Haloalkanes: Physical and Chemical Properties of Haloalkanes

Haloalkanes are hydrocarbons consisting of aliphatic alkanes with one or more hydrogen atoms replaced by halogens. In haloalkanes, the halogen atom gets attached to the sp3 hybridized carbon atom of the alkyl group. A few examples of haloalkanes are CH3Cl – Methyl Chloride, CH3CH2Br – Ethyl bromide, etc.

Haloalkanes and haloarenes are very useful organic compounds. These are used as solvents, propellants, and for many other industrial purposes. Haloalkanes and haloarenes are halogen derivatives of alkanes and arenes. These are also known as alkyl halides and aryl halides respectively. 

Here, we will discuss the physical and chemical properties of haloalkanes and haloarenes. Let us first start with a brief introduction of what haloarenes are. 

What are Haloarenes? 

Haloarenes are also hydrocarbons that consist of aromatic rings with one or more hydrogen atoms replaced by halogens. In haloarenes, the halogen atom gets attached to the sp3 hybridized carbon atom of the alkyl group. A few examples of haloarenes are given below – 

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What are the Physical Properties of Haloalkanes?

Haloalkanes possess the following physical properties –

• Haloalkanes are generally colourless and odourless compounds. 

• They are hydrophobic in nature. 

• Boiling point – The boiling point of haloalkanes is higher than alkanes if the number of carbon atoms is the same in both. The boiling point of haloalkanes also increases with the increasing number of halogens in haloalkanes. It means 1-Bromo-2-chloroethane will have a higher boiling point than chloroethane. Boiling points of haloalkanes scale with the atomic weight of halides. Although fluoroalkanes are exceptions. They show a lower boiling point than their analogous alkanes. It is due to the lower polarizability of fluorine. The boiling point of 2- methylpropane is -11.7℃ while 2- fluoropropane is -10℃. As the branching increases, boiling points of isomeric haloalkanes decrease. For example, the boiling point of 1-bromobutane is 375K while 2-bromopropane is 346K. 

CH3CH2CH2CH2Br   

B.P. =375K                   

• Melting points – Melting points of haloalkanes are higher than their analogous alkanes. Haloalkanes will have a higher melting point than those alkanes which have the same number of carbon atoms as haloalkanes. Although again fluoroalkanes are exceptions. They show a lower melting point than their analogous alkanes. For example, the melting point of methane is -182.5 ℃ while the melting point of tetrafluoromethane is -183.6 ℃.

• Haloalkanes are flammable but less flammable than alkanes. Because haloalkanes have fewer C-H bonds than alkanes. 

• Haloalkanes are polar in nature so they act as solvents. They are better solvents than alkanes. 

• Haloalkanes are more reactive than corresponding alkanes, although fluoroalkanes are exceptions. Reactivity increases with the increasing atomic weight of halogens. 

• Generally, Bromo, iodo, and polyhaloalkanes are heavier than water. The density of haloalkanes increases with an increase in the number of halogen atoms, carbon atoms, and atomic weight of halogen atoms. 

• Haloalkanes are slightly soluble in water. 

What are the Chemical Properties of Haloalkanes? 

Haloalkanes show the following chemical properties –

• Carbon attached to halogen in haloalkanes is generally electron-deficient that’s why haloalkanes are more reactive towards nucleophiles. For example, the reaction of tert-butyl bromide with hydroxide ion (a nucleophile). The reaction is given below –

(CH3)3CBr + OH- (CH3)3COH + Br-

The above reaction takes place by unimolecular nucleophilic substitution (SN1) reaction mechanism. 

• Haloalkanes show free radical reactions. The formation of Grignard reagent takes place by haloalkanes and Mg through a free radical reaction mechanism. The reaction mechanism is given below –

R−X + Mg → R−X•− + Mg•+

R−X•− → R• + X−

R• + Mg•+ → RMg+

RMg+ + X− → RMgX

Where R = alkyl group, X = halogen, RMgX = Grignard reagent 

• Haloalkanes react with Li and form organolithium compounds. General reaction is given below –

RX + 2Li  → RLi + LiX

They undergo oxidative addition reactions and give organometallic compounds. 

• In the Wurtz reaction, haloalkanes undergo coupling and give symmetrical alkanes. The general reaction is given below –

2R – X + 2Na  → R – R + 2NaX

• They undergo elimination reactions. For example, the reaction of bromoethane with NaOH. The reaction equation is given below –

C2H5Br + NaOH  → H2C=CH2 + NaBr + H2O

• Haloalkanes show substitution reactions. The halogen of haloalkane can be substituted by a nucleophile. The reaction is given below –

CH3Cl + OH-  →  CH3OH + Cl-

What are the Physical and Chemical Properties of Haloarenes?

Haloarenes are also known as aryl halides. They show the following physical and chemical properties –

• The most important member of haloarenes is aryl chlorides. Aryl chlorides are a wide class of haloarenes. They give many derivatives as well. 

• Chlorobenzene is a colourless haloarene. 

• It is liquid at room temperature. 

• It gives a sweet almond-like odour.

• It is insoluble in water and has a higher density than water. 

• Due to the resonance effect, sp2 hybridized carbon attached to halogen, instability of phenyl cation haloarenes are very less reactive towards nucleophiles. So, they rarely give nucleophilic substitution reactions.

• Haloarenes undergo electrophilic substitution reactions. For example, haloarenes undergo halogenation. Halogenation reaction of chlorobenzene is given below

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• Haloarenes undergo nitration. For example, when chlorobenzene reacts with nitric acid in presence of conc. sulfuric acid, gives 1-chloro-2-nitrobenzene and 1-chloro-4-nitrobenzene. The reaction is given below –

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• Haloarenes undergo sulfonation. For example, on heating chlorobenzene reacts with conc. Sulfuric acid gives 2-chlorobenzenesulfonic acid and 4- chlorobenzenesulfonic acid. Out of these products 4- chlorobenzenesulfonic acid is a major product. The reaction is given below –

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• Haloarenes give Freidel – Crafts reaction. The reaction is given below –

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• Haloarenes undergo Wurtz – Fittig reaction. The reaction of an alkyl halide with aryl halide and sodium metal, in presence of dry ether to form a substituted aromatic compound by the formation of a new carbon-carbon bond, is called Wurtz – Fittig reaction. The reaction is given below -

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• They undergo the Fittig reaction as well. In the Fittig reaction, two aryl halides react with sodium metal in presence of dry ether and form diphenyl. The reaction is given below –

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Conclusion

In everyday life, we use hydrocarbons a lot. These haloalkanes are very essential for the existence of human beings as well as for all living beings because they are used in medicines.