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    The rate determining step in the mechanism in part b (ii) involves attack by the nucleophile , suggest how the rate of reaction of propanone with HCN would compare with the rate of reaction of propanal with HCN

    part b ii was just nu addition of hcn to propanal to form 2 hydroxy butanitrile .

    I dont understand this question? any help
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    Just quoting in Danny Dorito so she can move the thread if needed :wizard:
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    (Original post by bobwibbles)
    The rate determining step in the mechanism in part b (ii) involves attack by the nucleophile , suggest how the rate of reaction of propanone with HCN would compare with the rate of reaction of propanal with HCN

    part b ii was just nu addition of hcn to propanal to form 2 hydroxy butanitrile .

    I dont understand this question? any help


    Part b (ii) gives you light on how the reaction normally works. The Nucleophile (HCN) attacks the C of the C=O group...

    Now compare the two structures of propanone and propanal, what do you notice is different?

    Spoiler:
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    Look around the C=O group cos that's the important bit of the molecule. Notice how propanone has two methyl groups surrounding it, whereas propanal has an ethyl and a hydrogen atom


    Spoiler:
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    What do you know about alkyl groups in terms of affecting the reactivity of compounds?


    HINT
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    Alkyl groups = positive inductive effect.. sound familiar?

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    (Original post by telepathicnine)


    Part b (ii) gives you light on how the reaction normally works. The Nucleophile (HCN) attacks the C of the C=O group...

    Now compare the two structures of propanone and propanal, what do you notice is different?

    Spoiler:
    Show


    Look around the C=O group cos that's the important bit of the molecule. Notice how propanone has two methyl groups surrounding it, whereas propanal has an ethyl and a hydrogen atom



    Spoiler:
    Show


    What do you know about alkyl groups in terms of affecting the reactivity of compounds?



    HINT
    Spoiler:
    Show


    Alkyl groups = positive inductive effect.. sound familiar?


    Tips for organic chemistry? I am really struggling :/
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    (Original post by shohaib712)
    Tips for organic chemistry? I am really struggling :/
    Is there something you're struggling specifically?

    Perhaps the steps in certain reaction mechanisms? Remembering which mechanism is which? etc
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    (Original post by telepathicnine)
    Is there something you're struggling specifically?

    Perhaps the steps in certain reaction mechanisms? Remembering which mechanism is which? etc
    Yes!
    remembering Mechanisms and general revision for that module

    +

    I did the 2016 ocr paper (breath) as a school moc and found it extremely dificult, I didn;t get the calculation questions (they had percentages etc..) which were really different to the past paper questions :/
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    (Original post by shohaib712)
    Yes!
    remembering Mechanisms and general revision for that module

    +

    I did the 2016 ocr paper (breath) as a school moc and found it extremely dificult, I didn;t get the calculation questions (they had percentages etc..) which were really different to the past paper questions :/
    There's several ways you can go about remembering mechanisms... the one I did it and I think would really work is ONE BIG FAT MINDMAP/NETWORK

    Lay out all the organic molecules you know: alcohols, amines, esters, ketones, benzenes, alkanes, allthatsht

    You can then begin to draw links between the molecules to understand how to get from one to the other, e.g. alcohol --> aldehyde --> carboxylic acid

    something like this
    https://www.thestudentroom.co.uk/att...0&d=1337182423


    And then once you have that, I would just memorise that everyday. The map will also help in those questions where they're like "Describe a series of reactions that turn A into B"


    When it comes to the actual steps of each mechanism, I think actually understanding it will help. the way I learned was
    1. examine what the reacting agents look like (look for electron pairs, look for reactive groups to give you an idea of what would happen)
    2. draw some arrows and just logically work out what should happen
    3. compare to what it says in the pathway and if you got the end molecule, bravo
    4. then you write the name

    For instance.


