Halogenoalkanes 

What is a halogenoalkane?

Halogenoalkanes are a family of compounds in which a halogen atom has replaced one or more hydrogen atoms in an alkane. 

They are classified as primary, secondary and tertiary halogenoalkanes:

• Primary halogenoalkane - the carbon atom attached to the halogenoalkane atom is only attached to 1 alkyl group
• Secondary halogenoalkane - the carbon atom attached to the halogenoalkane atom is attached to 2 alkyl groups 
• Tertiary halogenoalkane - the carbon atom attached to the halogenoalkane atom is attached to 3 alkyl groups.

Primary halogenoalkanes have the general formula C_nH_2n+1X, where X is a halogen atom.

Some halogenoalkane examples are iodoethane, CH₃CH₂I, which is a primary halogenoalkane, and 2-chloro-2-methylpropane, CH₃C(CH₃)ClCH₃, which is a tertiary halogenoalkane.

Why are halogenoalkanes reactive?

There is a large difference in the electronegativities between the carbon atom and the halogen atom in a halogenoalkane. This results in a polar carbon-hydrogen bond which makes them susceptible to attacks by nucleophiles and halogenoalkanes will undergo nucleophilic substitution reactions. The presence of the polar carbon-halogen bond is the reason why halogenoalkanes are more reactive than alkanes.

• More information: ‘The Key Reactions of the Halogenoalkanes’

How does bond enthalpy influence the rate of reaction?

The rate of substitution of different halogenoalkanes will vary. When substitution occurs, the carbon-halogen bond (C-X) is broken and the lower the bond enthalpy of the C-X bond, the more likely the halogenoalkane will undergo substitution.

The table below shows the bond enthalpies of the different carbon-halogen bonds:

Bond	Bond Enthalpy (kJ mol-1)
C 一 F	467 (strongest bond)
C 一 Cl	346
C 一 Br	290
C 一 I	228 (weakest bond)

 
The C-F bond is the strongest bond as it requires the most energy to break, therefore fluoroalkanes will have the slowest rate of reaction.

The C-I bond is the weakest bond as it requires the least amount of energy to break, therefore iodoalkanes will have the quickest rate of reaction.

• More information: ‘Reactivity of Halogenoalkanes’

How can the rate of the reaction be measured?

When halogenoalkanes react with an aqueous silver nitrate solution, a silver halide precipitate is formed. The faster the precipitate is formed, the quicker the rate of reaction.

From the three halogenoalkanes commonly used (chloroalkanes, bromoalkanes and iodoalkanes, the pale yellow precipitate silver iodide is produced the quickest and the white precipitate silver chloride is produced the slowest, confirming the reactivity of the halogenoalkanes.

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

• More information: ‘Reactivity of Halogenoalkanes’

Reactions of halogenoalkanes

The presence of the polar carbon-halogen bond causes halogenoalkanes to undergo two key types of reaction:

• Nucleophilic substitution
• Elimination reactions - a hydrogen halide is eliminated during the reaction and an alkene is produced

Nucleophilic substitution reactions of halogenoalkanes

In these reactions, the halogen atom is substituted by another atom or group of atoms. The most common products of these reactions are alcohols, nitriles and amines.

Halogenoalkane to alcohol

The halogenoalkane is heated with an aqueous solution of sodium hydroxide or potassium hydroxide with ethanol. The nucleophile is the hydroxide ion, -OH which replaces the halogen atom to form an alcohol.

The mechanism for this reaction is:

This is also an example of a hydrolysis reaction and the rate of hydrolysis depends on the type of halogen within the halogenoalkane.

The rate of hydrolysis of halogenoalkanes can be determined by heating a mixture of acidified silver nitrate solution and ethanol and adding a few drops of the halogenoalkane. The time taken to produce a precipitate is measured. 

The stronger the carbon-halogen bond, the quicker the rate of reaction. As seen earlier, the C-Cl bond is stronger than the C-I bond, so a pale yellow precipitate (silver iodide)  will be produced quicker than a white precipitate (silver chloride) as C-I will break more easily and will undergo nucleophilic substitution more readily.

NOTE: This reaction can also be performed with water but it is slow as water is not such a good nucleophile as hydroxide ions as it only has a partial negative charge, instead of a full negative charge:

• More information: ’The Key Reactions of the Halogenoalkanes’

Halogenoalkane to nitrile

Nitriles are produced when halogenoalkanes are heated under reflux with potassium cyanide in ethanol. The nucleophile in the cyanide ion, CN^–.

This is a useful reaction as it can be used to make a compound that has one more carbon atom than the starting halogenoalkane. The mechanism for this reaction is: 

Halogenoalkane to amine

Primary amines are produced when halogenoalkanes are heated under pressure with an ethanolic solution of excess ammonia. The nucleophile in the ammonia molecule, NH3. The mechanism for this reaction is:

• More information: ’The Key Reactions of the Halogenoalkanes’

Elimination reactions of halogenoalkanes

In these reactions, the halogenoalkane loses a hydrogen halide.

Halogenoalkane to alkene

Halogenoalkanes are heated with ethanolic sodium hydroxide. The carbon-halogen bond breaks heterolytically forming a halide ion and alkene. An example is:

The mechanism for this reaction is:

NOTE: The reaction conditions are very important as different products are formed when halogenoalkanes react with hydroxide ions, depending on the reaction conditions used.

• More information: ‘Elimination’ and ‘Elimination Reactions’

How do halogenoalkanes cause depletion of the ozone layer?

Common halogenoalkanes are chlorofluorocarbons (CFCs) which contain carbon, chlorine and fluorine atoms, e.g. CCl₃F. They are chemically inert and non-flammable and non-toxic. They have many uses due to their properties, e.g. as refrigerants, as propellants in aerosols and as solvents for dry cleaning.

Hydrofluorocarbons (HFCs) contain carbon, hydrogen and fluorine atoms, e.g. CHF₂CF₃, and are also chemically inert.

Ozone, O₃, is formed naturally in the upper atmosphere. It is beneficial because it absorbs ultraviolet radiation, preventing the majority of harmful UV radiation from reaching the Earth’s surface.

When CFCs reach the upper atmosphere, UV radiation breaks the C–Cl bonds to form chlorine radicals, Cl•. Chlorine radicals catalyse the decomposition of ozone and contribute to the hole in the ozone layer:

   Propagation step 1: Cl• + O₃ → ClO• + O₂ 

   Propagation step 2: ClO• + O₃ → 2O₂ + Cl•

Overall equation: 2O3 → 3O2

Scientific research has provided sufficient evidence of the harmful effects of CFCs which has led to legislation to ban the use of CFCs as solvents as refrigerants. Chlorine-free alternatives, such as HFCs, have been developed by chemists to replace CFCs.

• More information: ‘Uses’ and ‘Halogenoalkanes & The Ozone Layer’

What keyword definitions do I need to know for halogenoalkanes? 

Some keyword definitions you need to know are:

• Halogenoalkanes - 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 haloalkanes 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
• Elimination reaction - a reaction which involves the removal of a small molecule from one reactant molecule and produces two products
• 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

This is a quick summary of some key concepts on halogenoalkanes - 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!