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    Hi!

    I have to design a chem lab on nucleophilic substitution. I've got the basic idea [I'm going to react a bromoalkane with OH- ion.] but i need to be able to measure the rate of reaction in terms of the bromide ion. My teacher hinted that i should look at the test for bromine and then figure out how to work out the concentration?? I know that bromide reacts with silver nitrate to produce silver bromide as a precipitate but how does that help you measure the rate of a reaction?? Any ideas? or any other way in which i can measure the rate of reaction??
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    (Original post by sillynugget)
    Hi!

    I have to design a chem lab on nucleophilic substitution. I've got the basic idea [I'm going to react a bromoalkane with OH- ion.] but i need to be able to measure the rate of reaction in terms of the bromide ion. My teacher hinted that i should look at the test for bromine and then figure out how to work out the concentration?? I know that bromide reacts with silver nitrate to produce silver bromide as a precipitate but how does that help you measure the rate of a reaction?? Any ideas? or any other way in which i can measure the rate of reaction??
    Well imagine you have differently branced bromoalkane, ie isomers, this could affect the rate at which the nucleophile can come in and attack the bromine(either sn1 or sn2). The precipitate is formed because the Br- is displaced. So the time taken for the ppt to form can be related to the rate at which the Br on the bromoalkane is being displaced.
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    (Original post by shengoc)
    Well imagine you have differently branced bromoalkane, ie isomers, this could affect the rate at which the nucleophile can come in and attack the bromine(either sn1 or sn2). The precipitate is formed because the Br- is displaced. So the time taken for the ppt to form can be related to the rate at which the Br on the bromoalkane is being displaced.
    ok...but arent the NO3- ions going to interefere with the reaction?? I assume that for what you said to work you have to have the bromoalkane, sodium hydroxide (thats the source i'm using for the OH ion) and silver nitrate? So if they're in the same reaction, isnt it possible for NO3- to bond with the alkane instead of OH-?? And if you add silver nitrate later, then the Bromide (or well i suppose NaBr) is already present? so adding that would only give you the rate of reaction between silver nitrate and bromide right? I'm not very good at chemistry so i dont know? maybe i'm talking utter rubbish! :P

    and also, its not a very quantitative method?? ...IS there a quantitative method??

    i thought of using phenolphalein as an indicator for OH- ions and then over the course of the reaction, the colour should dissapear right? and then you could use a colorimeter to get the rate of reaction in terms of OH-?? but would that even work?? anyway, i want to know what my teacher was thinking of and she's not really allowed to tell me anything else
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    (Original post by sillynugget)
    ok...but arent the NO3- ions going to interefere with the reaction?? I assume that for what you said to work you have to have the bromoalkane, sodium hydroxide (thats the source i'm using for the OH ion) and silver nitrate? So if they're in the same reaction, isnt it possible for NO3- to bond with the alkane instead of OH-?? And if you add silver nitrate later, then the Bromide (or well i suppose NaBr) is already present? so adding that would only give you the rate of reaction between silver nitrate and bromide right? I'm not very good at chemistry so i dont know? maybe i'm talking utter rubbish! :P

    and also, its not a very quantitative method?? ...IS there a quantitative method??

    i thought of using phenolphalein as an indicator for OH- ions and then over the course of the reaction, the colour should dissapear right? and then you could use a colorimeter to get the rate of reaction in terms of OH-?? but would that even work?? anyway, i want to know what my teacher was thinking of and she's not really allowed to tell me anything else
    True, but will it be thermodynamically favourable? formation of a C-N bond is less favourable than forming a new C-O bond. O has smaller size than N, so C-O bond is shorter, hence a stronger and more favoured to be formed.

    Another thing to consider is the nucleophile. True, you can say that NO3- could compete with OH- as the nucleophile, but OH- is definitely a better nucleophile as it is smaller in size than NO3-, remember N has three oxygen flanking the side, and to act as a nucleophile, OH- is more favoured(steric reason).

    Furthermore, since you are to use same amount of AgNO3 each time, you are keeping this constant, so essentially, you can assume that the effect of NO3- is the same in every experiments, and hence, its effect can be neglected.

    Oh the time counted seems quantitative enough. With the time, you can do so much, trust me. Do this at least two times for the same concentration; you can then change concentration and also change the substrate(reactants), then you get a lot of graph by exploiting the time values in rate equation graphs. do also at different temperatures, you can get activation energy. by the way, there is bound to be errors which you ought to discuss in your report.

    No indicator is required, ppt of AgBr is quite easily seen. AgNO3 is colourless.
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    (Original post by shengoc)

    Oh the time counted seems quantitative enough. With the time, you can do so much, trust me. Do this at least two times for the same concentration; you can then change concentration and also change the substrate(reactants), then you get a lot of graph by exploiting the time values in rate equation graphs. do also at different temperatures, you can get activation energy. by the way, there is bound to be errors which you ought to discuss in your report.
    lol i didnt mean to say that the time counted would be completely useless...just that i was under the impression that you woud have to know how much of the precipitate was formed at regular time intervals?? so do you like sieve the precipitate out to get the mass of ppt formed with time??

    and my experiment's basically to do with effect of type of solvent used on the rate of SN1 and SN2 reactions? with water being the protic solvent and acetone the aprotic.
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    (Original post by sillynugget)
    lol i didnt mean to say that the time counted would be completely useless...just that i was under the impression that you woud have to know how much of the precipitate was formed at regular time intervals?? so do you like sieve the precipitate out to get the mass of ppt formed with time??

    and my experiment's basically to do with effect of type of solvent used on the rate of SN1 and SN2 reactions? with water being the protic solvent and acetone the aprotic.
    hmm, there might be a bit of subtlety here, let say you use primary, secondary, tertiary isomers of a particular halogenoalkane. very commonly, primary one undergoes sn2, while tertiary one favours sn1, but secondary one is the subtle one, you might have to refer to your experimental results to confirm if it is sn1 or sn2, and you can discuss little bit of this theoretically too.

