Haloalkanes and Haloarenes Class 12

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Haloalkanes and haloarenes are organic compounds in which one or more hydrogen atoms of an alkane or arene are replaced by halogen atoms. Haloalkanes are also known as alkyl halides, while haloarenes are also known as aryl halides.

Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes can be classified in a number of ways, including:

• By the type of halogen atom: Haloalkanes and haloarenes can be classified as fluoro-, chloro-, bromo-, or iodo- compounds, depending on the type of halogen atom present.
• By the number of halogen atoms: Haloalkanes and haloarenes can be classified as monohalo-, dihalo-, trihalo-, or polyhalo- compounds, depending on the number of halogen atoms present.
• By the nature of the carbon atom to which the halogen atom is attached: Haloalkanes can be classified as primary, secondary, or tertiary compounds, depending on the nature of the carbon atom to which the halogen atom is attached. In primary haloalkanes, the halogen atom is attached to a primary carbon atom (a carbon atom bonded to only one other carbon atom). In secondary haloalkanes, the halogen atom is attached to a secondary carbon atom (a carbon atom bonded to two other carbon atoms). In tertiary haloalkanes, the halogen atom is attached to a tertiary carbon atom (a carbon atom bonded to three other carbon atoms).

Haloalkanes and Haloarenes

Haloalkanes and Haloarenes Preparation 

Haloalkanes and haloarenes can be prepared by a variety of methods, including:

• From alkanes: Haloalkanes can be prepared from alkanes by halogenation, which is the process of reacting an alkane with a halogen atom. Halogenation can be carried out using a variety of methods, including free radical halogenation, electrophilic halogenation, and photochemical halogenation.
• From alkenes: Haloalkanes can also be prepared from alkenes by addition of halogens. The addition of halogens to alkenes is a syn-addition reaction, meaning that the halogen atoms add to the same side of the carbon-carbon double bond.
• From alcohols: Haloalkanes can be prepared from alcohols by dehydration or by reaction with a halogen acid.
• From alkyl halides: Haloalkanes can be converted to other haloalkanes by nucleophilic substitution reactions.
• From arenes: Haloarenes can be prepared from arenes by electrophilic aromatic substitution reactions.

Properties of Haloalkanes and Haloarenes

Haloalkanes and haloarenes have a number of physical and chemical properties that are characteristic of these classes of compounds.

• Physical properties: Haloalkanes and haloarenes are generally colorless liquids or solids. They are insoluble in water but soluble in organic solvents. The boiling point of haloalkanes and haloarenes increases with increasing molecular mass.
• Chemical properties: Haloalkanes and haloarenes are reactive compounds that can undergo a variety of chemical reactions, including nucleophilic substitution reactions, elimination reactions, and addition reactions.

Uses of Haloalkanes and Haloarenes

Haloalkanes and haloarenes have a wide range of uses in industry and in everyday life. Some of the most common uses of haloalkanes and haloarenes include:

• Solvents: Haloalkanes and haloarenes are used as solvents for a variety of organic substances.
• Refrigerants: Haloalkanes, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), were once widely used as refrigerants. However, the use of CFCs and HCFCs is now being phased out due to their environmental impact.
• Fire extinguishers: Haloalkanes, such as methyl bromide and Halon, are used as fire extinguishers because they can effectively extinguish fires caused by flammable liquids and gases.
• Pesticides: Haloalkanes, such as methyl bromide and chloropicrin, are used as pesticides to control weeds, insects, and rodents.
• Pharmaceuticals: Haloalkanes and haloarenes are used to synthesize a variety of pharmaceuticals, such as antibiotics, anesthetics, and pain relievers.

Haloalkanes and Haloarenes Class 12 Notes

Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are classified according to the type of halogen atom present and the number of halogen atoms present in the molecule.

