Unit 4 : HALOGENOALKANES (ALKYL HALIDES)
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 com
pounds 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.