Question

Explain mechanism for hydration of acid catalyzed ethene:

Answer 1

The mechanism for the hydration of ethene

This assumes that you know about the electrophilic addition reactions of ethene, and about the use of curly arrows in organic reaction mechanisms. If you aren't happy about either of these follow the link below before you go any further.

Note: Follow this link for the electrophilic addition reactions of ethene. You will find a link explaining the use of curly arrows in mechanisms on that page if you need it.
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All the steps in the mechanism below are shown as one-way reactions because it makes the mechanism look less confusing. It doesn't affect the argument, but in fact all the steps are reversible.

Step 1

All of the hydrogen atoms in the phosphoric(V) acid are fairly positively charged because they are attached to a very electronegative oxygen atom.

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One of these hydrogens is strongly attracted to the carbon-carbon double bond. The pi part of the bond breaks and the electrons in it move down to make a new bond with the hydrogen atom. That forces the electrons in the hydrogen-oxygen bond down entirely onto the oxygen.



Note: It is easy to see why the oxygen carries a negative charge. It has gained full control over the electron pair in the original bond - so has acquired an extra electron which originally belonged to the hydrogen.
The carbon atom has a positive charge because one of the electrons in the pi bond originally belonged to it. It loses that electron when the pi bond breaks.


Step 2

The carbocation (carbonium ion) formed reacts with one of the lone pairs on a water molecule. A carbocation is one which carries a positive charge on a carbon atom.



Note: The easiest way of remembering that the oxygen has to carry a positive charge is that you are reacting a positive ion with a neutral molecule. That means that there must be a positive charge on the product somewhere.
The only way that an oxygen atom can be joined to three things at the same time is if it carries a positive charge. A positive charge gives the oxygen the same electronic structure as a nitrogen atom which can, of course, form three bonds.


Step 3

Finally, one of the hydrogens on the oxygen is removed by reaction with the dihydrogenphosphate(V) ion, H2PO4-, formed in the first step.



The phosphoric(V) acid catalyst has been regenerated