NCERT Class 12 Chemistry Chapter 10 Notes Haloalkanes and Haloarenes - Download PDF

Haloalkanes and Haloarenes are an important part of our daily lives, they are everywhere from refrigerants in air conditioners to medicines. Haloalkanes and Haloarenes are organic compounds containing Halogens (Fluorine, Chlorine, Bromine, Iodine). These compounds play a crucial role in pharmaceuticals and industries. Haloalkanes and Haloarenes 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. So, Basically in these compounds, one or more Hydrogen atoms of Hydrocarbons are replaced by Halogens. Many organic compounds containing Halogen occur naturally and have clinical significance. Class 12 Chemistry Chapter 6 notes are prepared by subject experts in the easiest way.

Some of the widely used Haloalkanes are Chloroform (CHCl₃ ) and Carbon tetrachloride (CCl₄ ) and Haloarenes are DDT, Bromobenzene, etc. Haloalkanes And Haloarenes play a vital role in the Class 12 Chemistry curriculum. Chapter 6 starts with the basics and gradually moves to complex topics. NCERT notes have been created according to the latest CBSE curriculum.

Haloalkanes and haloarenes Class 12 notes cover a brief outline of topics such as preparation methods for haloalkanes and haloarenes, Chemical and Physical properties of haloalkanes, and haloarenes. On this page we provide a structured overview of the syllabus, guiding students through the essential topics and highlighting crucial points for effective revision.

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Topics of Chapter 6 Class 12 Chemistry

NCERT Class 12 Haloalkanes and Haloarenes Notes

1. Nomenclature

a)For haloalkanes-

Common name- alkyl halides

Example- methyl chloride, ethyl bromide, n-propyl iodide

IUPAC Name- haloalkane

Example- bromoethane, chloromethane, iodopropane.

b)For haloarenes-

Halo+ name of aromatic hydrocarbon

Example- chlorobenzene, Iodobenzene, 3-chlorotoluene.

2. Methods of Preparation of Haloalkanes

a)From phosphorus halides-

$\mathrm{R}-\mathrm{OH}+{\mathrm{PCl}}_5⟶\mathrm{RCl}+{\mathrm{POCl}}_3+\mathrm{HCl}$

b)From alkanes-

c)From alkenes- Haloalkenes are prepared by addition of halogens with alkenes

• Addition of halogens-

3. Methods of Preparation for Haloarenes

a)By direct halogenation of aromatic ring-

It is an electrophilic substitution reaction.

b)By the use of silver salt of aromatic acids-

This reaction is known as the Hunsdieker reaction

4. Physical Properties of Haloalkanes

• Haloalkanes are colourless and their taste sweet-smelling.

• Since haloalkanes cannot break the hydrogen bonds of water, these are slightly soluble in water.

• As the size of the alkyl group increases, density increases.

• The boiling, as well as melting points of haloalkanes, are higher than their parent hydrocarbon.

• These are not as flammable as hydrocarbons.

• Dipole moment decreases on going from methyl chloride, methyl bromide, and methyl iodide. However, the dipole moment of methyl fluoride is lower as compared to methyl chloride.

5. Physical Properties of Haloarenes

• Haloarenes are colourless in nature and heavier than water.

• Mostly soluble in organic solvents and insoluble in inorganic solvents.

• As the size of the halogen attached to benzene increases, the boiling point also increases.

• Out of the three isomers, the boiling point of the ortho isomer is the highest.

• The dipole moment of the ortho isomer is larger than the meta isomer. Para isomer has zero dipole moment.

6. Chemical Reactions of Haloalkanes

The reactions of haloalkanes may be divided into the following categories:

1. Nucleophilic substitution

2. Elimination reactions

3. Reaction with metals

1. Nucleophilic substitution reaction

a)Substitution by hydroxyl group-

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+{\mathrm{K}}^{+}{\mathrm{OH}}^{-}⟶{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}+\mathrm{KBr}$

b)Substitution by cyano group-

${\mathrm{CH}}_3-\mathrm{I}+{\mathrm{K}}^{+-}\mathrm{CN}⟶{\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{N}+\mathrm{KI}$

c)Substitution by isocyanide group-

d)Substitution by ammonia group-

The following reaction is known as the Hofmann ammonolysis reaction.

Nucleophilic substitution reaction are of two types

1) SN¹

2) SN²

Substitution nucleophilic reaction (SN¹ reaction)

• Rate is given as –

Rate=k[(CH₃Br]

• This reaction occurs in two steps. The first step leads to the formation of a carbocation and the second step leads to the nucleophilic attack on carbocation.

