The Characteristics and Properties of Alkyl Halides

Organic Chemistry is about the reactions to form the alkanes, alkenes, alkyne, alkyl halides, and many other organic compounds. Alkyl Halides are used on a large scale in several objects used in our day-to-day life. They are used as fire extinguishers, antibiotics and many more. Alkyl Halides consist of halogen groups, which give them the property of being polar. 

Alkyl Halides definition 

They are also called haloalkanes. When an alkane contains one or two halogen atoms as a substitution, it is called an alkyl halide. Fluorine, chlorine, iodine, or bromine are halogen atoms. The common formula for the alkyl halides is RX, where R is the substituted alkyl group and X is a halogen. The bond formed between carbon and halogens is polar.

Types of alkyl halides

Alkyl halides are divided according to the position of the halogen atom in a carbon chain. The types of alkyl halides are as follows.

1. Primary

When the halogen atom is attached to a carbon attached to only one alkyl group and has the first position in a chain, it is called a primary alkyl halide. CH3CH2Br is an example of primary alkyl halides.

2. Secondary 

When the halogen atom is attached to a carbon attached to two alkyl groups and has the second position in a chain, it is called a secondary alkyl halide. CH3CHBrCH3 is an example of primary alkyl halides.

3. Tertiary Alkyl Halides

When the halogen atom is attached to a carbon attached to three alkyl groups and has the third position in a chain, it is called a tertiary alkyl halide. CH3CH3CBrCH3 is an example of tertiary alkyl halides.

4. Allylic Halides 

When the halogen is attached to the carbon atom at the sp3 hybridised with a carbon-carbon double bond, it is called allylic halide.

5. Vinylic Halides

When the halogen is attached with the carbon atom, which is at the sp2 hybridised atom with the carbon-carbon double bond, it is called Vinylic Halides.

Characteristics

• Being colourless, odourless, and hydrophobic, they are similar to their parents.
• Due to strong intermolecular attraction, they have a higher melting point and boiling than their parent alkanes. Many alkyl halides of fluorine go against this trend.
• They are less flammable than alkanes, and thus, some of them are used in fire extinguishers.
• Other than haloalkanes of fluorine, all haloalkanes are more reactive than their parent alkanes.
• Many primary alkyl halides and the ones with the heavier halogens are alkylating agents. 

Formation of alkyl halides

From alkanes

In the case of the production of haloalkanes from alkanes, the hydrogen atom is substituted with the halogen atom.eg

CH4 +Cl2+energy→CH3Cl+HCl

From alkenes

The dry hydrogen halide (HX) reacts with the alkene to form mono-haloalkane. One bond forms with halogen and the other with hydrogen. It may also react with halogens to form double halo compounds.eg

H2C=CH2 + H-Cl → H2C-CH2Cl

From alcohols

Tertiary alcohol reacts with hydrochloric acid to form tertiary chloroalkane. In the case of a primary and secondary alcohol, a catalyst like zinc chloride is required to start the reaction.eg

CH3CH2OH + HI → CH3CH2 I + H2O

Preparation of Alkyl Halides by Free Radical Halogenation

A complex mixture of isomeric mono- and poly haloalkanes is formed upon free radical chlorination or bromination of alkanes.

CH3CH2CH2CH3 + Cl2  → CH3CH2CHClCH3 + CH3CH2CH2CHCl

Reacting properties of alkyl halides

1. Substitution reaction

Halogens react with nucleophiles to form saturated hydrocarbons and the halogenated product. The reaction is a substitution type of reaction. Eg.

  CH3CHBrCH3 + NaOH → CH3-CHOHCH3 +NaBr

2. Elimination reaction

In this reaction, the halogens and the adjacent proton get removed from the haloalkane to form an alkene. Dihaloalkanes are converted into alkenes by this reaction. This is an elimination reaction.

CH3CHBrCH3 + NaOH → CH2=CHCH3 +NaBr +H2O

3. Reaction with metals

Haloalkanes react with some metals to give alkyl metal compounds. For example, it undergoes a free radical reaction with magnesium to form alkyl magnesium compounds.eg

R- X + 2Na + X- R → R-R + 2NaX……( in presence of dry ether)

Conclusion

Thus, when an alkane contains one or two halogen atoms as a substitution, it is called an alkyl halide. Alkyl halides are divided according to the position of the halogen atom in a carbon chain, as given above. They are from the alkanes, alkenes, alkynes, biosynthesis process, alcohol, carboxylic acid, amines by executing various reaction types. It readily reacts with nucleophiles to give the saturated hydrocarbon and halogenated product. It also undergoes an elimination reaction and reaction with metals to form organometallic compounds. Also, they show many properties that are contrasting to their parents.

Do you still have questions on alkyl halides? Questions on the subject are answered below.