Haloalkanes: Classification, Preparation, Uses, Examples

Haloalkanes are organic compounds containing an alkane with one or more hydrogen atoms replaced by halogen atoms like Fluorine, Chlorine, Bromine, and Iodine. They are also known as alkyl halides. Similarly, if hydrogen of aromatic compounds is replaced by halogen, such compounds are known as aryl halides or haloarenes. Alkyl halides are named by the alkyl group followed by halides such as ‘chloropropane containing propane as alkane and chlorine as halogen group’.

These compounds are clinically tested and find wide applications in industry as well as in our daily life. In this article, we will uncover the essential methods of preparation, physical and chemical properties, and uses of haloalkanes and haloarenes.

What are Haloalkanes?

Haloalkanes or alkyl halides are organic compounds with an alkane-based compound with one or more hydrogen atoms replaced by halogen. The general representation of haloalkanes is \({\rm{R – X}}\) where \({\rm{R}}\) is represented as an alkyl group, and \({\rm{X}}\) is represented as halogens such as \({\rm{F, C l, B r, I}}\) Some examples of haloalkanes are Methyl Chloride or Chloromethane \(\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{Cl}}} \right){\rm{,}}\), Bromoethane or Ethyl Bromide \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{Br}}\), etc.

The organohalogen compounds occur in nature, such as fungi and lichens produce simple halogens like Chloromethane to some complex organohalogens, Leguminous plants produce a growth hormone ‘\(4\)-chloroindole-\(3\)-acetic acid, marine creatures produce natural organic bromides, and soil microorganisms produce Chlorine containing antibiotic- chloramphenicol. The human body produces iodine-containing hormone known as ‘thyroxine’, the deficiency of which causes a disease called goitre. Some synthetic halogen compounds, like chloroquine, are used to treat malaria, and some halogenated compounds are used as an anaesthetic during surgery. Thus now we can understand the importance of these haloalkanes and haloarenes in our lives.

Haloalkanes and Haloarenes

Hydrocarbons having aliphatic alkanes with one or more hydrogen atoms replaced by halogens are known as haloalkanes or alkyl halides. On the other hand, if the hydrogen of aromatic compounds is substituted by halogen, such compounds are known as aryl halides or haloarenes. Haloalkanes are derived from aliphatic hydrocarbons, whereas haloarenes are derived from aromatic hydrocarbons. For example, Bromopropane, Chloromethane, etc., are haloalkanes, while bromobenzene, chlorobenzene, etc., are haloarenes.

Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are classified on the following basis:

Nomenclature of Haloalkanes and Haloarenes

IUPAC Nomenclature system of haloalkanes is closely related to alkanes as replacing one or more hydrogen with halogen gives haloalkanes from alkanes. Whereas, in common names, they are named alkyl halides where alkyl is the name of the ‘alkyl’ group, and ‘halide’ is the stem of the name of halogen. Similarly, haloarenes are also named in the IUPAC nomenclature system. Dihalogen compounds are commonly named using prefix ortho, meta, para, and in IUPAC system numbering of carbon atoms as \(1,2; 1,3; 1,4\).

Nature of C-X Bond

Carbon-halogen bonds are polarised in nature as halogens are more electronegative than carbon. Thus carbon atoms carry a partial positive charge, whereas halogen atoms carry a partial negative charge.

Physical Properties of Haloalkanes

The physical properties of Haloalkanes are as follows:

1. Haloalkanes are primarily colourless and odourless compounds.
2. The boiling point and melting point of haloalkanes are greater than their respective alkanes.
3. The boiling point and melting point of haloalkanes increases with an increase in the number of halogen atoms in alkanes. \({\rm{R – l > R – Br > R – Cl > R – F}}\).
4. As the size of the halogen atom increases, the bond strength of haloalkanes decreases. Thus, the order of bond strength down the group is: \({\rm{C}}{{\rm{H}}_3}\;{\rm{F}} > {\rm{C}}{{\rm{H}}_3}{\rm{Cl}} > {\rm{C}}{{\rm{H}}_3}{\rm{Br}} > {\rm{C}}{{\rm{H}}_3}{\rm{I}}\)
5. The density of haloalkanes increases as we move down in homologous series,
6. They are hydrophobic.

