Why aryl halides are less reactive?

Aryl halides are much less reactive towards nucleophilic substitution reactions than haloalkanes. The less reactivity of aryl halides can be described as follows:

1. Withdrawal of electrons by Benzene and Stabilization by resonance:

In aryl halides, the electron pair of halogen atom is in conjugation with π electrons of benzene ring. Thus, halobenzene is a resonance hybrid of the following structures:

The contributing structures II, III and IV indicate that C-X bond has partial double character.

As a result, the C-X bond in halobenzene is shorter and hence, stronger as compared to that in alkyl halides. In chlorobenzene C-Clbond length is 1.69 Å as compared with 1.77 Å in methyl chloride. Thus, cleavage of C-X bond in halobenzene becomes difficult which makes it less reactive towards nucleophilic substitution.

2. Different hybrid state of carbon atom

In haloalkanes, the carbon atom bearing halogen is sp3 hybridized while halogen bearing carbon atom hybridized in halorens, sp3hybrid orbital is smaller in size due to greater s-character as compared with sp3 orbital. As a result bond formed by overlap of sp2 hybrid orbital is shorter in size and stronger than the bond formed by overlap of sp3 hybrid orbital. Therefore, C-X bond of haloalkanes is cleaved more easily than in haloalkanes by overlap of sp3 hybrid orbital. Therefore, C-X bond of haloalkanes is cleaved more easily than in haloarenes.

3. Polarity of C-X bond

The C-X bond in haloalkanes is more polar than the C-X bond in haloarenes. (This is supported by dipole moment of these bonds. Dipole moment of chlorobenzene is 1.7 D whereas that of haloalkanes falls in the range (2.0-2.2 D). So greater the polarity of bond hence higher is the reactivity.

The less polarity of C-X bond in aryl halides is due to the fact that electron withdrawing inductive effect of halogen is opposed by the electron releasing resonance effect, as is evident from the positive charge on X in stronger II, III and IV.

In a similar way we can explain the less reactivity of vinyl halides as compared with alkyl or allyl halides.

   Related Questions in Chemistry

  • Q : Liquid Vapour Free Energies The free

    The free energy of a component of a liquid solution is equal to its free energy in the equilibrium vapour.

    Partial molal free energies let us deal with the free energy of the components of a solution. We use these free energies, or simpler concentration ter

  • Q : Analytical chemistry 37% weight of HCl

    37% weight of HCl and density is 1.1g/ml. find molarity of HCl

  • Q : Particles of quartz Particles of quartz

    Particles of quartz are packed by:
    (i) Electrical attraction forces  (ii) Vander Waal's forces  (iii) Covalent bond forces  (iv) Strong electrostatic force of attraction

    Answer: (iii)

  • Q : Analytical chemistry 37% weight of HCl

    37% weight of HCl and density is 1.1g/ml. find molarity of HCl

  • Q : Problem related to molality Help me to

    Help me to solve this problem. What is the molality of a solution which contains 18 g of glucose (C6,H12, O6) in 250 g of water:  (a) 4.0 m (b) 0.4 m (c) 4.2 m (d) 0.8 m

  • Q : Problem based on molality of glucose

    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 : The Liver Is Responsible For Much Of

    The Liver Is Responsible For Much Of The Pentose Phosphate Activity Explain

  • Q : Meaning of Molar solution Molar

    Molar solution signifies 1 mole of solute present/existed in: (i) 1000g of solvent (ii) 1 litre of solvent (iii) 1 litre of solution (iv) 1000g of solution

  • Q : Relative lowering of vapour pressure

    explain the process of relative lowering of vapour pressure

  • Q : What is synthetic rubber and how it

    To meet human needs, scientists have started preparing synthetic rubbers. Besides having similar properties as natural rubbers they are tougher, more flexible and more durable than natural rubber. They are capable of getting stretched to twice its length. Though, it reverts to its original shape