Alkyl Halide Properties

Alkanes that contain more than one substituent of halogens are known as haloalkanes. They can also be called by other names, such as halogenoalkanes or alkyl halides. 

Alkyl halides are widely used for various commercial purposes. The properties of alkyl halides have also turned them into serious pollutants. For instance, ozone depletion is caused by chlorofluorocarbons, methyl bromide is a harmful fumigant, and iodine depletes the ozone layer. 

History of Alkyl Halide 

Alkyl halides have been present for centuries. Chloroethane was the first-ever known alkyl halide prepared in the 15th century. The properties of alkyl halide were noticed in this alkane. In the 19th century, systematic structures came out for these alkyl halides. Humans produce most alkyl halides. However, some non-artificial alkyl halides occur naturally on Earth. These are produced mainly by seaweeds, fungi, bacteria, etc. 

Reactions of alkyl halides

Alkyl halides are mostly reactive towards nucleophiles because they are polar molecules. The carbon with which the alkyl halide is connected is slightly electropositive. This results in carbon, which becomes electron-deficient, which then attracts the nucleophiles. 

• Substitution Reaction

For alkyl halides, substitution reaction means exchanging alkyl halide or halogen with other molecules, which then leaves a saturated hydrocarbon.

Example-  CH3Cl + OH–→ CH3OH + Cl–

• Elimination Reaction

An elimination reaction is also known as a dehydrohalogenation reaction. The halogen and a proton are removed to form an alkene in this reaction. Similarly, the process can be used to convert dihaloalkanes into alkynes. 

Example-  CH3CH2Cl + RO–→ CH3CH2OR + Cl–

• Hydrolysis 

In this reaction, water breaks a bond. This shows that the nature of alkyl halide is nucleophilic. If the hydrolysis of bromoethane is done, then we get ethanol. 

Example- CH3−Br+OH−→ CH3OH+Br−

• Finkelstein reaction

Iodine immediately substitutes chloroalkanes and bromoalkanes in this reaction. Iodoalkanes are formed, which can easily carry on the further reaction.

R-X + NaY → R-Y +NaX ( in the presence of acetone)

Applications of alkyl halides

Alkyl halides or haloalkanes go through many changes chemically, which can help produce many other chemicals and products in industries that are widely used in our lives. Some applications of alkyl halide properties are given below.

• Alkyl fluorides

 Many pharmaceuticals and other drugs contain fluorine. Most of these compounds are made up of alkyl halides in the form of alkyl fluorides—for example, desflurane, enflurane, methoxyflurane, etc.  

• Alkyl chlorides

The chlorinated hydrocarbons, which have relatively low weight (molecular), are used as solvents—for example, chloroform, dichloromethane, trichloroethane, etc. Chlorodifluoromethane is also used to produce Teflon. 

• Alkyl bromides

 Alkyl bromides are toxic and harmful, so their uses are limited. Methyl bromide is also used as a fumigant, but its uses and production are limited.

• Alkyl iodides

There are no large uses and applications for alkyl iodides. Methyl iodide is a popular alkyl iodide that can be used as methylating substituents in some organic processes. 

• Chlorofluorocarbons 

Chlorofluorocarbons are widely used as propellants and refrigerants. This is because they have low levels of toxicity and high vaporisation heat. These chlorofluorocarbons are limited because they contribute to ozone layer depletion. 

Alkyl halide properties importance 

• State: Alkyl halides are present in solid, liquid, and gaseous states. They occur as lower member gases as methyl chloride, ethyl bromide, ethyl chloride, etc. The higher members occur as liquids which are up to C18. Solid alkyl halides have greater members, such as above C18.

• Odour: All alkyl halides are primarily odourless, but alkyl halides till C18 have a pleasant and sweet smell.

• Colour: Pure alkyl halides are colourless. Iodoalkanes and bromoalkanes develop light colours when stored for longer periods. 

• Boiling points: Alkyl Halides have high boiling points compared with other halides. This is because they have high polarity and strong dipole interaction. The interaction is higher between the molecules of alkyl halides and London forces (van der Waals force).

• Solubility: They have dipole moments from 2.05 to 2.15 D. They are polar. They are unable to create bonds with water molecules and are not very soluble in water, but they are readily soluble in alcohols, benzenes, and others.

•  Density: Different alkyl halides show different nature of densities. For example, alkyl chlorides have a lighter density when compared with water. Alkyl iodides and alkyl bromides have densities comparatively heavier than water. You can see the order for density RI > RBr > RCl. You can notice the change in density that increases with the increase in halogens’ number or atomic mass.

Alkyl halides properties questions 

These are some of the typical questions asked on the topic.

• 2-Chlorobutane has less boiling point than 1-Chlorobutane. Explain why.

• Explain why chloroethane has less boiling point than bromoethane. 

• Chloroform is polar but still not soluble in water. Why?

• Which of the following has the lowest dipole moment – CH2Cl2, CHCl3, CCl4?

• Why does R-I have the highest density? 

• Arrange the following halogens in the decreasing order of boiling points. Bromobenzene, Chlorobenzene, and Iodobenzene. 

Conclusion 

Haloalkanes or alkyl halides are the alkanes that have substituents for halogens. These alkanes are used all over the world widely and commercially. They have many alkyl halide properties, which help produce other materials. Some alkyl halides are toxic and harmful, limiting or restricting their uses and production. Other alkyl halides, such as chlorofluorocarbons, etc., are dangerous as they lead to ozone layer depletion. Many such alkyl halides are used in air conditioners, refrigerators, perfumes or deodorants, etc., which harm our environment and ecosystem. 

Although many compounds and alkanes come under alkyl halides, they may not have the same properties. Every alkyl halide has different boiling points, solubility, density, polarity, state, colour, smell, etc., depending upon its molecular masses and other factors. 

Alkyl halides are very much reactive organic compounds. Alkyl halides have a general formula called  CnH2n+1-X,  where X are halogens such as Cl, Br, I, etc. They have wide classification based on their connectivity with primary, secondary and tertiary carbons.