Methods of Preparation of Haloalkanes

Haloalkanes can be prepared in various ways. These are discussed below:

Preparation of Haloalkanes from Alcohol

Haloalkanes are commonly prepared from alcohol. The hydroxyl group of alcohol is replaced by halogen in reaction with concentrated acids. Pure alkyl halides are extracted in this process, especially by using thionyl chloride. Preparation of alkyl chloride is done by passing dry hydrogen chloride gas through an alcoholic solution or heating it in concentrated aqueous acid.

Preparation of Haloalkanes by Radical Halogenation

Mono or poly-substituted haloalkanes are prepared by radical halogenation in the presence of light.

Preparation of Haloalkanes by Addition Reaction of Alkenes

In the reaction of haloalkanes with hydrogen chloride, hydrogen bromide, or hydrogen iodide, corresponding haloalkanes are prepared by an addition reaction. Dihalides are prepared by the addition reaction of alkenes with halogens.

Chemical Reactions of Haloalkanes

The various chemical reactions of haloalkanes can be divided into the following categories:

1. Nucleophilic Substitution Reactions
2. Elimination Reactions
3. Reaction with Metals

Let’s discuss them one by one.

1. Nucleophilic Substitution Reactions

As \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised carbon atom having halogen has a partially positive charge in it, the substitution of nucleophile takes place, and the halogen atom leaves as a halide ion. Nucleophilic Substitution Reactions are very common in alkyl halides and aryl halides. Based on the mechanism, they are divided into two types:

(i) \({{\rm{S}}_{\rm{n}}}{\rm{1}}\) type (Unimolecular nucleophilic reactions): This is a substitution unimolecular nucleophilic reaction taking place in a two-step mechanism. In the first step, carbocation is formed when halogen (leaving group) heterolytically breaks the bond with carbon, and in the second step, the nucleophile attacks this carbocation.

1. This reaction follows \(1\) order kinetics.
2. Reactivity order of haloalkanes towards the \({{\rm{S}}_{\rm{N}}}{\rm{1}}\) reaction is \({3^ \circ } > {2^ \circ } > {1^ \circ }\)
3. This reaction occurs in protic solvents.
4. Partial racemisation is possible as the nucleophile can attack from both sides.

(ii) \({{\rm{S}}_{\rm{N}}}2\) type (Bimolecular nucleophilic reactions): This is a substitution bimolecular nucleophilic reaction taking place in a single-step mechanism. Both bond breaking, as well as bond making, coincides, resulting in the inversion of configuration.

1. This reaction follows \({{\rm{2}}^{{\rm{nd}}}}\) order kinetics.
2. Reactivity order of haloalkanes towards the \({{\rm{S}}_{\rm{N}}}2\) reaction is \({1^ \circ } > {2^ \circ } > {3^ \circ }\)
3. This reaction occurs in non-polar or aprotic solvents.
4. Partial racemisation is possible as the nucleophile can attack from both sides.
5. Inversion of configuration (Walden Inversion) occurs.

2. Elimination Reactions

When alkyl halides with \({\rm{\beta }}\)-hydrogen atom are heated with alcoholic potassium hydroxide solution, there is the elimination of halogen atom from the \({\rm{\alpha }}\)-carbon atom and \({\rm{\beta }}\)-hydrogen atom from \({\rm{\beta }}\)-carbon that results in the formation of alkene. This reaction is commonly known as \({\rm{\beta }}\)-elimination since the \({\rm{\beta }}\)-hydrogen atom is eliminated.

3. Reaction with Metals

Many famous name reactions of alkyl halides come under this category. Let’s discuss them one by one:

1. Grignard’s Reagent – Organic halides react with the metal to form organometallic halides. One of the well-known, such compounds is Grignard’s Reagent represented as \({\rm{RMgX}}\). They are used to form hydrocarbons like alkanes from alkyl halides.

2. Wurtz Reaction – It is a type of coupling reaction where alkyl halides react with sodium metal in the presence of dry ether to give hydrocarbons having a double number of carbon atoms than in alkyl halide.

Solubility of Haloalkanes

The solubility of Haloalkanes are as follows:

Haloalkanes are Soluble in Organic Solvents

Haloalkanes are polar compounds containing polar covalent bonds. They can easily dissolve in organic solvents as the new force of attraction between the broken polar bonds, and the solvent is of almost the same strength.

Haloalkanes are Slightly Soluble in Water

Haloalkanes are not completely soluble in water because the bonds formed between ionic compounds formed by breaking polar covalent bonds and water are much weaker than hydrogen bonding between the water molecules. As a result, haloalkanes are slightly soluble in water.

