In our day to day life, we come across a number of alkyl halides like chloroform, freon, etc. In this chapter, we will discuss the different types, Isomerism, and the preparation of alkyl halides. After completion of this portion you may complete the reactions of alkyl halides, preparation, and reactions of chlorobenzene and Practice set.
1. TYPES OF HALIDES:
a. Haloalkane or alky halide:
When halogen atom(s) is bonded to an alkyl group, it is called haloalkanes or alkyl halides.
i) Primary alkyl halide:
If 2 directly bonded atoms remain connected with the carbon atom holding halogen, it is said to be 1o or primary alkyl halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 2 image 5 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-5.png)
ii) Secondary alkyl halides:
If a single directly bonded atom remain connected with the carbon atom holding halogen, it is said to be 2o or secondary alkyl halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 3 image 6 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-6.png)
iii) Tertiary alkyl halides:
If their is no directly bonded hydrogen connected with the carbon atom holding halogen, it is said to be 3o or tertiary alkyl halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 4 image 7 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-7.png)
b. Halocycloalkane or cycloalkyl halide:
When cycloalkanes are bonded with halogen, it is called halocycloalkane or cyclyoalkyl halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 5 image 8 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-8.png)
c. Allylic halide:
In allylic halides the halogen atom is linked to a sp3 hybridised carbon atom which is next to a carbon-carbon double bond.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 6 image 9 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-9.png)
d. Benzylic halide:
In Benzylic halides, the halogen atom is attached to a sp3 hybridised carbon atom which is next to an aromatic ring.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 7 image 10 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-10.png)
e. Propargyl halide:
In Propargyl halides, the halogen atom is linked to a sp3 hybridised carbon atom which is next to a carbon-carbon triple bond.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 8 image 11 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-11.png)
f. Vinyl halide:
In Vinyl halides, the halogen atom is attached to one of the carbon atoms of the carbon-carbon double bond.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 9 image 12 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-12.png)
g. Aryl halide:
In Aryl halides, the halogen atom is directly attached to aromatic ring.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 10 image 13 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-13.png)
h. Gem dihalide:
When halogen atoms are attached to the same atom, it is said to be a gem halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 11 image 14 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-14.png)
i. Vicinal dihalide:
When two halogen atoms are attached on adjacent carbon atoms it is called vicinal halide.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 12 image 15 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-15.png)
2. ISOMERISM IN ALKYL HALIDES
a. Chain isomerism:
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 13 image 16 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-16.png)
b. Position isomerism:
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 14 image 18 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-18.png)
c. Optical isomerism:
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 15 image 19 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-19.png)
Nature of C-X bond
Due to the electronegativity difference between carbon and halogen, the c-X bond is polar in nature.
- Bond length: C-F<C-Cl<C-Br<C-I ( increases with the size of the halogen)
- Bond energy: C-F>C-Cl>C-Br>C-I (Decreases with the size of the halogen)
- Dipole moment: C-Cl>C-F>C-Br>C-I (Decreases with the electron affinity of the halogen)
3. Preparation of Haloalkanes
a. From Alcohol:
1)When alcohol is treated with HBr or HI or HCl/Anh ZnCl2 the OH group gets substituted by the halogen atom.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 16 image 20 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-20.png)
i)HCl requires anh. ZnCl2 as a catalyst. A mixture of Conc.HCl and anh.ZnCl2 is known as Lucas reagent. This is called Groove’s process. For 30 alcohol, anh.ZnCl2 is not required.
ii)HCl<HBr<HI, reactivity order.
iii)1o<2o<3o, the reactivity for a specific HX.
iv)To prepare bromoalkane, NaBr/ H2SO4 is used.To prepare iodoalkane, KI/ H3PO4 is used.
2) When alcohol is reacted with PCl5 or PCl3 the OH group gets substituted by the chlorine atom.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 17 image 21 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-21.png)
i)PBr5 and PI5 are unstable compounds.
ii)As PBr3 and PI3 are unstable compounds, PBr3 and PI3 are produced insitu by the reaction of P4 and Br2 or I2.
3. When alcohol is reacted with SOCl2 / Pyridine, the OH group gets substituted by the chlorine atom.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 18 image 22 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-22.png)
i)When thionyl chloride is used, HCl and SO2 are produced as gaseous product , and can be easily separated.
ii)When pyridine is used, HCl reacts with pyridine.
iii)This process is called Darzens process. iv)RBr and RI cannot be prepared by this method as SOBr2 is less stable and SOI2 does not exist.
b.From Alkanes:
1.Chlorination and Bromination:
Alkanes when reacted with chlorine or bromine in the presence of diffused sunlight (Cl2/hν or Br2/hν), chloroalkane or bromoalkane is formed.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 19 image 23 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-23.png)
i)Benzylic ≈ allylic > tertiary > sec> pri > vinylic ≈ aryl . The reactivity of different types of hydrogens.
ii)Halogentaion of alkane follows free radical mechanism.
iii)F2>Cl2>Br2>I2 is the order of halogenations for a particular alkane.
iv)Direct fluorination is explosive. v)For halogenations on an aliphatic chain with more than 2 carbon, halogen always tries to avoid the peripheral carbons.
2.Fluorination:
Preparation of fluoride from bromide and chloride by using AsF3, SbF3, CoF2, AgF, Hg2F2 etc is called Swarts Reaction.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 20 image 24 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-24.png)
3.Iodination:
Iodination of alkyl chloride or bromide by NaI in acetone or methanol medium is called Finkelstein Reaction.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 21 image 28 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-28.png)
Iodination of an alkane is reversible in nature. Hence to make it irreversible HIO3 or HNO3 or HgO is used. CH4 + I2 →CH3I + HI
5HI + HIO3 → 3I2 + 3H2O
c. From Alkenes:
Alkenes when reacted with HCl or HBr or HI, respective haloalkane is produced.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 22 image 27 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-27.png)
i)Addition of HX will follow Markownikoff’s Rule.
ii)Addition of HBr in presence of organic peroxide is called peroxide effect or Kharasch effect where anti Markownikoff’s product is formed.
iii)The addition of HBr is a double step reaction, which is exothermic. But with HCl the second step is endothermic. With HI the first step is endothermic. iv)The peroxide effect will follow radical mechanism.
iv)Allylic bromination or chlorination is carried out by NBS/hν (for bromination) and SO2Cl2/hν/200oC/OPO ( for Chlorination)
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 23 image 29 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-29.png)
d. From Silver salt of carboxylic acid:
Silver carboxylates when treated with Br2 or Cl2 in presence of CCl4 produce alkyl bromide or chloride with a carbon less.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 24 image 30 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-30.png)
i)The reaction is called Hunsdiecker reaction.
ii) Aryl bromide can also be prepared by this method.
iii)The yield is 10>20>30
iv)Chlorine gives a poor yield
v)It is believed to follow free radical mechanism.
vi)Ehen the same reaction is carried with I2, ester is produced. This reaction is called Birnbaum-Simonini reaction.
![Haloalkanes and Haloarenes|Types|Isomerism|Preparations 25 image 31 ALL ABOUT CHEMISTRY](https://allaboutchemistry.net/storage/2020/06/image-31.png)