According to the type of halogen atom present:

Fluoroalkanes (CXF)
Chloroalkanes (CXCl)
Bromoalkanes (CXBr)
Iodoalkanes (CXI)
According to the number of halogen atoms present:

Monohaloalkanes (one halogen atom)
Dihaloalkanes (two halogen atoms)
Trihaloalkanes (three halogen atoms)
Tetrahaloalkanes (four halogen atoms)
Nomenclature

Haloalkanes and haloarenes are named according to the IUPAC system of nomenclature.

The following steps are involved:

Identify the longest carbon chain containing the halogen atom. This is the parent chain.

Number the carbon atoms in the parent chain starting from the end that is closest to the halogen atom.

Name the halogen atom as a prefix.

Name the parent chain as an alkane or arene, depending on the type of hydrocarbon.

If there are multiple halogen atoms, name them in alphabetical order and indicate their position on the carbon chain with numbers.

Examples: (Haloalkanes and Haloarenes)

 

CH3Cl: Chloromethane
CH3CH2Cl: Chloroethane
C6H5Cl: Chlorobenzene
CH3CH2CH2Br: 1-Bromopropane
CH3CBrCH3: 2-Bromopropane
C6H4Br2: 1,2-Dibromobenzene
Nature of C-X Bond

The C-X bond in haloalkanes and haloarenes is a polar covalent bond. The halogen atom is more electronegative than the carbon atom, so it attracts the shared pair of electrons more strongly. This results in a partial negative charge on the halogen atom and a partial positive charge on the carbon atom.

Methods of Preparation

Haloalkanes and haloarenes can be prepared by a variety of methods, including:

Direct halogenation of alkanes and arenes: This is the most common method of preparing haloalkanes and haloarenes. Alkanes and arenes can be halogenated with chlorine, bromine, and iodine in the presence of a catalyst.
Addition of hydrogen halides to alkenes and alkynes: Hydrogen halides can add to alkenes and alkynes in the presence of a catalyst to form haloalkanes.

From alcohols: Alcohols can be converted to haloalkanes by reacting them with hydrogen halides or phosphorus halides.
From Grignard reagents: Grignard reagents can be reacted with haloalkanes to form new haloalkanes.

Physical Properties

Haloalkanes and haloarenes are generally colorless liquids or solids. They have low boiling points and are insoluble in water. However, they are soluble in organic solvents.

Chemical Reactions

Haloalkanes and haloarenes undergo a variety of chemical reactions, including:

Nucleophilic substitution reactions: Haloalkanes and haloarenes can undergo nucleophilic substitution reactions with a variety of nucleophiles, such as hydroxide ions, alkoxide ions, and cyanide ions.
Elimination reactions: Haloalkanes and haloarenes can undergo elimination reactions in the presence of a strong base to form alkenes and alkynes.

Addition reactions: Haloalkanes and haloarenes can undergo addition reactions with a variety of reagents, such as hydrogen, ammonia, and Grignard reagents.

Polyhalogen Compounds

Polyhalogen compounds are haloalkanes and haloarenes that contain more than one halogen atom. They are generally more reactive than monohalogen compounds.

Important Uses of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are used in a wide variety of industrial and commercial applications, including:

Solvents: Haloalkanes and haloarenes are excellent solvents for a variety of substances, such as paints, varnishes, and adhesives.

Refrigerants: Haloalkanes, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are used as refrigerants in refrigerators and air conditioners.

However, CFCs and HCFCs are being phased out due to their harmful effects on the ozone layer.

Fire extinguishers: Haloalkanes, such as carbon tetrachloride and bromomethane, are used as fire extinguishers.

Pesticides: Haloalkanes are also used as pesticides.

Pharmaceuticals: Haloalkanes and haloarenes are used in the manufacture of a variety of pharmaceuticals, such as antibiotics.

Important questions and answers related to haloalkanes and haloarenes for a class 12 chemistry exam:

1. What are haloalkanes and haloarenes?

• Haloalkanes are organic compounds containing a carbon-halogen (C-X) bond, where X is a halogen atom (F, Cl, Br, or I).
• Haloarenes are aromatic compounds with one or more halogen substituents (F, Cl, Br, or I) attached to a benzene ring.