    I suppose you could measure out the mass of ppt produced, but as it is a kinetics experiment, working is time is really great way of simplification. I am giving an example here, say you record the time for reaction is the time between adding in your reactants and the time you see the first sign of ppt being formed. And you would have to do the same for all your other experiments, and not record the time till no more ppt is being formed. This is basically just to make sure you do the same procedure to all your experimental results, so the results are consistent and their values can be compared.
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    (Original post by shengoc)
    hmm, there might be a bit of subtlety here, let say you use primary, secondary, tertiary isomers of a particular halogenoalkane. very commonly, primary one undergoes sn2, while tertiary one favours sn1, but secondary one is the subtle one, you might have to refer to your experimental results to confirm if it is sn1 or sn2, and you can discuss little bit of this theoretically too.

    I suppose you could measure out the mass of ppt produced, but as it is a kinetics experiment, working is time is really great way of simplification. I am giving an example here, say you record the time for reaction is the time between adding in your reactants and the time you see the first sign of ppt being formed. And you would have to do the same for all your other experiments, and not record the time till no more ppt is being formed. This is basically just to make sure you do the same procedure to all your experimental results, so the results are consistent and their values can be compared.
    lol yh i was just going to use primary and tertiary at first but then my teacher said that i should also consider secondary :P
    so what you're saying is that you'll have two records of time - one when the first signs of ppt appear and the second when the last one appears?? and then you compare the data with the other experiments?? and when you do the graphs, it'll be a bar graph instead of a line graph (i've only ever done rate of reaction labs twice before and they were both concentration against time line graphs)

    or like...you just take one reading - when ppt starts forming and make different line graphs and compare the gradient of the lines to find out which one was a faster reaction??
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    (Original post by sillynugget)
    lol yh i was just going to use primary and tertiary at first but then my teacher said that i should also consider secondary :P
    so what you're saying is that you'll have two records of time - one when the first signs of ppt appear and the second when the last one appears?? and then you compare the data with the other experiments?? and when you do the graphs, it'll be a bar graph instead of a line graph (i've only ever done rate of reaction labs twice before and they were both concentration against time line graphs)

    or like...you just take one reading - when ppt starts forming and make different line graphs and compare the gradient of the lines to find out which one was a faster reaction??
    No, I am saying if you choose to record the time till a ppt first forms, then maintain the same style of recording the time with all your other experiments.

    If however you choose to record the time from adding reactants together till no more ppt is produced, this is difficult, since you already have ppt, how can you tell that no more ppt is being produced, plus there is no gas evolved in these sn1/sn2 reactions, so forget about this method.
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    If you wish to investigate the rate of reaction you must find a way to monitor the progress. For this you have to consider the process occurring:

    R-X + H2O --> ROH + HX

    Look at the equation...

    As the reaction proceeds there will be:
    1. an increase in acidity
    2. An increase in the concentration of ions

    You could follow the formation of acid by quenching with excess base and then titrating the excess against standard acid. This would be carried out by extracting samples at different times.

    You could monitor the ion concentration increase electronically by measurement of the conductivity of the solution, once again periodically.

    Forget precipitates. - they will not allow you to monitor the reaction and will only give qualitative results.
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    (Original post by charco)
    If you wish to investigate the rate of reaction you must find a way to monitor the progress. For this you have to consider the process occurring:

    R-X + H2O --> ROH + HX

    Look at the equation...

    As the reaction proceeds there will be:
    1. an increase in acidity
    2. An increase in the concentration of ions

    You could follow the formation of acid by quenching with excess base and then titrating the excess against standard acid. This would be carried out by extracting samples at different times.

    You could monitor the ion concentration increase electronically by measurement of the conductivity of the solution, once again periodically.

    Forget precipitates. - they will not allow you to monitor the reaction and will only give qualitative results.
    Hmm, i thought i nailed it right, but clearly your idea is much better, wonder why i didn't think of it.
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    (Original post by charco)

    You could follow the formation of acid by quenching with excess base and then titrating the excess against standard acid. This would be carried out by extracting samples at different times.

    You could monitor the ion concentration increase electronically by measurement of the conductivity of the solution, once again periodically.

    Forget precipitates. - they will not allow you to monitor the reaction and will only give qualitative results.
    i did think of that (sort of :P) but wouldnt it matter that the experiment is still going on in the little sampels that you extract??

    or do you think it would be better to measure the changes in pH using a pH meter??
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    (Original post by sillynugget)
    i did think of that (sort of :P) but wouldnt it matter that the experiment is still going on in the little sampels that you extract??
    This would be a very small error, depending on the overall rate of the reaction. If it's a slow reaction then no problem. You could use this as part of the evaluation.

    or do you think it would be better to measure the changes in pH using a pH meter??
    Depends on the sensitivity. You may find that the pH changes by only small quantities.

    Experimentation is all about trying things out.
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    (Original post by shengoc)
    Hmm, i thought i nailed it right, but clearly your idea is much better, wonder why i didn't think of it.
    [QUOTE = charco]
    Experimentation is all about trying things out. [/QUOTE]

    ok cool...thanks guys!! I will suggest the ideas to my teacher and see which one would be more feasible given the lab equipments that we have and stuff

    Thanks for your help!!!
 
 
 
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