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 : Inorganic Chemistry Inorganic

    Inorganic Chemistry:

    In the year 1869, Russian Chemist Dmitry Mendeleyev forms the periodic table of the element. Since Newlands did before him in the year 1863, Mendeleyev categorizes the el

  • Q : Describe Point Groups. For any

    For any symmetric object there is a set of symmetry operations that, together, constitute a mathematical group, called a point group.

    It is clear from the examples that most molecules have several elements of symmetry. The H2O

  • Q : Relationship between free energy and

    The free energy of a gas depends on the pressure that confines the gas.


    The standard free energies of formation, like those allow predictions to be made of the possibility of a reaction at 25°C for each reagent at 

  • Q : Mole fraction in vapours Choose the

    Choose the right answer from following. If two substances A and B have P0A P0B= 1:2 and have mole fraction in solution 1 : 2 then mole fraction of A in vapours: (a) 0.33 (b) 0.25 (c) 0.52 (d) 0.2

  • Q : Describe properties of carboxylic acids.

    1. Physical state: the first three aliphatic acids are colourless liquids with pungent smell. The next six are oily liquids with an odour of rancid butter while the higher members are colourless, odourless waxy solids. Benzoic acid is referred to

  • Q : What are heterogenous catalysis? Give

    When the catalyst exists in a different phase than that of reactants, it is said to be heterogeneous catalyst, and the catalysis is called heterogeneous catalysis. For example, SO2 can be oxidized to SO3

  • Q : Molecular Structure type The ionic

    The ionic radii of Rb+ and I- respectively are 1.46 Å and 2.16Å. The very most probable type of structure exhibited by it is: (a) CsCl type  (b) ZnS type  (c) Nacl type  (d) CaF2 type

    Q : Question on Mole fraction Mole fraction

    Mole fraction of any solution is equavalent to: (a) No. of moles of solute/ volume of solution in litter (b) no. of gram equivalent of solute/volume of solution in litters (c) no. of  moles of solute/ Mass of solvent in kg (d) no. of moles of any

  • Q : What is ortho effect? Orthosubstituted

    Orthosubstituted anilines are generally weaker bases than aniline irrespective of the electron releasing or electron withdrawing nature of the substituent. This is known as ortho effect and may probably be due to combined electronic and steric factors.

    The overall basic strength of ort

  • Q : Reactivity of allyl and benzyl halides

    why allyl halide and haloarenes are more reactive than alkyl halide towards nucleophilic substitution