• In SN¹ reaction alkyl halide reacts in the following manner-

• Tertiary alkyl halide> secondary alkyl halide> primary alkyl halide> primary alkyl halide> methyl halide

Substitution nucleophilic reaction (SN² reaction)

• Rate is given as -

Rate=k[CH₃Br][OH^-]

• This reaction occurs in a single concerted step.

• In SN² reaction alkyl halide reacts in the following manner-

Methyl halide > primary halide> secondary halide> tertiary halide

2. Elimination reaction

• Formation of alkene takes place.

• Also known as dehydrohalogenation reaction.

• The general reaction is given as –

• The reaction proceeds by the removal of hydrogen from a carbon atom situated next to a carbon atom carrying a halogen atom.

Example-

Saytzeff’s rule- This rule decides which alkene will be formed if there are chances of elimination of more than one hydrogen atom. That alkene will be formed in which more substituted alkene is formed.

3. Reaction with active metals- Haloalkanes reacts with metals such as Magnesium, Sodium.

a)Reaction with magnesium-

$\mathrm{RX}+\mathrm{Mg}\stackrel{\text{Dry ether}}{\to }\mathrm{RMgX}$

b)Reaction with sodium-

$\mathrm{RX}+2\mathrm{Na}+\mathrm{XR}\stackrel{\text{dry ether}}{\to }\mathrm{R}-\mathrm{R}+2\mathrm{NaX}$

• Reaction with active metals

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+2\mathrm{Li}\stackrel{\text{dry ether}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Li}+\mathrm{LiBr}$

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+\mathrm{Zn}\stackrel{\begin{array}{c}\text{inert}\\ \text{atmosphere}\end{array}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{ZnBr}$

4. Reduction

• The reaction of an alkyl halide with nickel, platinum, or palladium in the presence of a metal catalyst.

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+{\mathrm{H}}_2\underset{525\mathrm{K}}{\overset{\mathrm{Ni}\text{or}\mathrm{Pd}}{\to }}{\mathrm{CH}}_3{\mathrm{CH}}_3+\mathrm{HBr}$

• The reaction of ethyl alcohol with zinc-copper couple

$\mathrm{Zn}+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}⟶\left({\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{O}\right)\mathrm{Zn}+2\mathrm{H}$

• The reaction of hydroiodic acid in presence of red phosphorus.

7. Chemical Properties of Haloarenes

• Resonance effect- Due to resonance, haloarenes are not reactive towards nucleophilic substitution reactions.

• Hybridization of carbon atoms in haloarenes and haloalkanes- In haloalkanes carbon is sp³ hybridized whereas, in haloarenes, carbon is sp² hybridized.

• The polarity of carbon and hydrogen bond-

Sp² hybridized carbon of haloarene attached to halogen is has a lesser tendency to release its electrons to a halogen atom. Hence, the bond is less polar compared to the carbon and halogen bond of haloalkanes.

• Instability of phenyl cation- Phenyl cation is not stable since there is no resonance. Therefore, SN¹ mechanism cannot occur.

• Nucleophilic substitution reactions

Replacement by the hydroxy group-

Reaction for the replacement by the amino group-

Reaction for substitution with cyano group-

• Reactions with metal

Reaction with magnesium-

Reaction with lithium-

Reaction with sodium-

Reaction with copper powder-

• Reduction reaction-

• Electrophilic substitution reactions

Halogenation-

Nitration-

Sulphonation-

Alkylation-

Acylation-

8. Polyhalogen Compounds Compounds

Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds. Many of these compounds are useful in industry and agriculture.

1. Dichloromethane (Methylene chloride)
2. Triiodomethane (Iodoform)
3. Tetrachloromethane (Carbon tetrachloride)
4. Trichloromethane (Chloroform)
5. Freons
6. p,p’-Dichlorodiphenyltrichloroethane(DDT)

p,p’-Dichlorodiphenyltrichloroethane(DDT)

DDT, the first chlorinated organic insecticides. The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and it was also discovered to have a high toxicity towards fish. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolised very rapidly by animals; instead, it is deposited and stored in the fatty tissues.

Significance of NCERT Notes for Class 12 Chemistry Chapter 6

NCERT notes of Class 12 Chemistry Haloalkanes and haloarenes contain concepts in a systematic manner following NCERT and CBSE curriculum. These notes are very helpful for a quick revision for competitive exams like VITEEE, BITSAT, JEE Main, NEET, etc. Chapter 6 Chemistry Class 12 notes enhance problem-solving skills and analytical ability in students.

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