Haloalkanes

What is a haloalkane?

Haloalkanes are a family of compounds which have had one or more hydrogen atoms in an alkane replaced by a halogen atom. 

There are three types of haloalkanes: primary, secondary and tertiary haloalkanes. In a primary haloalkane, the carbon atom attached to the haloalkane atom is only attached to 1 alkyl group, In a secondary haloalkane, the carbon atom attached to the haloalkane atom is attached to 2 alkyl groups and in a tertiary haloalkane, the carbon atom attached to the haloalkane atom is attached to 3 alkyl groups.

The general formula of primary haloalkanes is C_nH_2n+1X, where X is a halogen atom.

Some haloalkane examples are chloroethane, CH₃CH₂Cl, which is a primary haloalkane, and 2-bromo-2-methylpropane, CH₃C(CH₃)BrCH₃, which is a tertiary haloalkane.

The large difference in the electronegativities between the carbon atom and the halogen atom results in a polar carbon-halogen bond. Because of this, haloalkanes are much more reactive than alkanes.

• For more specific information on this, go to this revision note: ‘Substitution Reactions of Haloalkanes’

What type of reactions do haloalkanes undergo?

Haloalkanes undergo nucleophilic substitution reactions. This is because the carbon atom in the carbon-halogen bond has a partial positive charge which can attract a nucleophile, such as hydroxide ions, water molecules and ammonia molecules. The halogen atom is substituted by another atom or group of atoms.

Typical products formed when haloalkanes undergo nucleophilic substitution are alcohols, amines and nitriles.

Some species are better nucleophiles than others, for example, the hydroxide ion is a better nucleophile as it has a full negative charge, whereas the oxygen atom in water only has a partial negative charge:

The general mechanism for nucleophilic substitution is:

• More information: ‘Nucleophilic Substitution of Haloalkanes’

Haloalkanes can also undergo elimination reactions in which a hydrogen halide is eliminated and an alkene is produced.

How do haloalkanes produce alcohols?

Alcohols are formed from haloalkanes in a hydrolysis reaction where the halogen atom is replaced by an -OH group, which acts as the nucleophile. This reaction is an example of nucleophilic substitution.

The haloalkane is heated with an aqueous solution of sodium hydroxide or potassium hydroxide with ethanol.

The nucleophilic substitution mechanism for the hydrolysis of bromoethane is shown below: 

• More information: ‘Nucleophilic Substitution of Haloalkanes’

Why do haloalkanes have different rates of hydrolysis?

During hydrolysis, the carbon-halogen bond is broken. The rate of hydrolysis is dependent on the carbon-halogen bond strength. The bond energies of these bonds are shown below:

This shows that the C-F bond is the strongest bond as it requires the most energy to break and that C-I bond is the weakest bond as it requires the least amount of energy to break.

As the C-I bond is the weakest bond, it will be more reactive and more likely to undergo nucleophilic substitution. Therefore, iodoalkanes will have the quickest rate of hydrolysis, whereas fluoroalkanes will have the slowest rate of hydrolysis.

So, the rate of hydrolysis increases as the carbon-halogen bond strength decreases.

• More information: "Hydrolysis of Primary Haloalkanes"’

How can the rate of hydrolysis of primary haloalkanes be measured?

The rate of hydrolysis of the primary haloalkanes, e.g. 1-chlorobutane, 1-bromobutane and 1-iodobutane, can be measured by heating the haloalkane with aqueous acidified silver nitrate solution and ethanol.

A precipitate reaction occurs as an insoluble silver halide is produced and the rate at which the precipitate is produced can be measured.

Silver iodide (a pale yellow precipitate) will be produced the fastest and silver chloride (a white precipitate) will be produced the slowest. This shows that, of the 3 haloalkanes tested, iodoalkanes are the most reactive and chloroalkanes are the least reactive.

• More information: ‘Hydrolysis of Primary Haloalkanes’

What are the environmental concerns with the use of haloalkanes?

Organohalogen compounds

These are a group of compounds that contain at least one halogen atom joined to a carbon atom. They have many uses but do not break down easily so are cause for environmental concerns.

Chlorofluorocarbons (CFCs)

CFCs are the more common fluorohaloalkanes which contain carbon, chlorine and fluorine atoms, e.g. CCl₃F.

CFCs are:

• Chemically inert
• Non-flammable
• Non-toxic

These properties led them to become widely used in refrigerators, as propellants in aerosols and as solvents in dry cleaning.

However, CFCs are responsible for breaking down the ozone layer:

• CFCs are broken down by UV radiation in the upper atmosphere, producing chlorine radicals in an initiation step
• Chlorine radicals react with ozone, breaking it down
• Chlorine radicals catalyse the reaction as they are regenerated in the second step of the reaction and can go on to break further ozone molecules down

The breakdown of ozone by chlorine radicals is shown below:

Hydrofluorocarbons (HFCs)

HFCs contain carbon, hydrogen and fluorine atoms, e.g. CH₃CH₂F. They are also chemically inert and are used in many applications.

As HFCs do not contain chlorine atoms, they do not breakdown the ozone layer and so are used as an alternative to CFCs.

• More information: ‘Breakdown of the Ozone Layer’

What other radicals break down the ozone layer?

Nitrogen oxide radicals are produced in the stratosphere which can also catalyse the breakdown of ozone. They are produced by lightning strikes and the high temperatures reached in aircraft engines. Nitrogen monoxide is a nitrogen oxide radical which has an unpaired electron. It can, therefore, also be used to catalyse the breakdown of ozone:

   Propagation step 1: NO• + O₃ → NO₂• + O2 

   Propagation step 2: NO₂•  + O  → NO• + O₂

   The overall equation is: O + O₃  → 2O₂

What keyword definitions do I need to know for haloalkanes? 

Some keyword definitions you need to know are:

• Haloalkanes - a family of compounds which have had one or more hydrogen atoms in an alkane replaced by a halogen atom (these are also referred to as halogenoalkanes by some exam boards and some resources refer to them as alkyl halides)
• Nucleophilic substitution - a reaction in which an electron-rich nucleophile displaces another atom or group of atoms
• Hydrolysis - a reaction that involves water (or the aqueous solution of a hydroxide) that results in the breaking of a bond and the formation of two products
• Organohalogen compounds - a group of compounds that contain at least one halogen atom joined to a carbon atom

This is a quick summary of some key concepts on haloalkanes - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!