    Name:  generalm1.GIF
Views: 152
Size:  1.0 KB

    Spoiler:
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    I can see this nucleophile has a lone pair of electrons so it's gonna be attracted to something that's positive. I can see that in this acyl chloride, the carbon atom has a positive charge (cos the oxygen and chlorine attracts the electrons from it)

    so... it makes sense to have the nucleophile electrons go to that carbon. Those electrons in that double bond will get repelled and so the oxygen has a negative charge. The nucleophile made an extra bond, so it's now positively charged..



    ^^ It's just that thought process which I think if you really understand helps to consolidate the mechanism. And then you could probably work it out yourself..
    Spoiler:
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    for instance, I start thinking that the H atom is gotta be removed somehow, and something has to happen to that oxygen atom with the charge...




    Once all that's in place, it's just a matter of practicing with a **** TON of questions. Seriously, organic chemistry is almost like maths: essentially the steps are the same, all that's changed is the species.
    And it doesn't matter what exam board those questions came from. I just went on physicsandmathstutor, found so many questions on organic chemistry and did them all. They might have been the wrong exam board but it doesn't matter because the actual chemistry is the same

    http://www.physicsandmathstutor.com/chemistry-revision/

    It might not work for you. I know people who did songs to memorise what each of them was, some even acted out the mechanism by pretending to be the molecule.

    Spoiler:
    Show


    Also, don't be thrown off by any complex looking stuff. You've probably done nucleophilic addition of aldehydes.

    So if you get some ****er like this..


    don't be thrown off, you can spot that there's a carbonyl group which gets attacked by nucleophiles, nothing's changed.

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    (Original post by telepathicnine)
    There's several ways you can go about remembering mechanisms... the one I did it and I think would really work is ONE BIG FAT MINDMAP/NETWORK

    Lay out all the organic molecules you know: alcohols, amines, esters, ketones, benzenes, alkanes, allthatsht

    You can then begin to draw links between the molecules to understand how to get from one to the other, e.g. alcohol --> aldehyde --> carboxylic acid

    something like this
    https://www.thestudentroom.co.uk/att...0&d=1337182423


    And then once you have that, I would just memorise that everyday. The map will also help in those questions where they're like "Describe a series of reactions that turn A into B"


    When it comes to the actual steps of each mechanism, I think actually understanding it will help. the way I learned was
    1. examine what the reacting agents look like (look for electron pairs, look for reactive groups to give you an idea of what would happen)
    2. draw some arrows and just logically work out what should happen
    3. compare to what it says in the pathway and if you got the end molecule, bravo
    4. then you write the name

    For instance.


    Name:  generalm1.GIF
Views: 152
Size:  1.0 KB

    Spoiler:
    Show




    I can see this nucleophile has a lone pair of electrons so it's gonna be attracted to something that's positive. I can see that in this acyl chloride, the carbon atom has a positive charge (cos the oxygen and chlorine attracts the electrons from it)

    so... it makes sense to have the nucleophile electrons go to that carbon. Those electrons in that double bond will get repelled and so the oxygen has a negative charge. The nucleophile made an extra bond, so it's now positively charged..




    ^^ It's just that thought process which I think if you really understand helps to consolidate the mechanism. And then you could probably work it out yourself..
    Spoiler:
    Show




    for instance, I start thinking that the H atom is gotta be removed somehow, and something has to happen to that oxygen atom with the charge...





    Once all that's in place, it's just a matter of practicing with a **** TON of questions. Seriously, organic chemistry is almost like maths: essentially the steps are the same, all that's changed is the species.
    And it doesn't matter what exam board those questions came from. I just went on physicsandmathstutor, found so many questions on organic chemistry and did them all. They might have been the wrong exam board but it doesn't matter because the actual chemistry is the same

    http://www.physicsandmathstutor.com/chemistry-revision/

    It might not work for you. I know people who did songs to memorise what each of them was, some even acted out the mechanism by pretending to be the molecule.

    Spoiler:
    Show



    Also, don't be thrown off by any complex looking stuff. You've probably done nucleophilic addition of aldehydes.

