What are 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.
FAQs on Haloalkanes: Physical and Chemical Properties of Haloalkanes
1. What is the fundamental difference between an alkane and a haloalkane?
The fundamental difference lies in their chemical composition and polarity. An alkane is a saturated hydrocarbon containing only carbon and hydrogen atoms connected by single bonds, making it non-polar. A haloalkane is a derivative of an alkane where one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br, or I). This substitution introduces a polar carbon-halogen (C-X) bond, making the haloalkane molecule polar and significantly altering its physical and chemical properties compared to the parent alkane.
2. What are the main physical properties of haloalkanes as per the Class 12 syllabus?
The key physical properties of haloalkanes for Class 12 students include their boiling point, solubility, and density.
- Boiling Points: Haloalkanes have higher boiling points than alkanes of similar mass due to their polarity, which results in stronger intermolecular forces (dipole-dipole interactions). The boiling point increases with the size of the halogen atom (R-I > R-Br > R-Cl > R-F).
- Solubility: Despite being polar, haloalkanes are only sparingly soluble in water. This is because they cannot form strong hydrogen bonds with water molecules. They are, however, soluble in organic solvents.
- Density: Simple chloroalkanes are generally lighter than water, but bromo-, iodo-, and polychloro-derivatives are denser. The density increases as the atomic mass of the halogen increases.
3. Which are the most important chemical reactions of haloalkanes?
For the CBSE Class 12 curriculum, the most important chemical reactions of haloalkanes are:
- Nucleophilic Substitution Reactions: This is a characteristic reaction where the halogen atom is replaced by a nucleophile. It proceeds via two main mechanisms: SN1 (unimolecular) and SN2 (bimolecular).
- Elimination Reactions: Also known as dehydrohalogenation, this reaction involves the removal of a hydrogen atom from the β-carbon and a halogen atom from the α-carbon to form an alkene, typically in the presence of an alcoholic base like KOH.
- Reaction with Metals: This includes the formation of highly useful Grignard reagents by reacting a haloalkane with magnesium in dry ether, and the Wurtz reaction, where two alkyl halides react with sodium to form a higher alkane.
4. Why do haloalkanes have higher boiling points than their parent alkanes?
Haloalkanes have higher boiling points primarily due to stronger intermolecular forces. Alkanes are non-polar, so their molecules are held together only by weak van der Waals forces. In contrast, the carbon-halogen bond in haloalkanes is polar, creating a molecular dipole. This leads to much stronger dipole-dipole interactions between haloalkane molecules, in addition to the van der Waals forces. More energy is required to overcome these combined forces, resulting in a higher boiling point.
5. If haloalkanes are polar, why are they not readily soluble in water?
This is a common point of confusion. While haloalkanes are polar, their solubility in water is very low. The reason lies in the energy exchange during dissolution. For a haloalkane to dissolve in water, the strong hydrogen bonds between water molecules must be broken, and the dipole-dipole attractions between haloalkane molecules must also be overcome. The new attractions formed between haloalkane and water molecules are not strong enough to release sufficient energy to compensate for breaking these original bonds. Therefore, haloalkanes are not readily soluble in water.
6. How does changing the halogen from Fluorine to Iodine affect the reactivity of a haloalkane?
The identity of the halogen atom significantly affects a haloalkane's reactivity, primarily in nucleophilic substitution reactions. The reactivity is determined by the strength of the carbon-halogen (C-X) bond. As we go down the group from Fluorine to Iodine:
- The atomic size of the halogen increases.
- The C-X bond length increases.
- The C-X bond strength decreases.
Therefore, the C-I bond is the weakest and easiest to break, making iodoalkanes the most reactive. The order of reactivity is R-I > R-Br > R-Cl > R-F.
7. Why are haloarenes much less reactive than haloalkanes towards nucleophilic substitution?
Haloarenes are significantly less reactive than haloalkanes due to several factors:
- Resonance Effect: The lone pair of electrons on the halogen atom participates in resonance with the benzene ring. This gives the C-X bond a partial double-bond character, making it stronger and harder to break.
- Difference in Hybridisation: The carbon atom in the C-X bond of a haloarene is sp² hybridised, which is more electronegative than the sp³ hybridised carbon in a haloalkane. This holds the C-X bond more tightly.
- Instability of Phenyl Cation: In an SN1-type reaction, the resulting phenyl cation would be highly unstable and does not form readily.
- Electronic Repulsion: The electron-rich benzene ring tends to repel the incoming electron-rich nucleophile.
8. What are some key examples of haloalkanes and their real-world applications?
Haloalkanes have several important real-world applications. Some key examples include:
- Dichloromethane (CH₂Cl₂): Widely used as an industrial solvent and a paint stripper.
- Trichloromethane (Chloroform, CHCl₃): Formerly used as an anaesthetic, it is now primarily used as a solvent for fats, waxes, and resins.
- Iodoform (CHI₃): Was used as an antiseptic due to the liberation of free iodine, although its use has declined.
- Freons (e.g., CCl₂F₂): These are chlorofluorocarbons (CFCs) that were extensively used as refrigerants and propellants in aerosols before being phased out due to their role in ozone layer depletion.
