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 : Describe physical adsorption and its

    When the forces of attraction existing between adsorbate and adsorbent are van der Waal's forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or van der Waal's adsorption. Since the forces existing between adsorbent and adsorbate are very w

  • Q : Preparation of ammonium sulphate Select

    Select the right answer of the question. Essential quantity of ammonium sulphate taken for preparation of 1 molar solution in 2 litres is: (a)132gm (b)264gm (c) 198gm (d) 212gm

  • Q : Explain polyhalogen compounds with

    Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Most of these compounds are valuable in industry and agriculture. Some important polyhalogen compounds are described as follows:

    Q : Strength of the Hydrochloric acid

    Provide solution of this question. 1.0 gm of pure calcium carbonate was found to need 50 ml of dilute HCL for complete reaction. The strength of the HCL solution is specified by : (a) 4 N (b) 2 N (c) 0.4 N (d) 0.2 N

  • Q : Molar concentration of hydrogen 20 g of

    20 g of hydrogen is present in 5 litre of vessel. Determine he molar concentration of hydrogen: (a) 4  (b) 1 (c) 3 (d) 2 Choose the right answer from above.

  • Q : How haloalkanes are prepared from

    This is the common method for preparing haloalkanes in laboratory. Alcohols can be converted to haloalkanes by substitution of - OH group with a halogen atom. Different reagents can be used to get haloa

  • Q : Haloalkanes define primary secondary

    define primary secondary and tertiary alkyl halides with examples

  • Q : Molarity in Nacl The molarity of 0.006

    The molarity of 0.006 mole of NaCl in 100 solutions will be: (i) 0.6 (ii) 0.06 (iii) 0.006 (iv) 0.066 (v) None of theseChoose the right answer from above.Answer: The right answer is (ii) M = n/ v(

  • Q : Decinormal concentration of Sulfuric

    Give me answer of this question. The volume of water to be added to 100cm3 of 0.5 N N H2SO4 to get decinormal concentration is : (a) 400 cm3 (b) 500cm3 (c) 450cm3 (d)100cm3

  • Q : Volumes of solution after concentration

    Hydrochloric acid solution A and B encompass concentration of 0.5N and 0.1N  corresspondingly. The volumes of solutions A and B needed to make 2liters of 0.2N of HCL are: (i) 0.5l of A + 1.5l of B (ii) 1.5l of A + 0.5 l of B  (iii) 1.0 l of A + 1.0l of B&nbs

©TutorsGlobe All rights reserved 2022-2023.