    So if you get some ****er like this..


    don't be thrown off, you can spot that there's a carbonyl group which gets attacked by nucleophiles, nothing's changed.


    Lol thanks man!
    Do you need to know the equations of the reactions as well (or just the diagrams?)
    How would you draw the sigma/pi bond in alkenes/alkanes if you were asked in a question?
    And Why is there only 1[O] in alcohol to aldehyde reaction but 2[O] to form a carboxylic acid?
    Do we need to know about hydrogenation/cracking in the new spec?
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    (Original post by telepathicnine)


    Part b (ii) gives you light on how the reaction normally works. The Nucleophile (HCN) attacks the C of the C=O group...

    Now compare the two structures of propanone and propanal, what do you notice is different?

    Spoiler:
    Show


    Look around the C=O group cos that's the important bit of the molecule. Notice how propanone has two methyl groups surrounding it, whereas propanal has an ethyl and a hydrogen atom



    Spoiler:
    Show


    What do you know about alkyl groups in terms of affecting the reactivity of compounds?



    HINT
    Spoiler:
    Show


    Alkyl groups = positive inductive effect.. sound familiar?



    Yes that does sound familiar.
    So the alkyl groups release electrons stabilising the centre carbon in propanone or is that less as effective as an ethyl on the centre carbon? And the more electron stable thr carbon the slower the reaction or?

    Sorry im not so good with this atm.
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    (Original post by shohaib712)
    Lol thanks man!
    Do you need to know the equations of the reactions as well (or just the diagrams?)
    How would you draw the sigma/pi bond in alkenes/alkanes if you were asked in a question?
    And Why is there only 1[O] in alcohol to aldehyde reaction but 2[O] to form a carboxylic acid?
    Do we need to know about hydrogenation/cracking in the new spec?
    You can derive the equations of the reactions from the diagram, IF you understand how structural formula works... when you do the mechanism, you tend to draw the structure in displayed formula which makes it easier to visualise. Hence, draw the structural formula from that.
    Spoiler:
    Show


    Like propanone you'd draw it like this to show all the arrows and partial charges.
    But it's structural formula would be CH3COCH3



    So when they ask you to write the equation, the mechanism, and the mechanism name, the order to do things would be
    1. the mechanism
    2. the mechanism name
    3. the equation
    (at least how I see it)

    Not all sigma bonds need to be drawn in a mechanism. For instance that propanone above, I didn't draw the sigma bonds between the C and the H in CH3 because it doesn't add anything to the mechanism.
    Basically, draw the sigma bonds around the reacting part, the part that's relevant, everything is just some group that tags along

    In pi bonds, you should always draw the bond (at least that's how I learned it), for EVERYTHING, in your mechanisms, in your equation, because it removes any ambiguity whatsoever. That's why ethene should be CH2=CH2
    Or if you wanna be super specific...
    H2C=CH2



    Regarding your question about oxidiation of primary alcohols, the easiest way to think of it is to think of the structures of the aldehyde and the carboxylic acid.

    Spoiler:
    Show

    The first [O] rips off an H atom from the Carbon and from the OH, which is why you get that C=O forming.

    The second [O] then puts itself between the last C-H to get C-O-H






    I had to learn about hydrogenation and cracking in my specification, I am not sure if it has changed so you'd need to look at your new spec, find it online, find any terms to do with cracking/hydrogenation and that'll answer it. If you do need to know, I suspect it's only minor, I only had to learn why you do cracking/hydrogenation and an overview of how to do it.
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    (Original post by bobwibbles)
    Yes that does sound familiar.
    So the alkyl groups release electrons stabilising the centre carbon in propanone or is that less as effective as an ethyl on the centre carbon? And the more electron stable thr carbon the slower the reaction or?

    Sorry im not so good with this atm.
    Yeah you're correct! Those alkyl groups release their electrons and stabilise that positive charge on the carbon atom.

