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

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

Important Topics - Organic compounds Containing Halogens

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.
  
• 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:
  
  (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.
  

(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.

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

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

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:
  
  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.
  
• From alkene: Alkyl halide can be prepared from alkenes by reaction with hydrogen halides or Br₂/hv.
  
  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.
  
• 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.
  
• 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.
  
  (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.
  
• 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.
  
• 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.
  
  (b) Friedel-Crafts acylation: In this reaction, the aromatic halide reacts with an acyl group and forms the product as given below.
  

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.
  
• 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) :

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.

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