Haloalkanes and Haloarenes - Notes, Topics, Formula, Books, FAQs

Haloalkanes and Haloarenes - Notes, Topics, Formula, Books, FAQs

Edited By Team Careers360 | Updated on Jul 10, 2025 01:56 PM IST

From refrigerants in air conditioners to medicines, Haloalkanes and Haloarenes are everywhere around us. These words are made up of two words: the first is ‘Halo’, which means Halogens, and the second word is ‘alkanes’ or ‘arenes’, which means aliphatic or aromatic hydrocarbons. These are organic compounds containing Halogens (Fluorine, Chlorine, Bromine, Iodine). These compounds play a crucial role in pharmaceuticals and industries. In these compounds, one or more Hydrogen atoms are replaced by Halogens. Many organic compounds containing halogens occur naturally and have clinical significance. Some of the widely used Haloalkanes are Chloroform (CHCl3) and Carbon tetrachloride (CCl4), and Haloarenes are DDT, Bromobenzene, etc.

This Story also Contains
  1. Important Topics - Organic compounds Containing Halogens
  2. Overview of the Chapter
  3. Haloalkanes and haloarenes can be classified mainly on the basis of two aspects:
  4. Physical properties
  5. Chemical Reactions
  6. Applications of Haloalkanes and Haloarenes
  7. Importanat PYQS
  8. How to Prepare for Hydrocarbons
  9. Prescribed Books
Haloalkanes and Haloarenes - Notes, Topics, Formula, Books, FAQs
Haloalkanes and Haloarenes - Notes, Topics, Formula, Books, FAQs

Important Topics - Organic compounds Containing Halogens

Alkyl Halides

Alkyl halides are organic compounds containing a halogen atom (F, Cl, Br, or I) bonded to an alkyl group. Alkyl halides are classified as primary, secondary, or tertiary based on the carbon atom attached to the halogen. Alkyl halides are versatile intermediates in organic synthesis, used in the production of polymers, refrigerants, and agrochemicals.

Physical & Chemical Properties of Haloalkanes

Haloalkanes exhibit unique physical properties like higher boiling points compared to alkanes due to the polar C-X bond and their larger molecular mass. Chemically, they undergo nucleophilic substitution (SN1 and SN2 mechanisms) and elimination reactions. The reactivity depends on the halogen and the carbon skeleton.

Haloarenes

Haloarenes are aromatic compounds where one or more hydrogen atoms in an aromatic ring are replaced by halogen atoms. They are less reactive than haloalkanes due to resonance in the benzene ring, which delocalizes the electrons and stabilizes the C-X bond. Haloarenes find applications in pesticides, dyes, and pharmaceuticals.

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Markovnikov Rule

Markovnikov's Rule predicts the outcome of electrophilic addition reactions to alkenes. It states that when a protic acid (HX) adds to an asymmetric alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halide (or other substituent) attaches to the carbon with fewer hydrogen atoms. This rule is explained by the stability of the carbocation intermediate, which favors the more substituted and stable carbocation.



Overview of the Chapter

Haloalkanes and haloarenes can be classified mainly on the basis of two aspects:

  • On the basis of the number of halogen atoms
    Halogens may be classified as mono, di or poly halogen compounds depending on the number of halogen compounds attached to the carbon compound.

    Halogens Halogens
  • Halogen attached to sp3 or sp2 carbon
    (a) Alkyl Halide: In this class, the halogen atom is bonded to the sp3 hybridized carbon. These can further be classified into three categories as follows:

    Alkyl Halide
    (b) Allylic Halides: In this class, the halogen atom is attached to the sp3 hybridised carbon which is attached to the carbon-carbon double bond.
    Alkyl Halide

(c) Benzylic halides: These are the halides in which the halogen atom is attached to sp3 hybridised carbon which is attached to an aromatic ring.
Benzylic halides


