--%>
Assignment Help - homework help

How haloalkanes are prepared from hydrocarbons?

Alkyl halides can be prepared from alkanes through substitution and from alkenes through addition of halogen acids or through allylic substitution.
    
From alkanes

When alkanes are treated with halogens, chlorine or bromine, in the presence of light or heat, they undergo free radical substitution and a mixture of mono- and poly- substituted products are obtained.

2242_alkyl halide.png 

Although, the substitution beyond monohalogenation may be suppressed by using alkane in excess yet the method is not of much practical use because of the difficulties of separation of such a mixture.

In case of higher alkanes, different isomeric products are formed even when mono-substitution is carried out.

1702_alkyl halide1.png 

In general, the ease of substitution of different types of hydrogen atoms is:

Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl

The iodination of alkanes is reversible and is done by heating with iodine in the presence of oxidising agents like conc. HNO3, HIO4 orHIO3. The function of using such agents is to oxidize HI formed during the reaction to iodine, and hence shift the equilibrium in the forward direction.

264_alkyl halide2.png 

Due to formation of polysubstituted products and isomeric products, this method is not suitable for the laboratory preparation of pure haloalkanes. However, this method can be used for laboratory preparation of certain specific alkyl halides as given below:
    
When all the hydrogen atoms in the alkane are equivalent are equivalent, then it can form only one product on monosubstitution. In such cases this method may be applied.

924_alkyl halide3.png 
    
Allylic and benzylic halides can be prepared from alkenes and arenes respectively by this method because allylic and benzylic hydrogen atoms are substituted much more readily than vinylic and aryl hydrogen atoms.

260_alkyl halide4.png 

In such cases vinylic aryl hydrogens being less reactive do not participate in free radical substitution.

Allylic and benzylic hydrogen atoms are substituted very easily because their substitution proceeds via allylic and benzylic free radicals as intermediates. These intermediates are stabilized by resonance and hence being stable are formed at faster rate.
    
By halide exchange

Iodoalkanes can be obtained by treating bromo or chloroalkanes with a solution of sodium iodine in acetone or methanol. For example,

1778_alkyl halide5.png 

The reaction is known as Finkelstein reaction. This reaction is based on the fact that NaI is soluble in acetone but NaBr and NaCl are not. As a result, equilibrium in the above reaction is very much in favour of forward reaction. The reaction gives best result with primary halides.

Fluoroalkanes are difficult to prepare directly by the action of alkanes with fluorine. It is because fluoride has gor a high reactivity towards the hydrogen. It extracts all the hydrogen atoms from hydrocarbon molecule.

CH4 + 2F2  71_potassium permangnate3.png  4HF + C

However, Fluoroalkanes can be obtained by treating alkyl halides with salts like AgF, Hg2F2, CoF3 or SbF3. This reaction is known as Swarts reaction.

CH3Br + AgF 71_potassium permangnate3.png CH3F + AgBr

2CH3CH2Cl + Hg2F2  71_potassium permangnate3.png  2CH3CH2F + Hg2Cl2

For replacement of two or three halogen atoms at the same carbon CoF3 or SbF3 is used.

67_alkyl halide6.png

   Related Questions in Chemistry

  • Q : Symmetry Elements The symmetry of the

    The symmetry of the molecules can be described in terms of electrons of symmetry and the corresponding symmetry operations.Clearly some molecules, like H2O and CH4, are symmetric. Now w

    		•
  • Q : Problem on normality Help me to solve

    Help me to solve this problem. 0.5 M of H2AO4 is diluted from 1 lire to 10 litre, normality of resulting solution is : (a)1 N (b) 0.1 N (c)10 N (d)11 N

    		•
  • Q : Acid Solutions Choose the right answer

    Choose the right answer from following. Volume of water needed to mix with 10 ml 10N NHO3 to get 0.1 N HNO3: (a) 1000 ml (b) 990 ml (c) 1010 ml (d) 10 ml

    		•
  • Q : Electrochemistry ( electrolysis of

    1. Define Faraday's first law of electrolysis 2. define Faraday's second law of electrolysis

    		•
  • Q : Acid value definition The acid value

    The acid value definition is the number milligrams of KOH needed to neutralize the acid present in one gram oil and fats however why not employ NaOH for the neutralization?

    		•
  • Q : Explain the catalyst definition and

    Catalyst is a substance which accelerates the rate of a chemical reaction without undergoing any change in its chemical composition or mass during the reaction. The phenomenon of increasing the rate of a reaction with the help of a catalyst is known as catalysis.

    		•
  • Q : Describe Transformation Matrices. Each

    Each symmetry operation can be represented by a transformation matrix.You have seen what happens when a molecule is subjected to the symmetry operation that corresponds to any of the symmetry elements of the point group to which the molecule belongs. The m

    		•
  • Q : Problem on Redlich-Kwong equation i)

    i) Welcome to Beaver Gas Co.! Your first task is to calculate the annual gross sales of our superpure-grade nitrogen and oxygen gases. a) The total gross sales of N2 is 30,000 units. Take the volume of the cylinder to be

    		•
  • Q : Explain the molecular mass with respect

    During the formation of polymers, different macromolecules have different degree of polymerisation i.e. they have varied chain lengths. Thus, the molecular masses of the individual macromolecules in a particular sample of the polymer are different. Hence, an average value of the molecular mass is

    		•
  • Q : Molarity of Sulfuric acid Choose the

    Choose the right answer from following. What is the molarity of H2SO4 solution, that has a density 1.84 gm/cc at 35c and contains solute 98% by weight: (a) 4.18 M (b) 8.14 M (c)18.4 M (d)18 M

    		•