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 : M ive me answer of this question. When

    ive me answer of this question. When mercuric iodide is added to the aqueous solution of potassium iodide, the: (a) Freezing point is raised (b) Freezing point is lowered (c) Freezing point does not change (d) Boiling point does not change

  • Q : Describe various systems for

    Common system

    According to this system, the individual members are named according to alkyl groups att

  • Q : What is Flash Photolysis Reactions.

    An example illustrates the type of mechanism that can be written to explain the development of flash photolysis reactions.

    Often, as the reactions in the ozone layer of the earth's atmosphere, we are interested in the kinetic behavior of species that are not a

  • Q : Moles of HCl present in .70 L of a .33

    Detail the moles of HCl which are present in .70 L of a .33 M HCl solution?

  • Q : Explosions produce carbon dioxide

    Illustrate all the explosions produce carbon dioxide?

  • Q : Question on colligative property Choose

    Choose the right answer from following. Which of the following is a colligative property: (a) Osmotic pressure (b) Boiling point (c) Vapour pressure (d) Freezing point

  • Q : Problem associated to vapour pressure

    Provide solution of this question. 60 gm of Urea (Mol. wt 60) was dissolved in 9.9 moles, of water. If the vapour pressure of pure water is P0 , the vapour pressure of solution is:(a) 0.10P0 (b) 1.10P0 (c) 0.90P0 (d) 0.99P0

  • Q : Explain Ionic Bond with examples. The

    The bonding in ionic molecules can be described with a coulombic attractive term.

    For some diatomic molecules we take quite a different approach from that used in preceding sections to describe the bonding. Ionic bonds are interpreted in terms of the coulom

  • Q : Number of electrons in the benzene

    Describe the number of electrons in the benzene? Write a short note on it?

  • Q : Film Mass Transport Sulfur trioxide

    Sulfur trioxide (SO3) is manufactured by the gas-phase oxidation of SO2 over a platinum catalyst:

    SO2 + ½ O2 à SO3

    The catalyst is a non-porous ext

2015 ©TutorsGlobe All rights reserved. TutorsGlobe Rated 4.8/5 based on 34139 reviews.