Optimum pH range for formation of Schiff bases

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What is the optimum range of pH for the formation of Schiff Bases by the addition of hydroxylamine to an aldehyde or a ketone?



According to Peter Sykes' A Guidebook to Mechanisms in Organic Chemistry,



"The initial addition step or the dehydration step can be made rate limiting at will, depending on the pH of the solution. At neutral and alkaline pHs it is generally dehydration, that is the slow and rate limiting step while at acidic pHs it is the initial addition which is slow and rate limiting. This clearly has significance in preparatory terms, and formation of such derivatives of carbonyl compounds tends to exhibit pH optima."



My doubts are :



  1. How does the pH change the rate limiting step? That is, why is dehydration the rate limiting step at neutral and alkaline pHs and why is addition the rate limiting step at acidic pHs?


  2. What is the significance in preparative terms that Sykes is taking about?


  3. How will this significance determine the pH and what is that pH (or the range of pHs)?






organic-chemistry reaction-mechanism ph







share|improve this question

























    up vote
    1
    down vote

    favorite












    What is the optimum range of pH for the formation of Schiff Bases by the addition of hydroxylamine to an aldehyde or a ketone?



    According to Peter Sykes' A Guidebook to Mechanisms in Organic Chemistry,



    "The initial addition step or the dehydration step can be made rate limiting at will, depending on the pH of the solution. At neutral and alkaline pHs it is generally dehydration, that is the slow and rate limiting step while at acidic pHs it is the initial addition which is slow and rate limiting. This clearly has significance in preparatory terms, and formation of such derivatives of carbonyl compounds tends to exhibit pH optima."



    My doubts are :



    1. How does the pH change the rate limiting step? That is, why is dehydration the rate limiting step at neutral and alkaline pHs and why is addition the rate limiting step at acidic pHs?


    2. What is the significance in preparative terms that Sykes is taking about?


    3. How will this significance determine the pH and what is that pH (or the range of pHs)?






    organic-chemistry reaction-mechanism ph







    share|improve this question























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      What is the optimum range of pH for the formation of Schiff Bases by the addition of hydroxylamine to an aldehyde or a ketone?



      According to Peter Sykes' A Guidebook to Mechanisms in Organic Chemistry,



      "The initial addition step or the dehydration step can be made rate limiting at will, depending on the pH of the solution. At neutral and alkaline pHs it is generally dehydration, that is the slow and rate limiting step while at acidic pHs it is the initial addition which is slow and rate limiting. This clearly has significance in preparatory terms, and formation of such derivatives of carbonyl compounds tends to exhibit pH optima."



      My doubts are :



      1. How does the pH change the rate limiting step? That is, why is dehydration the rate limiting step at neutral and alkaline pHs and why is addition the rate limiting step at acidic pHs?


      2. What is the significance in preparative terms that Sykes is taking about?


      3. How will this significance determine the pH and what is that pH (or the range of pHs)?






      organic-chemistry reaction-mechanism ph







      share|improve this question













      What is the optimum range of pH for the formation of Schiff Bases by the addition of hydroxylamine to an aldehyde or a ketone?



      According to Peter Sykes' A Guidebook to Mechanisms in Organic Chemistry,



      "The initial addition step or the dehydration step can be made rate limiting at will, depending on the pH of the solution. At neutral and alkaline pHs it is generally dehydration, that is the slow and rate limiting step while at acidic pHs it is the initial addition which is slow and rate limiting. This clearly has significance in preparatory terms, and formation of such derivatives of carbonyl compounds tends to exhibit pH optima."



      My doubts are :



      1. How does the pH change the rate limiting step? That is, why is dehydration the rate limiting step at neutral and alkaline pHs and why is addition the rate limiting step at acidic pHs?


      2. What is the significance in preparative terms that Sykes is taking about?


      3. How will this significance determine the pH and what is that pH (or the range of pHs)?






      organic-chemistry reaction-mechanism ph




      organic-chemistry reaction-mechanism ph






      share|improve this question













      share|improve this question











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      asked 2 hours ago









      Anubhab Das

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          1 Answer
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          1. Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate.


          2. Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.


          3. The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.






          share|improve this answer




















          • Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
            – Anubhab Das
            16 mins ago










          Your Answer





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          active

          oldest

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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          3
          down vote



          accepted










          1. Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate.


          2. Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.


          3. The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.






          share|improve this answer




















          • Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
            – Anubhab Das
            16 mins ago














          up vote
          3
          down vote



          accepted










          1. Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate.


          2. Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.


          3. The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.






          share|improve this answer




















          • Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
            – Anubhab Das
            16 mins ago












          up vote
          3
          down vote



          accepted







          up vote
          3
          down vote



          accepted






          1. Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate.


          2. Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.


          3. The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.






          share|improve this answer












          1. Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate.


          2. Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.


          3. The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 2 hours ago









          Waylander

          4,9321719




          4,9321719











          • Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
            – Anubhab Das
            16 mins ago
















          • Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
            – Anubhab Das
            16 mins ago















          Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
          – Anubhab Das
          16 mins ago




          Can you tell me how to find the relevant literature? I am new to this and can't seem to find any papers or any book on the web that carries the information I need. I specifically require the pH when the substrate is acetaldehyde.
          – Anubhab Das
          16 mins ago

















           

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