Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl Halides From Alkanes
In this article, we shall study methods of preparation of alkyl halides from alkanes. Alkyl halides can be prepared from alkanes by their halogenation.
General Reaction:
R – H + X– X → R – X + H – X
Alkane Halogen Alkyl halide Halogen acid
Conversion of -H (Alkanes) into – X (Alkyl halides)
Order of Reactivity for Halogens:
F2 > > Cl2 > Br2 > I2
Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl
Preparation of Alkyl Chlorides From Alkanes:
General Reaction:
When alkane is treated with chlorine in presence of ultraviolet light or diffused sunlight alkyl chlorides or chloroalkanes are obtained. The reaction gives a mixture of all possible chloroalkanes.
R – H + Cl– Cl R – Cl + H – Cl
Alkane Chlorine Alkyl chloride Hydrogen chloride
As the reaction is taking place in the presence of ultraviolet light or diffused sunlight the reaction is known as photohalogenation of alkanes.
Example –1: Preparation of methyl chloride (Chloromethane) from methane
CH4 + Cl2 CH3Cl + HCl
Methane Chlorine Methyl Chloride Hydrogen Chloride
Example –2: Preparation of ethyl chloride (Chloroethane) from ethane
C2H6 + Cl2 C2H2Cl + HCl
Ethane Chlorine Ethyl Chloride Hydrogen Chloride
Preparation of Alkyl Bromides From Alkanes:
General Reaction :
When alkane is heated with bromine in presence of anhydrous AlBr3 as catalyst alkyl bromide or bromoalkane is obtained.
R – H + Br– Br R – Br + H – Br
Alkane Bromine Alkyl bromide Hydrogen bromide
Example- 1: Preparation of ethyl bromide (Bromoethane) from ethane:
C2H6 + Br2 C2H2Br + HBr
Ethane Bromine Ethyl bromide Hydrogen bromide
Example- 2: Preparation of methyl bromide (Bromomethane) from methane:
CH4 + Br2 CH3Br + HBr
Methane Chlorine Methyl Chloride Hydrogen Chloride
Preparation of Alkyl Iodides From Alkanes:
With iodine reaction is reversible. Thus we can not get a good yield of alkyl iodide. Hence direct iodination is difficult.
R – H + I – I ⇌ R – I + H – I
Alkane Iodine Alkyl iodine Hydrogen iodine
If the reaction is carried out in the presence of iodic acid HIO3, or mercuric oxide HgO, or dilute HNO3 which can oxidise HI formed and the lodoalkane can be obtained. The iodination reaction stops at mono-iodo state.
General Reaction Using Iodic Acid HIO3:
5 R-H + 2 I2 + HIO3 → 5 R-I + 3 H2O
Alkane Iodine Iodic acid Alkyl iodide
General Reaction Using Mercuric Oxide HgO:
2R-H + 2 I2 + HgO → 2 R-I + Hg I2 + H2O
Alkane Iodine mercuric oxide Alkyl iodide
General Reaction Using Dilute Nitric Acid HNO3:
8R-H + 4 I2 + HNO3 → 8R-I + 3H2O + NH3
Alkane Iodine nitric acid Alkyl iodide ammonia
Example-1: Preparation of ethyl iodide (Iodoethane) using HIO3:
5 C2H6 + 2 I2 + HIO3 → 5 C2H5I + 3 H2O
Ethane Iodine Iodic acid Ethyl iodide
Example-2: Preparation of ethyl iodide (Iodoethane) using HgO:
2C2H6 + 2 I2 + HgO → 2 C2H5I + HgI2 + H2O
Ethane Iodine mercuric oxide Ethyl iodide
Example-3: Preparation of ethyl iodide (Iodoethane) using HNO3:
8C2H6 + 4 I2 + HNO3 → 8C2H5I + 3H2O + NH3
Ethane Iodine Nitric acid Ethyl iodide Ammonia
Drawbacks/Disadvantages of direct halogenation (Photohalogenation):
After forming alkyl halide the reaction proceeds further and poly-substitution takes place and gives a mixture of di, tri, tetra etc. haloalkanes.
CH4 + Cl2 CH3Cl (Chloromethane) + HCl
CH3Cl + Cl2 → CH2Cl2 (Dichloromethane) + HCl
CH2Cl2 + Cl2 → CHCl3 (Trichloromethane) + HCl
CHCl3 + Cl2 → CCl4 (Tetrachlromethane) + HCl
This reaction continues till all halogens in the alkane are replaced one by one by chlorine or bromine. Thus we get mixture of di, tri, tetra etc. haloalkanes.
The mixture contains less amount of alkyl chlorides or bromides. Chlorination and bromination reactions are not selective. Hence they may give isomers of the monohalogen derivatives of alkanes. Besides the separation of constituents is difficult.
Iodination of alkanes is reversible reaction it requires additional reagents like mercuric oxide or iodic acid or nitric acid for obtaining alkyl iodes. Hence direct halogenation is not the suitable method for the preparation of alkyl halides.
Notes:
- Thermal chlorination of alkanes is called Hare and Mc Bee reaction. It is carried about at 673K.
- Diffused sunlight causes homolytic fission of halogen, hence the reaction is a free radical substitution.
- If alkanes are used in excess, the major product is monohalogen derivatives of alkanes. i.e. alkyl halides.
- The ease of substitution of different types of a hydrogen atom is Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl
- Thus, an alkane containing a tertiary hydrogen atom would be more reactive towards substitution reaction than with only primary hydrogen atom or primary and secondary hydrogen atoms. Isobutane would undergo substitution more rapidly than propane or ethane.
- Alkyl fluorides are not prepared by fluorination of alkanes, because the reaction is highly explosive in nature.
- Propane on chlorination gives a mixture of 2- Chloropropane (55 %) and 1- Chloropropane (45 %)
- Butane on chlorination gives a mixture of n-butyl chloride (28 %) and sec-butyl chloride (72 %)
- iso-Butane on chlorination gives a mixture of tert-butyl chloride (64 %) and isobutyl chloride (36 %)
- Bromination of alkanes is electrophilic substitution reaction, as it forms AlBr4 ion and Br+ ion. Br+ ion takes part in substitution.
Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl Halides From Alkanes