Halogenoalkanes

Halogenoalkane definition

Halogenoalkanes are also known as haloalkanes or alkyl halides. Simply put, they are alkanes, but they contain a halogen atom instead of one (or more!) of their hydrogen atoms. The halogen atom is referred to as X.

Halogenoalkane general formula

The general molecular formula for a halogenoalkane with a single halogen atom is C_nH_2n+1X. Examples include chloroethane, C₂H₅Cl, and bromomethane, CH₃Br.

Halogenoalkane nomenclature

Halogenoalkanes are named using standard nomenclature rules and the appropriate prefix, as shown in the table below.

If you’re not familiar with the basics of naming organic molecules, take a quick look at Organic Compounds. But for those of you who feel confident, let’s use some examples to practice applying our knowledge of nomenclature.

Here’s another example:

Primary, secondary, and tertiary halogenoalkanes

Halogenoalkanes are classified as primary, secondary, or tertiary. This depends on the number of alkyl groups attached to the C-X bonded carbon. Classification is referred to using the degree symbol (°), as shown below. Alkyl groups are often called R groups when discussing organic molecules.

• In a primary halogenoalkane (1°), the carbon bonded to the halogen atom is also bonded to either zero or one R group.
• In a secondary halogenoalkane (2°), the carbon bonded to the halogen atom is also bonded to two R groups.
• Likewise, in a tertiary halogenoalkane (3°), the carbon bonded to the halogen atom is also bonded to three R groups.

Here are some examples of primary, secondary, and tertiary halogenoalkanes. We've highlighted the halogen atom in blue and the R groups in red, to help you identify them within the molecule.

Examples of halogenoalkane classification. StudySmarter Originals

Properties of halogenoalkanes

Halogenoalkanes have slightly different properties to alkanes due to their polar C-X bond. This is all thanks to the different electronegativities of carbon and the halogens, which are shown in the table below.

You can see that all of the halogens are more electronegative than carbon. This means that the halogen atom becomes partially negatively charged and the carbon atom partially positively charged. We represent the partial charges using the delta symbol (δ), positioned above each atom:

The C-X bond polarity. StudySmarter Originals

Because of this polarity, halogenoalkanes experience permanent dipole-dipole forces between molecules. These are stronger than van der Waal forces and require more energy to overcome, influencing some of the physical properties of the molecules. Let's explore them now.

Melting and boiling points

Halogenoalkanes have higher boiling points than alkanes of similar chain length. This is due to two factors.

• Halogenoalkanes have a higher molecular mass than their respective alkanes. For example, methane (with a chain length of one carbon atom) has a molecular mass of 16.0, but chloromethane (also with a chain length of one carbon atom) has a molecular mass of 50.5. This means that chloromethane contains more electrons and can form larger temporary dipoles, and so experiences greater van der Waals forces between molecules.
• As explored above, C-X bonds are polar and contain a dipole moment. This means that halogenoalkanes additionally experience permanent dipole-dipole forces.

The increased strength of the intermolecular forces means more energy is required to separate the molecules. Halogenoalkanes, therefore, have higher boiling points than similar alkanes.

Methane and chloromethane. Note chloromethane's permanent dipoles and higher molecular mass. StudySmarter Originals

In addition, longer halogenoalkanes have higher boiling points than shorter halogenoalkanes. They are larger molecules and experience greater van der Waals attraction. On the other hand, a branched chain hydrocarbon has a lower boiling point than a similar unbranched one. This is because the molecules cannot pack together as tightly, so the attraction between molecules is weaker.

For example, consider 1-chlorobutane and 1-chloromethylpropane. Both have the same molecular mass, but whilst the former is a straight-chain molecule, the latter is branched. This means that it can't pack together as closely - note how three 1-chlorobutane molecules can fit into the same space as just two 1-chloromethylpropane molecules.

Straight and branched halogenoalkanes. StudySmarter Originals

The boiling point of halogenoalkanes also varies depending on the halogen in the molecule. It is affected by two factors.

• As you move down the group in the periodic table, the halogen atoms increase in atomic mass. This means that they will experience stronger van der Waals forces.
• We saw above that as you move down the group, the halogen’s electronegativity decreases. This makes the C-X bond less polar and reduces the permanent dipole-dipole forces between molecules.

These two factors seem to oppose each other, so what is the trend in boiling points?

Well, melting and boiling points increase as you move down group 7 in the periodic table. The large increase in the number of electrons is therefore more important than the reduced permanent dipole strength. You can see this in the table below. Although chlorine has a greater electronegativity than iodine, 1-iodopropane has a higher boiling point than 1-chloropropane. This is because 1-iodopropane has more electrons and hence experiences stronger van der Waals attraction.

Solubility

Halogenoalkanes are insoluble in water, despite their polarity. They are not polar enough to hydrogen bond with water molecules. However, they are highly soluble in organic solvents.

Production of halogenoalkanes

We can produce halogenoalkanes using a variety of different methods:

• Free radical substitution of alkanes, using Cl₂ or Br₂ in the presence of UV light.
• Electrophilic addition of alkenes using a halogen, X₂, or hydrogen halide, HX.
• Substitution of alcohols, using various different reactants and conditions.

Reactivity of halogenoalkanes

Because of their polar C-X bond, halogenoalkanes are commonly attacked by nucleophiles.

Nucleophiles are negatively charged or partially negative charged molecules with at least one lone pair of electrons. They are attracted to positive, or partially positive, atoms such as the carbon in the C-X bond.

Common reactions involving halogenoalkanes include:

• Nucleophilic substitution with :OH^- to form an alcohol.
• Nucleophilic substitution with ammonia to form an amine.
• Nucleophilic substitution with :CN^- to form a nitrile.
• Elimination with :OH^- to form an alkene.

Factors affecting halogenoalkane reactivity

Two factors influence halogenoalkane reactivity:

• Bond polarity.
• Bond strength.

Bond polarity

We learned earlier that the electronegativity of the halogen atom decreases as you move further down the group in the periodic table. This makes the C-X bond less polar. The carbon atom, now less positively charged, is less subject to attack by nucleophiles, so the bond is less reactive.

Bond strength

As you move further down group 7 in the periodic table, the C-X bond enthalpy decreases. This is because the halogen atom becomes larger and the shared pair of electrons is further from its nucleus. Thus, there is a weaker attraction between the electrons and the nucleus, making the bond easier to break and more reactive.

You might wonder which factor is more important. Well, experiments show that reactivity increases as you move down the group. This means that bond strength is a more important factor than bond polarity when it comes to reactivity.

A graph showing the relative strength of the C-X bond. StudySmarter Originals

Uses of halogenoalkanes

Finally, let's take a moment to consider some of the uses of halogenoalkanes.

• At the start of this article, we discussed how halogenoalkanes are useful components of many drugs, such as the antidepressant fluoxetine and anesthetic isoflurane.
• Halogenoalkanes are also used as solvents and fumigants.
• The non-stick coating Teflon is a polymer based on the halogenoalkane tetrafluoroethene.
• Chlorofluorocarbons (CFCs) rose to fame in the 20th century as popular features of many aerosols and refrigerants. However, CFCs are extremely dangerous due to their effect on the ozone layer. As a result, they are now banned in many countries. Alternatives include hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).