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Question:

Ethene reacts with $\ce{Cl2}$ in $\ce{H2O}$ to form A. A on reaction with $\ce{H2SO4}$ followed by heating gives B. B on reaction with alcoholic $\ce{KOH}$ gives C. Find C.

My answer is:

A: $\ce{CH2(OH)CH2Cl}$
B: $\ce{CH2=CHCl}$
C: ethyne.

Explanation:

Ethene on reaction with chlorine water undergoes addition reaction to give A. A on reaction with $\ce{H2SO4}$ followed by heating undergoes dehydration to give B. B on reaction with alcoholic $\ce{KOH}$, undergoes elimination to give ethyne.

But the answer given is $\ce{CH2=CH-O-CH=CH2}$.

Where I am going wrong and what are the products?

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  • $\begingroup$ I think diethyl ether is thermodynamically favourable because ethylene is not that stable. (less substituted alkene). Try this: google.co.in/… $\endgroup$ – Aditya Dev Jan 6 '16 at 8:22
  • $\begingroup$ I don't know about this question, feels way too prone to "my school syllabus teaches that this reaction occurs with conc. H2SO4/ethanolic KOH, so only this reaction happens". $\endgroup$ – orthocresol Jan 6 '16 at 10:02
  • $\begingroup$ @Aditya Dev thank you very much for replying but can you please show the mechanism of how it happens?(just tell me what happens step by step). $\endgroup$ – Ajay Sabarish Jan 6 '16 at 12:49
  • $\begingroup$ @orthocresol thank you very much for replying,do you mean to say only ethyne is formed? $\endgroup$ – Ajay Sabarish Jan 6 '16 at 12:50
  • $\begingroup$ It's the B product that's confusing. To get the required product, eliminate water from two molecules of A. Then alcoholic KOH will give the product that's given in your book. But, the more important part is "why intermolecular reaction takes place". $\endgroup$ – Aditya Dev Jan 6 '16 at 12:53
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The first reaction is a hydrochlorination reaction:

step 1

The second reaction is an acid catalyzed ether synthesis. The the solution is full of other 2-chloroethanol, which is capable of acting as a nucleophile in an $\mathrm{S_N2}$ reaction to eject $\ce{H2O}$. This is reaction will have a much lower energy barrier than the dehydration you proposed, which requires the extraction of an aliphatic hydrogen. The ether synthesis mechanism can be seen here:

step 2

The final step is a series of two $\mathrm{E2}$ reactions (shown condensed here):

$\hspace{4cm}$enter image description here

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