Chapter 10

Haloalkanes and Haloarenes

Haloalkanes and haloarenes can be generally classified as mono, di, or poly halogen (tri-, tetra-, etc.) compounds, depending on the number of halogen atoms (1, 2, or more) in their structures. The replacement of hydrogen atoms in an aliphatic or aromatic hydrocarbon by halogen atoms results in the formation of alkyl halide or haloalkane and aryl halide or haloarene, respectively.

 

As the halogen atoms are more electronegative than carbon, the carbon-halogen bond of haloalkanes is polarised. In this, the halogen atom bears a partial negative charge, and the carbon atom bears a partial positive charge. Haloalkanes are formed by the free radical halogenation of alkanes, with the addition of halogen acids to alkenes, and replacement of –OH group of alcohols with halogens using thionyl chloride, phosphorus halides, or halogen acids. Haloarenes are formed by electrophilic substitution to arenes. Fluorides and iodides are mainly formed by using the halogen exchange method.

 

The boiling points of organohalogen compounds, organic compounds that contain chlorine, bromine, fluorine atoms, are comparatively higher than the corresponding hydrocarbons because of their powerful dipole-dipole and van-der-Waals forces of attraction. Haloalkanes and haloarenes are slightly soluble in water but completely soluble in organic solvents. Various halogen-containing organic compounds occur in nature. Some of them have wide applications in the medical or industrial industry and even in our everyday lives.

 

In the haloalkanes and haloarenes NCERT chapter, you will learn about the important preparation methods, properties, applications, and effects of these organohalogen compounds.

Haloalkanes and haloarenes

The compounds are called haloalkanes and haloarenes when hydrocarbons with one or more hydrogen atoms are displaced by halogen atoms. The compound will be called haloalkane when an aliphatic hydrocarbon replaces a hydrogen atom with a halogen atom. And if the aromatic hydrocarbon is displaced by a halogen, the compound then created will be haloarene. 

Haloalkanes are mainly formed from open-chain hydrocarbons (alkanes), while haloarenes are formed from aromatic hydrocarbons. 

Haloarenes and haloalkanes have different characteristics and formation processes.

Haloarene is also named aryl halide. In the haloalkane formula (R-X), X stands for halogen group. It is connected to an alkyl group hybridised sp3 atom, whereas, in haloarene (Ar – X), it is attached to an aryl group hybridised sp2, hybridised atom.

Example of haloarenes: Bromobenzene − C6H5Br (sp2C).

Examples of haloalkanes: Ethyl bromide − CH3CH2 − Br (sp3C).

Haloalkanes are organic chemical compounds that are created by replacing one or more hydrogen atoms from the alkane group with halogen group atoms (with any Group 17 elements, such as— Iodine, chlorine, fluorine, bromine, and so on). Haloalkanes have all the single chemical bonds bound to the carbon atom with the saturated organic compounds. In haloalkanes, the halogen atom is attached to a single carbon atom.

Aryl halides or haloarenes are aromatic compounds that are created when the halogen group replaces one or more hydrogen atoms connected to an aromatic ring. Aryl chlorides are one of the key components of the haloarene class. This compound and its derivatives are commonly used for various industrial purposes. 

Some examples of haloarenes include Chlorobenzene, Bromobenzene, Iodobenzene, Chlorotoluene, etc.

On the basis of the hybridisation of a carbon atom with the halogen atom, these mono, halo compounds can be categorized into various groups and subclasses.

 

  1. Compounds with sp3 C—X Bond [X (Halogen Group) = Cl, I, F, Br. These compounds can again be divided into:

 

  • i) Allylic halides: Allylic halides has the carbon-carbon double bond structure and is created by halogen group bonding with sp3 hybridised carbon atoms next to the structure of a carbon-carbon double bond(C=C). 

  • ii) Alkyl halides/haloalkanes (R – X): The halogen atom is related to an alkyl group in this category. CnH2n+1 X is the homologous formula for these compounds. They can again be divided into three groups– primary, secondary, and tertiary, based on the carbon atom to which the carbon-bearing halogen (X) atom is bound.

  • iii)Benzylic halides: Such compounds are created by adding the halogen atom to an sp3-hybridised carbon atom. To create benzyl halides, an sp3 hybridised carbohydrate should be next to an aromatic ring.

    2. Compounds that have sp2C-X bond: Such compounds have aryl halides and vinyl halides.

    i) Aryl halides: These compounds are created when the halogen group is connected in an aromatic ring to an sp2 hybrid carbon         atom.

   ii) Vinyl halides: This class of compounds are created by adding a halogen atom to an Sp2 hybridised C atom in carbon by                   carbon double bond.

Haloalkanes and haloarenes are used for various industrial and day-to-day applications. They are used as pharmaceuticals, flame retardants, solvents, propellants, coolants, and much more.