(d) Vinylic Halides: In this type of halide, the halogen atom is attached to sp2 carbon.
Vinylic Halides
(e) Aryl Halides: In this type of halide, the halogen atom is sp2 carbon atom of an aromatic ring.
Aryl Halides

Physical properties

  • Melting and boiling point: The melting and boiling point of haloalkanes and haloarenes are determined by the intermolecular forces of attraction between them. The boiling point of alkyl halides follow this order:
    R-I > R-Br > R-Cl > R-F
    The boiling point is always determined by vander Waal forces. As the molecular size increases, the boiling point increases. For example, in the case given below:

    boiling point
    As branching increases in alkyl halides, then the boiling point decreases.
  • Density: The density increases as the size of the haloalkanes and haloarenes increases. It also increases with the increase of the atomic mass of halogen.

Preparation of Alkyl halides

  • Alkyl halides are prepared in many ways. The most common method to prepare alkyl halides is from alcohols. In this method, the alcohols are reacted with halogen acids, thionyl chloride or phosphorus halides, then the hydroxyl group is replaced with a halogen group.
    alkyns
  • From alkene: Alkyl halide can be prepared from alkenes by reaction with hydrogen halides or Br2/hv.
    brb
    There are other important methods are also available for the preparation of alkyl halides such as, by free radical halogenation and Sandmeyer's reaction.

Chemical Reactions

  • Reaction with NaCN, AgCN, NaNO2, AgNO2: In this reaction, the halide group is replaced by CN or NO2. If alkyl halide is reacting with NaNO2 then ONO replaces the halide group but if AgNO2 is reacting then NO2 replaces the halide group.
    crv
  • Finkelstein Reaction: This reaction is used for the preparation of alkyl halide from another alkyl halide. In this reaction, alkyl halide is treated with metal halide salt in the presence of acetone.
    23
  • Substitution Reactions:
    Alkyl halides undergo substitution reactions either via SN1 mechanism or SN2 mechanism
    (a) SN1 reaction: This mechanism is favourable when the tertiary alkyl halide is present. This mechanism occurs in two steps viz, breaking of halide bond and then the formation of the carbocation.

    24
    (b) SN2 reaction: This reaction is favorable when the primary alkyl halide is present. In this mechanism, the breaking of the halide bond and the formation of the carbon-OH bond both occur at the same time. Since this reaction mechanism is bimolecular thus it is known as the SN2 mechanism.
    25
  • Elimination reaction:
    This reaction occurs when the alkyl halide having β-hydrogen atom is reacting with alcoholic potassium hydroxide. An example of this reaction mechanism is given below. When there is a possibility of formation of more than one alkene, then the most stable alkene is the major product.

    26
  • Friedel-Crafts reactions:
    These are those reactions in which an aromatic halide is treated with alkyl halide or acyl halide to attach substituents. These reactions are of two types i.e, alkylation and acylation. Both these reactions follow the electrophilic aromatic substitution mechanism. In both these reactions, the most stable product is the major product.
    (a) Friedel-Crafts alkylation: In this reaction, an aromatic halide reacts with an alkyl halide and forms the product as given below in the reaction.

    27
    (b) Friedel-Crafts acylation: In this reaction, the aromatic halide reacts with an acyl group and forms the product as given below.
    28

Applications of Haloalkanes and Haloarenes

There are some important applications of haloalkanes and haloarenes viz:

  • Some haloalkanes are used as synthetic intermediate compounds in labs.
  • They are used in fire extinguishers.
    fire extinguishers
  • The derivatives of these compounds are used for medicinal purposes.
  • Some naturally occurring haloalkanes act as protective agents against ocean attackers.
  • They are used as propellants.

Importanat PYQS

Question: Given below are two statements :

Statement (I) : SN2 reactions are 'stereospecific', indicating that they result in the formation only one stereo-isomers as the product.