2. Explain the nomenclature of haloalkanes.

• Haloalkanes are named as alkyl halides, where the halogen substituent is considered a functional group. The halogen is named as a prefix (fluoro-, chloro-, bromo-, iodo-) followed by the alkane name.

3. What is the order of reactivity of halogens in haloalkanes towards nucleophilic substitution?

• The reactivity order is F > Cl > Br > I. Fluorine is the most reactive halogen due to its small size and high electronegativity.

4. Describe SN1 and SN2 reactions in haloalkanes.

• SN1 is a unimolecular nucleophilic substitution reaction where the rate-determining step involves the formation of a carbocation intermediate.
• SN2 is a bimolecular nucleophilic substitution reaction where the nucleophile directly displaces the leaving group in a single step.

5. Explain the stability of carbocations in haloalkanes.

• Carbocations are stabilized by adjacent alkyl groups through hyperconjugation, leading to the order: 3° > 2° > 1° > methyl in stability.

6. What is the Wurtz reaction?

• The Wurtz reaction is a method for the synthesis of alkanes from haloalkanes by coupling two alkyl halides in the presence of sodium or lithium metal.

7. How can you distinguish between primary, secondary, and tertiary haloalkanes?

• Primary haloalkanes have one carbon directly bonded to the halogen, secondary have two, and tertiary have three carbons bonded to the halogen.

8. What is the mechanism of electrophilic aromatic substitution in haloarenes?

• Electrophilic aromatic substitution involves the attack of an electrophile on an aromatic ring. The mechanism includes the formation of a sigma complex intermediate.

9. Explain the reactivity of different halogens in haloarenes.

• Similar to haloalkanes, the reactivity order in haloarenes is F > Cl > Br > I due to halogen’s electron-withdrawing effects.

10. How can you prepare haloarenes from benzene? – Haloarenes can be prepared by electrophilic substitution reactions, where a halogenating agent (e.g., Cl2, Br2, FeBr3) replaces a hydrogen atom on the benzene ring.

Haloalkanes and Haloarenes Class 12 Notes

 

Haloalkanes:

1. Introduction:
   • Haloalkanes are organic compounds containing a carbon-halogen (C-X) bond, where X is a halogen (F, Cl, Br, or I).
2. Nomenclature:
   • Named as alkyl halides, where the halogen is a substituent.
   • Halogen prefix (fluoro-, chloro-, bromo-, iodo-) + alkane name.
3. Reactivity Order of Halogens:
   • F > Cl > Br > I (Most to least reactive).
   • Due to electronegativity and size factors.
4. Nucleophilic Substitution Reactions:
   • SN1 (Unimolecular): Carbocation intermediate, often in 3° substrates.
   • SN2 (Bimolecular): Direct displacement of leaving group, common in 1° substrates.
5. Carbocation Stability:
   • Stabilized by adjacent alkyl groups: 3° > 2° > 1° > methyl.
6. Preparation Methods:
   • From alcohols by reaction with HX (hydrogen halides).
   • By replacing active hydrogen in alkanes via free radical halogenation.
7. Wurtz Reaction:
   • Converts alkyl halides to higher alkanes by coupling using sodium or lithium metal.

Haloarenes:

1. Introduction:
   • Haloarenes are aromatic compounds with halogen substituents on a benzene ring.
2. Nomenclature:
   • Halogen prefix (fluoro-, chloro-, bromo-, iodo-) + benzene name.
3. Reactivity of Halogens:
   • Similar to haloalkanes, reactivity order is F > Cl > Br > I.
4. Electrophilic Aromatic Substitution:
   • Mechanism where an electrophile replaces a hydrogen atom on the aromatic ring.
   • Requires a Lewis acid catalyst (e.g., FeBr3, AlCl3).
5. Preparation of Haloarenes:
   • Direct halogenation of benzene using halogen (Cl2, Br2) and a catalyst.
   • From diazonium salts (Sandmeyer reaction).