    And as you said the more electron stable the carbon is, the slower the reaction.

    Spoiler:
    Show

    Why? Well.. nucleophiles love really positively charged things, they've got a lone pair of electrons: negatively charged species.
    If the carbon atom grabs electrons from alkyl groups and becomes more neutral, more electron stable, that nucleophile is less likely to attack it.
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    (Original post by telepathicnine)
    You can derive the equations of the reactions from the diagram, IF you understand how structural formula works... when you do the mechanism, you tend to draw the structure in displayed formula which makes it easier to visualise. Hence, draw the structural formula from that.
    Spoiler:
    Show



    Like propanone you'd draw it like this to show all the arrows and partial charges.
    But it's structural formula would be CH3COCH3




    So when they ask you to write the equation, the mechanism, and the mechanism name, the order to do things would be
    1. the mechanism
    2. the mechanism name
    3. the equation
    (at least how I see it)

    Not all sigma bonds need to be drawn in a mechanism. For instance that propanone above, I didn't draw the sigma bonds between the C and the H in CH3 because it doesn't add anything to the mechanism.
    Basically, draw the sigma bonds around the reacting part, the part that's relevant, everything is just some group that tags along

    In pi bonds, you should always draw the bond (at least that's how I learned it), for EVERYTHING, in your mechanisms, in your equation, because it removes any ambiguity whatsoever. That's why ethene should be CH2=CH2
    Or if you wanna be super specific...
    H2C=CH2



    Regarding your question about oxidiation of primary alcohols, the easiest way to think of it is to think of the structures of the aldehyde and the carboxylic acid.

    Spoiler:
    Show


    The first [O] rips off an H atom from the Carbon and from the OH, which is why you get that C=O forming.

    The second [O] then puts itself between the last C-H to get C-O-H







    I had to learn about hydrogenation and cracking in my specification, I am not sure if it has changed so you'd need to look at your new spec, find it online, find any terms to do with cracking/hydrogenation and that'll answer it. If you do need to know, I suspect it's only minor, I only had to learn why you do cracking/hydrogenation and an overview of how to do it.
    Thanks so much!
    Could you explain the pi bond/sigma bonds, I dont get it. :/ If a question asked you to draw the formation of sigma and pi bonds in terms of s/p orbitals. How would you draw it?
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    (Original post by shohaib712)
    Thanks so much!
    Could you explain the pi bond/sigma bonds, I dont get it. :/ If a question asked you to draw the formation of sigma and pi bonds in terms of s/p orbitals. How would you draw it?
    OH.

    Honestly that's sort of beyond me now. I have never ever been asked to draw bonds in terms of orbitals, which is a bit too complex I find. That's quite a horrid thing to ask cos like S and p orbitals change in atoms, and they give them different names.


    All I really know is that sigma bonds are single bonds whic are an overlap between two orbitals, the way I think of it is like a venn diagram with the overlap having the highest probability of finding an electron within it.

    http://web.chem.ucla.edu/~harding/IG...gma_bond06.png

    Pi bonds are kinda different. They're formed from spare orbitals. Let's take ethene. What's actually happening is that each carbon atom is forming three sigma bonds (two hydrogen atoms and a bond to the carbon atom).

    Three bonds indicates trigonal planar so the three orbitals are positioned planar around the carbon.

    But you know carbon makes four bonds. This last orbital appears above and below the plane (I don't know, it just does).

    When you mash the two carbons together, the two orbitals form a bond above and below the plane. this is the pi bond.

    http://www.chemtube3d.com/images/ethene_n.png

    It's quite hard to explain it actually... I suggest reading these two pages cos they really really helped me to understand it (it had no benefit exam-wise, but pi bonds were just really interesting)

    First this one
    http://www.chemguide.co.uk/basicorg/...thane.html#top

    Then this one
    http://www.chemguide.co.uk/basicorg/...thene.html#top
 
 
 
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