Polyhalogen Compounds

Compounds formed by multiple substitutions of halogens, i.e. more than one halogen atom, are known as polyhalogen compounds; such compounds are helpful to us. Some of them are discussed below:

1. Trichloromethane (Chloroform) – Chloroform \(\left( {{\rm{CHC}}{{\rm{l}}_3}} \right)\) is primarily used in making Freon-refrigerants. Earlier it was used as anaesthesia in surgery. 

2. Freons – The chlorofluorocarbon compounds of methane and ethane are together known as freons (dichlorodifluoromethane). These are used in aerosol propellants, refrigeration, and air conditioners. 

3. DDT (\(1, 1, 1\)-trichloro-\(2, 2\)-diphenylmethane) – DDT is one of the first chlorinated organic insecticide. Due to its stability and fat solubility, it got banned. 

4. Perchloroethane \(\left( {{{\rm{C}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{6}}}} \right)\) – It is used as a moth or insect repellent and is also known as artificial camphor.

Summary

We can conclude that haloalkanes are derived from alkanes, with at least one hydrogen replaced by halogen. Haloalkanes are aliphatic, while haloarenes are aromatic. These compounds are beneficial in clinical as well as industrial uses. Haloalkanes have a high boiling point and melting point than alkanes. Haloalkanes undergo characteristic reactions like \({\rm{S}}{{\rm{N}}^{\rm{1}}}\) and \({\rm{S}}{{\rm{N}}^{\rm{2}}}\) reactions, Elimination reactions, Wurtz reactions, etc. Grignard’s reagent is a famous organometallic halide used various in chemical reactions.

We also studied the uses of haloalkanes in our day-to-day life, such as they are widely used for commercial purposes like they are used as flame retardants, refrigerants, fire extinguishers, propellants, solvents, and in the pharmaceuticals industry.

FAQs on Haloalkenes

Q.1. What are the types of Haloalkanes?
Ans:  All haloalkanes contain a halogen atom such as fluorine, Chlorine, bromine, or iodine attached to an alkyl group. Based on the number of adjacent carbon atoms, they are classified into three different kinds of haloalkanes as Primary haloalkanes, Secondary haloalkanes, and Tertiary haloalkanes. 

Q.2. What are examples of Haloarenes?
Ans: Examples of haloarenes are chlorobenzene, bromobenzene, \(1,2\)-dichlorobenzene, o-bromo toluene, fluorobenzene, etc.

Q.3. How are Haloalkanes classified?
Ans: Based on the number of halogens present, haloalkanes are classified as mono haloalkane (having \(1\) halogen atom), haloalkane (having \(2\) halogen atoms), trihaloalkane (having \(3\) halogen atoms), and polyhaloalkanes. Based on the nature of the C-X bond, they are also classified as primary, secondary, and tertiary haloalkanes 

Q.4. What is a Haloalkane? Identify one of them?
Ans: A haloalkane can be identified as a chemical compound derived from an alkane, having one halogen atom replaced by a hydrogen atom. They are also known as alkyl halides. Haloalkanes can be identified in the laboratory by silver nitrate test.

Q.5. What are Haloalkanes used for?
Ans: Haloalkanes have multiple uses in our daily life, such as they are widely used for commercial purposes- flame retardants, refrigerants, fire extinguishers, propellants, solvents, and the pharmaceuticals industry.

Q.6. Are Haloalkanes toxic?
Ans: Some of the haloalkanes like chlorofluorocarbons are toxic and are responsible for ozone layer depletion.

Q.7. Why are Haloalkanes not soluble in water?
Ans: Haloalkanes are not soluble in water because the bonds formed between ionic compounds formed by breaking polar covalent bonds and water are much weaker than hydrogen bonding between the water molecules. 

Q.8. Why are haloarenes less reactive than haloalkanes?
Ans: Haloarenes are less reactive than haloalkanes because they are more stable as they have a resonance hybrid structure. The C – X bond has a partial double bond character, making the haloarene more stable and less reactive. 

Q.9. How do you cook Haloalkanes?
Ans: Haloalkanes are cooked or prepared from alcohol. The hydroxyl group of alcohol is replaced by halogen on reaction with concentrated acids. Pure alkyl halides are extracted in this process. Alkyl chloride is prepared by passing dry hydrogen chloride gas through a solution of alcohol or by heating it in concentrated aqueous acid.

Students can make haloalkanes and haloarenes notes from the above questions to revise the concepts quickly. We hope this article on Haloalkanes has helped you. If you have any queries, drop a comment below, and we will get back to you.