--%>
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 : Entropy on molecular basis. The

    The equation S = k in W relates entropy to W, a measure of the number of different molecular level arrangements of the system.In the preceding developments it was unnecessary to attempt to reach any "explana

    		•
  • Q : Molal concentration Select the right

    Select the right answer of the question. If one mole of any substance is present in of solvent, then: (a) It shows molar concentration (b) It shows molal concentration (c) It shows normality (d) It shows strength.

    		•
  • Q : Explain Second Order Rate Equations.

    Integration of the second order rate equations also produces convenient expressions for dealing with concentration time results.A reaction is classified as second order if the rate of the reaction is proportional to the square of the concentration of one o

    		•
  • Q : Value of molar solution Select the

    Select the right answer of the question. Molar solution contains: (a)1000g of solute (b)1000g of solvent (c)1 litre of solvent (d)1 litre of solution

    		•
  • Q : Problem based on molarity Select the

    Select the right answer of the question. If 18 gm of glucose (C6H12O6) is present in 1000 gm of an aqueous solution of glucose, it is said to be: (a)1 molal (b)1.1 molal (c)0.5 molal (d)0.1 molal

    		•
  • Q : Problem on Clausius equation of state

    If a gas can be described by the Clausius equation of state: P (V-b) = RT Where b is a constant, then:  (a) Obtain an expression for the residual vo

    		•
  • Q : Explain Phase Rule The relation between

    The relation between the number of phases, components and the degrees of freedom is known as the phase rule. One constituent systems: the identification of an area on a P-versus-T with one phase of a component system illustrates the two degrees of freedom that

    		•
  • Q : Molecular weight of solute Select right

    Select right answer of the question. A dry air is passed through the solution, containing the 10 gm of solute and 90 gm of water and then it pass through pure water. There is the depression in weight of solution wt by 2.5 gm and in weight of pure solvent by 0.05 gm. C

    		•
  • Q : HCl polarity Illustrate HCl is polar or

    Illustrate HCl is polar or non-polar?

    		•
  • 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

    		•