give mechanism of hydration
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This assumes that you know about the electrophilic addition reactions of ethene, and about the use of curly arrows in organic reaction mechanisms.
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.
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.
Note: This is a bit of a simplification. The concentrated phosphoric(V) acid probably contains some water, in addition to the water in the steam. It is likely that some of the hydrogen ions in this last step will be removed by water molecules, and possibly transferred to a dihydrogenphosphate(V) ion to make phosphoric(V) acid in a later step. Other hydrogen ions will be removed directly by the dihydrogenphosphate(V) ions as shown above. Don't worry about this. Your examiners will almost certainly not be looking for this sort of fine detail.
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.
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.
Note: This is a bit of a simplification. The concentrated phosphoric(V) acid probably contains some water, in addition to the water in the steam. It is likely that some of the hydrogen ions in this last step will be removed by water molecules, and possibly transferred to a dihydrogenphosphate(V) ion to make phosphoric(V) acid in a later step. Other hydrogen ions will be removed directly by the dihydrogenphosphate(V) ions as shown above. Don't worry about this. Your examiners will almost certainly not be looking for this sort of fine detail.
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