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    Hi,

    I was looking at the alkaline hydrolysis of polyesters with NaOH. Previously i had looked at the alkaline hydrolysis of esters with NaOH and established that when dissolved the OH- ion was a powerful enough nucleophile to attack the ester and break it up into the alcohol and sodium salt of the carboxylic group, Hence i dont see how water comes in...

    So looking at polyesters, my book suggests the conditions for alkaline hydrolysis as NaOH/H20. But surely its the same case as before when the Oh- ion will attack the carbon. The only thing i can think of is water being used as a solvent to break up the NaOh but then again could you not just use another solvent?

    Thanks for your help
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    (Original post by 111davey1)
    Hi,

    I was looking at the alkaline hydrolysis of polyesters with NaOH. Previously i had looked at the alkaline hydrolysis of esters with NaOH and established that when dissolved the OH- ion was a powerful enough nucleophile to attack the ester and break it up into the alcohol and sodium salt of the carboxylic group, Hence i dont see how water comes in...

    So looking at polyesters, my book suggests the conditions for alkaline hydrolysis as NaOH/H20. But surely its the same case as before when the Oh- ion will attack the carbon. The only thing i can think of is water being used as a solvent to break up the NaOh but then again could you not just use another solvent?

    Thanks for your help
    Hydrolysis means breaking apart with water.

    The hydroxide ions increase the rate of hydrolysis.
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    (Original post by charco)
    Hydrolysis means breaking apart with water.

    The hydroxide ions increase the rate of hydrolysis.
    Thanks, the mechanism i saw though for the alkaline hydrolysis of esters showed a hydroxide ion attacking the carbon. The tetrahedral intermediate formed then collapsed which ultimatly broke the bond.

    With acid hydrolysis the acid speeds up the rate because it protonates the ester making it more electrophilic so the weak nucleophile (H20) can attack it.

    im just wondering how water is involved in the alkaline hydrolysis of esters and polyesters because from the mechanism it doesnt seem to be involved, but from my book it insists that what you need is NaOh/H20.

    This is what i found on a website:

    Technically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water or by dilute acids such as dilute hydrochloric acid.

    The alkaline hydrolysis of esters actually involves reaction with hydroxide ions, but the overall result is so similar that it is lumped together with the other two.
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    (Original post by 111davey1)
    Thanks, the mechanism i saw though for the alkaline hydrolysis of esters showed a hydroxide ion attacking the carbon. The tetrahedral intermediate formed then collapsed which ultimatly broke the bond.

    With acid hydrolysis the acid speeds up the rate because it protonates the ester making it more electrophilic so the weak nucleophile (H20) can attack it.

    im just wondering how water is involved in the alkaline hydrolysis of esters and polyesters because from the mechanism it doesnt seem to be involved, but from my book it insists that what you need is NaOh/H20.

    This is what i found on a website:

    Technically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water or by dilute acids such as dilute hydrochloric acid.

    The alkaline hydrolysis of esters actually involves reaction with hydroxide ions, but the overall result is so similar that it is lumped together with the other two.
    You can argue that the nucleophile is water in all cases.

    H-O-H <==> H+ + OH-

    As the lone pair on the oxygen attacks the carbon of the carbonyl group one hydrogen is lost.

    This is clearly slower than when you increase the concentration of hydroxide ions.
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    (Original post by 111davey1)
    Thanks, the mechanism i saw though for the alkaline hydrolysis of esters showed a hydroxide ion attacking the carbon. The tetrahedral intermediate formed then collapsed which ultimatly broke the bond.

    With acid hydrolysis the acid speeds up the rate because it protonates the ester making it more electrophilic so the weak nucleophile (H20) can attack it.

    im just wondering how water is involved in the alkaline hydrolysis of esters and polyesters because from the mechanism it doesnt seem to be involved, but from my book it insists that what you need is NaOh/H20.

    This is what i found on a website:

    Technically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water or by dilute acids such as dilute hydrochloric acid.

    The alkaline hydrolysis of esters actually involves reaction with hydroxide ions, but the overall result is so similar that it is lumped together with the other two.
    As Charco has said, the hydroxide is in equilibrium with the water through proton exchange.

    The hydroxide does the attacking because it is the most nucleophilic (more nucleophilic than water certainly)

    the reagents will be listed as NaOH/H2O because you must carry out the hydrolysis in water.

    Using another solvent could cause issues with getting the right product.

    Any suitable solvent would have to be able to stabilise OH- and Na+ ions so you'd have to use something polar, and ideally something that can hydrogen bond.

    An obvious choice might be to use an alcohol solvent but this causes a couple of problems.

    lets say the ester is ethyl ethanoate. If you chose methanol as the solvent you'd set up the equilibrium
    CH3OH + OH- <---> CH3O- + H2O

    and seeing as the methanol is in huge excess the equilibrium would be pulled all the way to the right, so you wouldn't get hydrolysis, you'd just end up turning your ethyl ethanoate into methyl ethanoate!

    If you were to choose ethanol solvent then the same problem would arise, you get rapid exchange of the ethoxide group, but no hydrolysis (because the alcohol, in huge excess, would always substitute first).

    If you chose a non alcoholic solvent, chances are, if it could dissolve NaOH and stabilise the resulting ions, it would probably be a nucleophile and would spoil your chances of hydrolysing anything.

    So the NaOH increases the rate of hydrolysis because it is more nucleophilic, but the water as solvent is crucial in achieving the right product!

    Sorry for the long explanation, hope that makes sense!
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    (Original post by MexicanKeith)
    As Charco has said, the hydroxide is in equilibrium with the water through proton exchange.

    The hydroxide does the attacking because it is the most nucleophilic (more nucleophilic than water certainly)

    the reagents will be listed as NaOH/H2O because you must carry out the hydrolysis in water.

    Using another solvent could cause issues with getting the right product.

    Any suitable solvent would have to be able to stabilise OH- and Na+ ions so you'd have to use something polar, and ideally something that can hydrogen bond.

    An obvious choice might be to use an alcohol solvent but this causes a couple of problems.

    lets say the ester is ethyl ethanoate. If you chose methanol as the solvent you'd set up the equilibrium
    CH3OH + OH- <---> CH3O- + H2O

    and seeing as the methanol is in huge excess the equilibrium would be pulled all the way to the right, so you wouldn't get hydrolysis, you'd just end up turning your ethyl ethanoate into methyl ethanoate!

    If you were to choose ethanol solvent then the same problem would arise, you get rapid exchange of the ethoxide group, but no hydrolysis (because the alcohol, in huge excess, would always substitute first).

    If you chose a non alcoholic solvent, chances are, if it could dissolve NaOH and stabilise the resulting ions, it would probably be a nucleophile and would spoil your chances of hydrolysing anything.

    So the NaOH increases the rate of hydrolysis because it is more nucleophilic, but the water as solvent is crucial in achieving the right product!

    Sorry for the long explanation, hope that makes sense!
    Thanks for your time.
    So i can think of hydrolysis as breaking down of a compound in water which can be catalysed by adding H+ and OH- ions, even though the H+ and OH- ions go about that in different ways.
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    (Original post by 111davey1)
    Thanks for your time.
    So i can think of hydrolysis as breaking down of a compound in water which can be catalysed by adding H+ and OH- ions, even though the H+ and OH- ions go about that in different ways.
    That'd be a good way of looking at it


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