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OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD watch

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    (Original post by Noura*)
    Hi guys, hope revision is going well.
    I have a question, I hope someone can help me.

    From the Jan 12 F324 paper, in Q2, they ask how we can make 4-chlorophenlyamine from benzene. So I said we can make nitrobenzene, reduce it to get phenyl amine and then just react it with Cl-2. The mark scheme says you can't react it with Cl2, rather you have to do HCL and tin (???), why is this?
    The tin and concentrated HCl is used to reduce nitrobenzene to phenylamine. You also need a halogen carrier (e.g. AlCl3) to react benzene/phenylamine with Cl2.
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    (Original post by Noura*)
    Hi guys, hope revision is going well.
    I have a question, I hope someone can help me.

    From the Jan 12 F324 paper, in Q2, they ask how we can make 4-chlorophenlyamine from benzene. So I said we can make nitrobenzene, reduce it to get phenyl amine and then just react it with Cl-2. The mark scheme says you can't react it with Cl2, rather you have to do HCL and tin (???), why is this?
    You need the HCl and tin to reduce the nitrobenzene to phenylamine

    To add the chlorine we have to use a halogen carrier
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    Name:  Screen Shot 2016-06-10 at 13.50.31.png
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    thanks for your replies

    Maybe I don't understand the m.s but I mean after you've reduced nitrobenzene
    using tin and conc HCL why do you use it again to substitute in Cl? And also, bcos you have NH2 there already electron density is increased so you wouldn't need a halogen carrier I assumed. I used sequence 2
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    Any halogen that's added to a Benzene ring needs a halogen carrier. The NH2 isn't enough to polarise the chlorine, so although the density has increased, to give it that extra polarity, a halogen carrier is required which will accept a lone of electrons to increase polarity even more.

    I also forgot to mention, you're adding the Cl2 to the other side of the benzene ring, the one that doesn't have the Amine group. Which also means that side has a reduced electron density...so a halogen carrier is needed.
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    (Original post by Noura*)
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    thanks for your replies

    Maybe I don't understand the m.s but I mean after you've reduced nitrobenzene
    using tin and conc HCL why do you use it again to substitute in Cl? And also, bcos you have NH2 there already electron density is increased so you wouldn't need a halogen carrier I assumed. I used sequence 2
    Ah I see. The answers for stage three are the other way round.

    And yes, NH2 does increase the electron density but the question was asked on the basis that we didn't know that (it's not on the spec afaik)
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    (Original post by Serine Soul)
    Ah I see. The answers for stage three are the other way round.

    And yes, NH2 does increase the electron density but the question was asked on the basis that we didn't know that (it's not on the spec afaik)
    There was actually a question recently on a paper that asked about it (Jan 2013)! They did say that the phenylamine behaved similarly to phenol though, so I guess that you didn't really have to know exactly what was going on.
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    (Original post by Serine Soul)
    Ah I see. The answers for stage three are the other way round.

    And yes, NH2 does increase the electron density but the question was asked on the basis that we didn't know that (it's not on the spec afaik)
    Oh right, thank you!
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    (Original post by kawehi)
    There was actually a question recently on a paper that asked about it (Jan 2013)! They did say that the phenylamine behaved similarly to phenol though, so I guess that you didn't really have to know exactly what was going on.
    Yeah, I was thinking about that!
    Just ignore the increased electron density in benzene ring due to amine everyone, we're not really supposed to know it and it may just leave you majorly confused in the exam
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    How would you guys go about answering this question? I kind of get the mark scheme but I don't think that I would have gotten there in the actual exam! (June 12 q4b)
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    (Original post by kawehi)
    How would you guys go about answering this question? I kind of get the mark scheme but I don't think that I would have gotten there in the actual exam! (June 12 q4b)
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    I didn't get that the first time I did it but looking at it now, isn't it simply being oxidised to a carboxylate ion and reduced to an alcohol?

    Can't remember the mark schemr myself now
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    (Original post by kawehi)
    How would you guys go about answering this question? I kind of get the mark scheme but I don't think that I would have gotten there in the actual exam! (June 12 q4b)
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    Would I be right in saying you would just form C6H5COO–K+.... i can't see the second product...
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    (Original post by Serine Soul)
    I didn't get that the first time I did it but looking at it now, isn't it simply being oxidised to a carboxylate ion and reduced to an alcohol?

    Can't remember the mark schemr myself now
    Yeah, it is! Are we supposed to know that KOH would do that, though?
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    (Original post by kawehi)
    Yeah, it is! Are we supposed to know that KOH would do that, though?
    I don't think we're supposed to know it

    I guess all we had to do is see it being 'oxidised' and 'reduced' and show the compounds formed

    Strange question though. I never would have got that in the exam
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    (Original post by RayMasterio)
    Would I be right in saying you would just form C6H5COO–K+.... i can't see the second product...
    Also C6H5CH2OH so that you get a reduced product!

    (Original post by Serine Soul)
    I don't think we're supposed to know it

    I guess all we had to do is see it being 'oxidised' and 'reduced' and show the compounds formed

    Strange question though. I never would have got that in the exam
    Ah ok, weird.. I was wondering if there was something in the spec I'd completely missed
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    (Original post by kawehi)
    Also C6H5CH2OH so that you get a reduced product!



    Ah ok, weird.. I was wondering if there was something in the spec I'd completely missed
    Just out of interest what would be the mechanism for this reaction? I can see the products formed and that makes sense but I don't entirely understand how they got there.
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    (Original post by ForeverDay)
    Just out of interest what would be the mechanism for this reaction? I can see the products formed and that makes sense but I don't entirely understand how they got there.
    For the reduction of the aldehyde to the alcohol, that'd be nucleophilic substitution with sodium borohydride followed by water. I don't think we've learnt a mechanism for oxidation but it'd just be a standard oxidation reaction of an aldehyde to a carboxylic acid.

    Posted from TSR Mobile
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    (Original post by pineneedles)
    For the reduction of the aldehyde to the alcohol, that'd be nucleophilic substitution with sodium borohydride followed by water. I don't think we've learnt a mechanism for oxidation but it'd just be a standard oxidation reaction of an aldehyde to a carboxylic acid.

    Posted from TSR Mobile
    Oh okay, thank you. 😀
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    (Original post by ForeverDay)
    Just out of interest what would be the mechanism for this reaction? I can see the products formed and that makes sense but I don't entirely understand how they got there.
    I just googled it and apparently it's called the Cannizzaro reaction? There's a mechanism on the wikipedia page
    https://en.wikipedia.org/wiki/Cannizzaro_reaction

    (Original post by pineneedles)
    ...
    In the original question, the only reactant (besides benzaldehyde) was KOH, so there weren't any other oxidising/reducing agents!
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    (Original post by kawehi)
    I just googled it and apparently it's called the Cannizzaro reaction? There's a mechanism on the wikipedia page
    https://en.wikipedia.org/wiki/Cannizzaro_reaction



    In the original question, the only reactant (besides benzaldehyde) was KOH, so there weren't any other oxidising/reducing agents!
    Thanks, I did try google it but I couldn't find it so that was v helpful.
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    (Original post by ForeverDay)
    Just out of interest what would be the mechanism for this reaction? I can see the products formed and that makes sense but I don't entirely understand how they got there.
    Definitely wouldn't worry about mechanisms for something like that, it'd probably be something incredibly complex anyway but for those 'suggest' kind of questions I think you're just supposed to apply your knowledge to something you haven't seen/don't explicitly know to answer it without understanding what's going on
 
 
 
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