Is it the double bond attacking the $\ce{X+}$ ion or the other way around?

Also, does it form a cyclic transition state if it isn't bromine or chlorine?

Eg. cyclohexene $+ \,\ce{Cl+ ->}$ cyclic transition state

But would the same thing happen if I put any other electrophile?


These kind of reactions are called Electrophilic addition reactions. These are usually given by alkenes and alkynes. In your example, the electrophile is $\ce{Cl+}$. The alkene will act as a source of π electrons and will donate it to the electrophile which is in need of electrons. So, alkene is called as nucleophile in this reaction.

Is it the double bond attacking the X+ ion or the other way around?

It depends on how you look at it. We can say that alkene polarizes the $\ce{Cl-Cl}$ bond by pushing electrons into the anti-bonding molecular orbital. This weakens the bond and hence electrophile is getting generated. In the next step,the electrophile attacks the double bond [or double bond attacks the electrophile] and hence electrophile gets added across across the double bond

Why does the 3-membered cyclic transition state form?

The cyclic transition state is just a temporary relief for the electrophile that's just been added. The π molecular orbital donates electrons to the electrophile.

While the p-orbitals in the π-bond overlap with the electrophile, each p-orbital forms a σ-bond.

But would the same thing happen if I put any other electrophile?

Generally,electrophilic addition reactions take place through carbocation intermediate. But this reaction can take place through cyclic 3-membered intermediate,if the electrophile has a lone pair.

Apart from $\ce{Cl+,Br+}$ even $\ce{Hg^2+}$ (when the reagent is $\ce{Hg(OAc)2/H3O+}$) and $\ce{NO+}$ (when the reagent is $\ce{NOCl}$) can form the 3-membered cyclic intermediate.

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    $\begingroup$ This is my first answer.Can anyone verify whether what I have written is correct? Please suggest some tips on answering and tell what I can improve on,so that I can write better answers in the future.Thanks :) $\endgroup$ – Chem-Learner May 22 '20 at 8:40
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    $\begingroup$ Include references if you have them $\endgroup$ – Waylander May 22 '20 at 9:17
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    $\begingroup$ thanks at lot actually this helped :) $\endgroup$ – ganuwoahh May 22 '20 at 14:55
  • $\begingroup$ @Waylander,Thanks for the suggestion.Unfortunately,I don't have any such references for this.If you have them, you can add them so that the answer is complete.Thanks $\endgroup$ – Chem-Learner May 22 '20 at 18:41
  • $\begingroup$ @ganuwoahh Thanks for the feedback.It's nice to hear that it helped you :) $\endgroup$ – Chem-Learner May 22 '20 at 18:42

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