunburn, genetic damage in living organisms and a greater risk of skin cancer in humans.

Question 16

Describe the formation of ozone in the stratosphere.

Answer 16

• In the stratosphere, ozone is continually being formed and broken down by the action of ultraviolet radiation. Initially very high energy UV breaks oxygen molecules into oxygen radicals:
  O2 → 2O
• A steady state is then set up involving O₂ and the oxygen radical in which ozone froms and then breaks down. In this steady state, the rate of formation of oxone is the same as the rate at which it is broken down:
  O₂ + O ⇌ O₃

Question 17

Describe the stability of CFCs.

Answer 17

CFCs are very stable because of the strength of the carbon-halogen bonds within their molecules. The CFCs remain stable until they reach the stratosphere. Here the CFCs begin to break down, forming chlorine radicals, which are thought to catalyse the breakdown of the ozone layer.

Question 18

Describe the depletion of CFCs in the ozonelayer.

Answer 18

Question 19

Are CFCs responsible for all ozone-depleting reactions?

Answer 19