Chemistry

UNIT 4: HALOGENOALKANES (ALKYL HALIDES)

Key unit competency

The learner should be able to relate the physical and chemical properties of halogenoalkanes to their reactivity and their uses

Learning objectives

• Define halogenoalkanes and homologous series.

• Explain the reactivity of halogenoalkanes.

• Explain the physical properties of halogenoalkanes.

• Describe preparation methods for halogenoalkanes.

• Explain different mechanisms in halogenoalkanes.

• Explain the uses and dangers associated with halogenoalkanes.

• Draw displayed structural formulae of halogenoalkanes and give names

using IUPAC system.

• Classify halogenoalkanes according to developed formula as primary,

secondary and tertiary.

• Write reaction mechanisms of halogenoalkanes as SN1, SN2, E1 and E2.

• Test for the presence of halogenoalkanes in a given sample organic

compound.

• Appreciate the uses and dangers of halogenoalkanes in everyday life.

• Develop the awareness in protecting the environment.

• Develop team work approach and confidence in group activities and

presentation sessions.

4.1. Definition and nomenclature of halogenoalkanes

1. Definition

Halogenoalkanes compounds are compounds in which the halogen atoms like

chlorine, bromine, iodine or fluorine are attached to a hydrocarbon chain. When

the halogen atom is attached to a hydrocarbon chain the compound is called a halogenoalkane or haloalkane or an alkyl halide. 

Halogenoalkanes contain halogen atom(s) attached to the sp³ hybridised carbon atom of an alkyl group. 

2. Nomenclature of halogenoalkanes

Halogenoalkanes are organic compounds that contain a halogen atom: F, Cl, Br, I.

They are named using the prefixes fluoro-, chloro-, bromo- and iodo-.

Numbers are used if necessary to indicate the position of the halogen atom in the

molecule.

4.2. Classification and isomerism

4.2.1. Classification of halogenoalkanes

There are three types of halogenoalkanes:

A primary halogenoalkane has a halogen atom attached to the ended carbon atom

of the chain. A secondary halogenoalkane has a halogen atom attached to a carbon

bonded to two other carbon atoms while a tertiary halogenoalkane has a halogen

atom attached to a carbon bonded to three other carbon atoms.

4.2.2. Isomerism

Halogenoalkanes exhibit both chain and position isomerism.

Example: Molecular formula C₄H₉Br

a. Chain isomerism: This arises due to arrangement of carbon atoms in chains of

different size.

b. Position isomerism: This arises due to the different positions taken by the

halogen atom on the same carbon chain.

The following compounds are position isomers: CH₃ CH₂ CH₂ CH₂-Br and CH₃ CH₂ CH Br CH₃; because the atoms of bromine are on different positions of the chain.

Hence, all isomers of the compound with molecular formula C₄H₉ Br are the following.

4.3. Physical properties of halogenoalkanes

1. Volatility

Volatility is a property that shows if a substance transforms easily or not into vapour

or gaseous form. This property depends on the nature of the bonds that make up

the molecule of the substance. Generally non polar covalent compounds are more

volatile than polar covalent compounds. We know that halogens when bonded to

other atoms form polar bonds because they possess high electronegativities: F =

4.0, Cl = 3.0, Br = 2.8, I = 2.5, and C = 2.5.

The more the difference of electronegativities of the atoms that form the bond,

the more polar is the bond. This explains the high polarity of C-F bond with an

electronegativity difference of 1.5, and the low polarity of C-Cl and C-Br bonds where

the electronegativity differences are 0.5 and 0.3 respectively.

The presence of polarity or charge distribution results into more attraction between

polar molecules called dipole-dipole attraction forces, one type of Van der Waals

forces, as shown below:

The dashed line represents the attraction forces between the polar molecules or

dipoles.

Therefore, more energy must be supplied to separate polar molecules and this

explains why melting and boiling temperatures of fluoroalkanes and chloroalkanes

are higher than those of alkanes of similar molecular mass.

As we have already learnt, molecules of organic halogen compounds are generally

polar. Due to the greater polarity as well as higher molecular mass as compared

to the parent hydrocarbons, the intermolecular forces of attraction (dipole-dipole

and Van der Waals) are stronger in the halogen derivatives. That is why the boiling

points of chlorides, bromides and iodides are considerably higher than those of the

hydrocarbons of comparable molecular mass (Table 4.1).

Chloromethane, bromomethane, chloroethane and some chlorofluoromethanes

are gases at room temperature. Higher members are liquids or solids.

The attractions get stronger as the molecules get bigger in size. The pattern of

variation of boiling points of different halides is depicted in Figure 4.1. For the same

alkyl group, the boiling points of alkyl halides increase in the order: RF <RCl < RBr, <

RI This is because with the increase in size and mass of halogen atom, the magnitude of Van der waal forces increases.

2. Solubility

The solubility is the capacity of a substance to dissolve in a given solvent; in chemistry the most common solvent we refer to is water. It is a result of the interaction between the molecules of the substance, a solute, and the molecules of the solvent.

Polar molecules can interact with water molecules, but the attractive forces set

up between water molecules and molecules concerned are not as strong as the

hydrogen bonds present in water. Halogenoalkanes therefore, although they

dissolve more than alkanes, are only slightly soluble in water.

3. State

The state of matter is the physical appearance of that matter: solid, liquid and

gaseous.

Chloromethane, bromomethane, chloroethane and chloroethene are colourless

gases at room temperature and pressure. The higher members are colourless

liquids with a sweet pleasant smell

4. Density

The density is a measure of the quantity of matter by volume unit. Cotton wool is

less dense than sand because if you compare the quantity of matter cotton wool

and sand contained in for instance 1m³, you find that there more matter in sand than in cotton wool.

The density of halogenoalkanes increases in the order RCl < RBr < RI, since the

atomic weight of halogens increases in order Cl < Br < I. Iodo, bromo and polychloro

derivatives are denser than water but chloro derivatives are less dense than water.

4.4. Preparation methods of halogenoalkanes

1. From alkenes and alkynes

Direct halogenation of alkanes in the presence of ultraviolet light gives alkyl halides and a hydrogen halide.