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Hello there. Here's a problem that appeared on one of my tests.

What is the order of reactivity of halogens in electrophilic addition reaction via formation of cyclohalonium intermediate?

I tried to predict it using the mechanism, but couldn't reach a conclusion. Is this based on thermodynamic and kinetic data?

Here's the mechanism involving bromine, for reference:

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Could someone please provide a detailed explanation for the above problem, with data if possible?

Thanks a lot.

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  • $\begingroup$ related chemistry.stackexchange.com/questions/41130/… $\endgroup$ – Mithoron Jan 3 '18 at 16:16
  • $\begingroup$ Good to learn something new, but it didn't answer my question. $\endgroup$ – cogito_ai Jan 3 '18 at 16:18
  • $\begingroup$ Edit your question to remove salutation/thanks etc. Also add what you think about possible answer. This is rather important question so it should be rather improved then tempt to be closed as homework. $\endgroup$ – Mithoron Jan 3 '18 at 16:22
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ans is d Normally not possible to F2 because it is highly exothermic with I2 reversible reaction takes place due to more E.N of Cl2 compare to Br2. It does not give e pair to carbocation

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Ans is (D) Since F+ is not possible and in case. of I2 it will be reversible so In I2 &F2 the order is I2>F2 So ans is (D)

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    $\begingroup$ So how does Fluorine react ? $\endgroup$ – Maurice Mar 22 at 10:20
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I would say (B).

The mechanism involves the breaking of the Halogen–Halogen bond. The strength of this bond decreases down the group because atomic radius increases. The attraction of the halogen nuclei on the electrons in the covalent bond decreases as the valence electrons are further away from the nucleus.

Therefore, the I–I bond is weakest while F–F is strongest. Hence, I2 is most reactive and F2 is least.

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