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Haloalkanes and Haloarenes are an important part of our daily lives; they are everywhere, from refrigerants in air conditioners to medicines. These are organic compounds containing Halogens (Fluorine, Chlorine, Bromine, Iodine). These compounds play a crucial role in pharmaceuticals and industries. 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 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. 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, 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.
Also, students can refer,
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.
a)From phosphorus halides-
R−OH+PCl5⟶RCl+POCl3+HCl
b)From alkanes-
c)From alkenes- Haloalkenes are prepared by the addition of halogens to alkenes
Addition of halogens-
a)By direct halogenation of the aromatic ring-
It is an electrophilic substitution reaction.
b)By the use of silver salt of aromatic acids-
This reaction is known as the Hunsdiecker reaction
Haloalkanes are colourless and their a sweet-smelling taste.
Since haloalkanes cannot break the hydrogen bonds of water, they 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 than compared of methyl chloride.
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.
The reactions of haloalkanes may be divided into the following categories:
1. Nucleophilic substitution
2. Elimination reactions
3. Reaction with metals
Nucleophilic substitution reaction
a)Substitution by hydroxyl group-
CH3CH2Br+K+OH⟶CH3CH2OH+KBr
b)Substitution by cyano group-
CH3−I+K+−CN⟶CH3−C≡N+KI
c)Substitution by isocyanide group-
d)Substitution by ammonia group-
The following reaction is known as the Hofmann ammonolysis reaction.
Nucleophilic substitution reactions are of two types
1) SN1
2) SN2
Substitution nucleophilic reaction (SN1 reaction)
The rate is given as –
Rate=k[(CH3Br]
This reaction occurs in two steps. The first step leads to the formation of a carbonation, and the second step leads to the nucleophilic attack on the 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)
The 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
Elimination reaction
Formation of an alkene takes place.
Also known as the 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 the more substituted alkene is formed.
Reaction with active metals- Haloalkanes react with metals such as Magnesium and sodium.
a)Reaction with magnesium-
b)Reaction with sodium-
Reaction with active metals
CH3CH2Br+2Li
CH3CH2Br+Zn
Reduction
The reaction of an alkyl halide with nickel, platinum, or palladium in the presence of a metal catalyst.
CH3CH2Br+H2
The reaction of ethyl alcohol with the zinc-copper couple
The reaction of hydroiodic acid in the presence of red phosphorus.
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 the carbon and hydrogen bond-
Sp2 hybridized carbon of haloarene attached to a halogen has a lesser tendency to release its electrons to a halogen atom. Hence, the bond is less polar compared to the carbon-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 the 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-
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.
p,p’-Dichlorodiphenyltrichloroethane(DDT)
DDT, the first chlorinated organic insecticide. 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 the lice that carry typhus. However, problems related to the 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.
Question1. The products
(A)
(B)
(C)
(D)
Answer:
These reactions are the preparation of haloalkanes from alcohols.
(A)
(B)
Hence, the correct answer is option (D).
Question 2. The incorrect statement regarding chirality is :
(A) The product obtained by
(B) Enantiomers are superimposable mirror images on each other.
(C) A racemic mixture shows zero optical rotation.
(D)
Answer: Chirality refers to the property of asymmetry in a molecule, where the molecule and its mirror image cannot be superimposed onto each other. In simpler terms, a chiral molecule cannot be rotated or flipped in a way that matches its mirror image. This property is essential in fields such as chemistry and biochemistry, particularly in understanding how molecules interact with each other and with biological systems.
Therefore, Enantiomers are non-superimposable mirror images of each other.
Hence, the correct answer is option (B).
To effectively solve questions of the chapter on haloalkanes and haloarenes, follow a systematic and structured approach that helps understand the concepts and the mechanism of reactions:
6.1 Classification
6.2 Nomenclature
6.3 Nature of C-X bond
6.4 Methods of Preparation of Haloalkanes
6.5 Preparation of Haloarenes
6.5 Physical properties
6.7 Chemical reaction
6.8 Polyhalogen compounds
The hyperlinks of ncert notes chapter-wise are given below:
The hyperlinks of ncert solutions subject-wise are given below:
The hyperlinks of the NCERT exemplar solutions, subject-wise, are given below:
Haloalkanes and Haloarenes are organic compounds containing halogens (Fluorine, Chlorine, Bromine, Iodine). One or more hydrogen is replaced by halogens in Hydrocarbons to form Haloalkanes and Haloarenes.
Haloalkanes are classified based on several Halogen atoms bonded to the Carbon atom. They are classified into primary, secondary and tertiary haloalkanes, depending on how many carbon atoms are attached to the carbon bearing the halogen.
Haloarenes derived from aromatic compounds, follow a similar classification as monohaloarenes. Di haloarenes can exist in ortho (o-), meta (m), or para (p) positions based on the placement of halogens on the benzene ring.
The IUPAC nomenclature of haloalkanes
Haloalkanes can be prepared using various methods, primarily involving substitution or addition reactions.
Haloalkanes and haloarenes have numerous applications in daily life due to their chemical properties.
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