Haloalkanes and Haloarenes Important Questions

Some important questions and answers on haloalkanes and haloarenes for class 12:

Q. What is the difference between haloalkanes and haloarenes?

A. Haloalkanes are hydrocarbons in which one or more hydrogen atoms have been replaced by halogen atoms. Haloarenes are hydrocarbons in which one or more hydrogen atoms have been replaced by halogen atoms on an aromatic ring.

Q. Why are haloalkanes more reactive than haloarenes?

A. Haloalkanes are more reactive than haloarenes because the carbon atom bonded to the halogen atom is sp3 hybridized in haloalkanes, while it is sp2 hybridized in haloarenes. This results in a more polar C-X bond in haloalkanes, which makes them more susceptible to nucleophilic attack.

Q. What is the order of reactivity of haloalkanes in nucleophilic substitution reactions?

A. The order of reactivity of haloalkanes in nucleophilic substitution reactions is:

RI > RBr > RCl > RF

This is because the iodide ion is a better leaving group than the bromide ion, which is a better leaving group than the chloride ion, and so on.

Q. What is the difference between SN1 and SN2 reactions?

A. SN1 reactions are two-step reactions in which the first step is the formation of a carbocation intermediate. SN2 reactions are one-step reactions in which the nucleophile attacks the carbon atom bonded to the halogen atom simultaneously with the departure of the leaving group.

Q. Which type of haloalkane is more likely to undergo an SN1 reaction?

A. 3° haloalkanes are more likely to undergo SN1 reactions than 1° or 2° haloalkanes because the 3° carbocation intermediate is more stable.

Q. Which type of haloalkane is more likely to undergo an SN2 reaction?

A. 1° haloalkanes are more likely to undergo SN2 reactions than 2° or 3° haloalkanes because the steric hindrance is less.

Q. What are some important uses of haloalkanes and haloarenes?

A. Haloalkanes and haloarenes are used in a wide variety of applications, including:

Solvents
Solvents
Refrigerants
Insecticides
Plastics
Pharmaceuticals
Dyes
Explosives
Here are some additional important questions that may be asked in class 12 exams:

Q. What is the Williamson synthesis?

A. The Williamson synthesis is a method for preparing ethers by reacting a sodium alkoxide with a haloalkane.

Q. What is the Reimer-Tiemann reaction?

A. The Reimer-Tiemann reaction is a method for preparing salicylaldehyde by reacting a phenol with chloroform in the presence of sodium hydroxide.

Q. What is the Wurtz reaction?

A. The Wurtz reaction is a method for preparing alkanes by reacting two alkyl halides with sodium metal in dry ether.

Q. What is the Friedel-Crafts reaction?

A. The Friedel-Crafts reaction is a method for preparing alkyl and acyl derivatives of benzene by reacting benzene with an alkyl halide or an acyl halide in the presence of a strong Lewis acid catalyst.

Q. What is the Kolbe reaction?

A. The Kolbe reaction is a method for preparing salicylic acid from sodium phenoxide by reacting it with carbon dioxide in the presence of sodium hydroxide.

These are just a few examples of important questions and answers on haloalkanes and haloarenes for class 12. It is important to have a good understanding of all the concepts covered in this chapter in order to do well in exams.

Haloalkanes and Haloarenes Class 12 NCERT Solutions

important NCERT questions and answers on haloalkanes and haloarenes for Class 12:

Question 1: What are the different types of haloalkanes?

Answer: Haloalkanes can be classified into three types:

• Primary haloalkanes: Haloalkanes in which the halogen atom is attached to a primary carbon atom are called primary haloalkanes.
• Secondary haloalkanes: Haloalkanes in which the halogen atom is attached to a secondary carbon atom are called secondary haloalkanes.
• Tertiary haloalkanes: Haloalkanes in which the halogen atom is attached to a tertiary carbon atom are called tertiary haloalkanes.

Question 2: What is the order of reactivity of haloalkanes in SN1 reactions?

Answer: The order of reactivity of haloalkanes in SN1 reactions is tertiary > secondary > primary. This is because the formation of the carbocation intermediate is more favorable in the case of tertiary haloalkanes.