Statement (II) : SN1 reactions generally result in formation of product as racemic mixtures. In the light of the above statements, choose the correct answer from the options given below :

(1) Statement I is true but Statement II is false

(2) Statement I is false but Statement II is true

(3) Both Statement I and Statement II is true

(4) Both Statement I and Statement II is false

Answer:

SN2→ Inversion

SN1→ Racemisation

Hence, the answer is the option (3).


Question: Given below are two statements :
Statement (I) : Alcohols are formed when alkyl chlorides are treated with aqueous potassium hydroxide by elimination reaction.

Statement (II) : In alcoholic potassium hydroxide, alkyl chlorides form alkenes by abstracting the hydrogen from the β-carbon.

In the light of the above statements, choose the most appropriate answer from the options given below :

(1) Both Statement I and Statement II are incorrect

(2) Statement I is incorrect but Statement II is correct

(3) Statement I is correct but Statement II is incorrect

(4) Both Statement I and Statement II are correct

Answer:

Statement (I) :

R−Cl→(aq,KOH)R−OH this reaction is nucleophilic substitution (SN2 or SN1), not elimination.

Statement (II) :

Screenshot%20(66)

Hence, the correct answer is option (2).

Practice more questions from the link given below

For more questions to practice, the following MCQs will help in the preparation for competitive examinations

How to Prepare for Hydrocarbons

  • This chapter is part of the organic chemistry and possesses a very high weightage of marks in board exams and other competitive exams like JEE and NEET. It is completely theory-based. You are not supposed to memorize any formula, but the reaction mechanisms and named reactions are very important.
  • For preparing this chapter, firstly, you must have a basic understanding of the chapter "Organic Chemistry- Some Basic Principles and Techniques". After this, you must read this chapter, "Haloalkanes and Haloarenes" from NCERT thoroughly, with every concept grasped completely on your own.
  • Practice lots of examples related to this chapter because then you will be able to understand the most basic concepts related to the whole of organic chemistry.

Prescribed Books

For this chapter, first, the NCERT book is best for initial level preparation as well as for board exams. Now, after this, if you want to prepare for competitive exams like JEE and NEET, then these are the best books for you: Morrison and Boyd, and R.K. Gupta by Arihant Publication. Meanwhile, in the preparation, you must continuously give mock tests for the depth of knowledge. Our platform will help you to provide a variety of questions for deeper knowledge with the help of videos, articles, and mock tests.

Also read,


Frequently Asked Questions (FAQs)

1. What are haloalkanes?

Haloalkanes, also known as alkyl halides, are organic compounds that contain at least one halogen atom bonded to an alkane carbon atom. They can be classified into primary, secondary, and tertiary haloalkanes based on the degree of substitution of the carbon atom attached to the halogen.

2. How do haloarenes differ from haloalkanes?

Haloarenes are compounds where the halogen atom is attached to an aromatic ring, such as benzene. In contrast, haloalkanes have the halogen attached to a saturated carbon chain. This structural difference can lead to distinct physical and chemical properties, including reactivity and boiling points.

3. What are some common uses of haloalkanes?

Haloalkanes are widely used in the chemical industry and household applications. They can serve as solvents, refrigerants, and intermediate compounds in the synthesis of pharmaceuticals, agrochemicals, and other organic compounds. Their ability to interact with various functional groups makes them versatile in chemical reactions.

4. Why are haloalkanes considered to be environmentally hazardous?

Many haloalkanes are toxic and can contribute to environmental issues, especially when they are volatile or persist in the environment. Some members of this class, such as chlorofluorocarbons (CFCs), can deplete the ozone layer, while others may be harmful to aquatic life or human health if released into the environment.

5. What safety precautions should be taken when handling haloalkanes?

When handling haloalkanes, it's important to work in a well-ventilated area or under a fume hood to avoid inhalation of fumes. Personal protective equipment, such as gloves and goggles, should also be worn to prevent skin and eye exposure, as many haloalkanes can be irritants or harmful.

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