  • Haloalkanes and haloarenes are used for various industrial and day-to-day applications. They are used as pharmaceuticals, flame retardants, solvents, propellants, coolants, and much more.

  • Alkyl halides are mainly used for non-polar compounds as solvents.

  • Haloalkanes and haloarenes are widely used as starting constituents for the synthesis of various organic compounds.

  • The derivatives of these compounds, such as Chloramphenicol, a chlorine-containing antibiotic, are very effective for the treatment of typhoid disease.

  • Some synthetic halogen compounds such as chloroquine are widely used to treat malaria.

  • DDT is mostly used as a pesticide.

  • Halothanes are commonly used as an anaesthetic during surgery.

Haloalkanes and haloarenes have wide industrial applications. These halocarbons are also strong environmental pollutants and toxins. Some haloalkanes that contain chlorine or bromine have harmful environmental effects, including ozone depletion. The most common compound in this group is chlorofluorocarbons (CFCs). Another compound, methyl bromide, is a harmful fumigant that negatively impacts the environment. Another compound, CCl4, found in paints, thinners, cleaning agents, and industrial adhesives, can be detrimental to heart health. It also leads to ozone layer depletion. Some evidence shows negative effects of CCl4 that cause nausea, vomiting, or sometimes permanent nerve cell damage.

1. Write the equations for the preparation of 1-iodobutane from

(i) 1-butanol

(ii) but-1-ene

(iii) 1-chlorobutane

 

 

 

2. Out of C6H5CHClC6H5 and C6H5CH2Cl, which can be more easily hydrolysed by aqueous KOH.

1. Write the equations for the preparation of 1-iodobutane from

(i) 1-butanol

(ii) but-1-ene

(iii) 1-chlorobutane

 

 

 

2. Out of C6H5CHClC6H5 and C6H5CH2Cl, which can be more easily hydrolysed by aqueous KOH.

3. What are ambident nucleophiles? Explain with an example.

 

 

4. Which compound in the following pairs will react faster in SN2 reaction with –OH? (i) (CH3 ) 3CCl or CH3Cl (ii) CH3Br or CH3 I

 

 

 

5. Explain the IUPAC names for the below compounds:

(i) CH3CH(Cl)CH(Br)CH3

(ii) CHF2CBrClF

(iii) ClCH2C≡CCH2Br

(iv) (CCl3 ) 3CCl

(v) CH3C(p-ClC6H4 ) 2CH(Br)CH3

(vi) (CH3 ) 3CCH=CClC6H4 I-p

1. The equations for the preparation of 1-iodobutane from (i) 1-butanol (ii) but-1-ene (iii) 1-chlorobutane.

 

 (i) 〖CH〗_3 〖CH〗_2 〖CH〗_2 〖CH〗_2 OH+KI+H_3 PO_4 

                 〖CH〗_3 〖CH〗_2 〖CH〗_2 〖CH〗_2 I+H_2 O+〖KH〗_2 PO_4

(ii) 〖CH〗_3 〖 CH〗_2-CH=〖CH〗_2+HBr Peroxide/ 

                                 〖CH〗_3 〖CH〗_2 〖CH〗_2 〖CH〗_2 Br

                                      NaI/Acetone

〖CH〗_3 〖CH〗_2 〖CH〗_2 〖CH〗_2 -I+NaBr

 

(iii) 〖CH〗_3 〖 CH〗_2 〖 CH〗_2 〖CH〗_2 CL+KIAcetone

                                 〖CH〗_3 〖CH〗_2 〖CH〗_2 〖CH〗_2 I+KCI

 

2. Because of the formation of a secondary carbocation stabilised by resonance with two phenyl groups, C6H5CHClC6H5​ is easily hydrolysed by aqueous KOH. However, for C6​H5​CH2​Cl, a primary carbocation is created during hydrolysis that is stabilised by resonance with just one phenyl group, which makes it less stable and less readily formed.

 

3. Ambident nucleophiles consists of two nucleophilic sites using which they can attack. Cyanide ion is an example of ambident nucleophile. Nitrite-ion can attack from oxygen which leads to the formation of alkyl nitrites. It can also attack through nitrogen and form nitroalkanes.

 

4. (i) And CH3Cl will react faster than (CH3)3CCl because the order of reactivity in SN2 reaction is: Primary halide > Secondary halide > Tertiary halide. (ii) CH3I will react faster than CH3Br as the order of reactivity is R-I > R-Br > R-Cl > R-F.

 

5. The IUPAC names of the compounds are as follows: (i) 2-Bromo-3-chlorobutane, (ii) 1-Bromo-1-chloro-1,22-trifluoroethane, (iii) 1-Bromo-4-chlorobut-2-yne, (iv) 2-(Trichloromethyl)-1,1,1,2,3,3,3-heptachloropropane, (v) 2-Bromo-3,3-bis (4-chlororphenyl) butane, (vi) 1-Chloro-1-(4-iodophenyl)-3,3-dimethylbut-1-ene.

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