Question 3: What is the order of reactivity of haloalkanes in SN2 reactions?

Answer: The order of reactivity of haloalkanes in SN2 reactions is primary > secondary > tertiary. This is because the steric hindrance is less in the case of primary haloalkanes.

Question 4: What are the different methods of preparing haloalkanes?

Answer: Haloalkanes can be prepared by the following methods:

• From alkanes: Haloalkanes can be prepared from alkanes by halogenation reaction. In this reaction, an alkane is treated with a halogen (such as Cl2, Br2, or I2) in the presence of sunlight or a catalyst.
• From alcohols: Haloalkanes can be prepared from alcohols by dehydration reaction. In this reaction, an alcohol is treated with an acid catalyst (such as H2SO4 or HCl) to form a haloalkane and water.
• From alkenes: Haloalkanes can be prepared from alkenes by addition reaction. In this reaction, an alkene is treated with a halogen acid (such as HCl or HBr) to form a haloalkane.

Question 5: What are the different types of haloarenes?

Answer: Haloarenes are aromatic compounds in which the halogen atom is attached directly to the carbon atom of the aromatic ring. Haloarenes can be classified into two types:

• Primary haloarenes: Haloarenes in which the halogen atom is attached to a primary carbon atom of the aromatic ring are called primary haloarenes.
• Secondary haloarenes: Haloarenes in which the halogen atom is attached to a secondary carbon atom of the aromatic ring are called secondary haloarenes.

Question 6: What are the different methods of preparing haloarenes?

Answer: Haloarenes can be prepared by the following methods:

• From hydrocarbons: Haloarenes can be prepared from hydrocarbons by direct halogenation reaction. In this reaction, a hydrocarbon is treated with a halogen (such as Cl2 or Br2) in the presence of a catalyst (such as Fe or AlCl3).
• From haloalkanes: Haloarenes can be prepared from haloalkanes by the Friedel-Crafts reaction. In this reaction, a haloalkane is treated with an aromatic hydrocarbon in the presence of a Lewis acid catalyst (such as AlCl3) to form a haloarene.

Question 7: What are the different types of reactions undergone by haloalkanes and haloarenes?

Answer: Haloalkanes and haloarenes undergo the following types of reactions:

• Substitution reactions: In substitution reactions, a halogen atom is replaced by another atom or group.
• Elimination reactions: In elimination reactions, a halogen atom and a hydrogen atom are removed from adjacent carbon atoms to form an alkene.
• Addition reactions: In addition reactions, a halogen atom adds to a multiple bond to form a saturated compound.

Question 8: What are the important uses of haloalkanes and haloarenes?

Answer: Haloalkanes and haloarenes are widely used in a variety of applications, including:

• Solvents: Haloalkanes, such as chloroform and carbon tetrachloride, are used as solvents for cleaning and degreasing.
• Fuels: Haloalkanes, such as methyl bromide and chlorodifluoromethane, are used as fuels in refrigeration and air conditioning systems.
• Pesticides: Haloalkanes, such as DDT and BHC, are used as pesticides.
• Pharmaceuticals: Haloalkanes, such as halothane and methoxyflurane, are used as anesthetics.
• Plastics: Haloarenes, such as PVC and polystyrene, are used in the manufacture of plastics.

Notes of Haloalkanes and Haloarenes Class 12 PDF Download

Download PDF Notes of Haloalkanes and Haloarenes

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Conclusion:

Haloalkanes and haloarenes are important classes of organic compounds with a wide range of uses. They are essential components of many products that we use in our everyday lives. However, it is important to note that haloalkanes and haloarenes can also be toxic and environmentally harmful. Therefore, it is important to use them safely and responsibly.

are important classes of organic compounds with a wide range of uses. They are essential components of many products that we use in our everyday lives. However, it is important to note that haloalkanes and haloarenes can also be toxic and environmentally harmful. Therefore, it is important to use them safely and responsibly.