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    (Original post by Gulzar)
    How much further detail do we need to know including limitation and retention time?
    limitation of tlc:
    similar compounds have similar Rf values

    Unknown compounds would no reference of Rf values.

    difficulty in finding a solvent which can separate all components.

    limitation of gc:
    similar retention time

    no all components separate fully

    unknown compounds would have no reference.



    for retention time all we basically need to know is the definition which is the time the component spends in the column or even simpler...injection to detection
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    (Original post by otrivine)
    I see!! makes sense now, thanks
    can someone explain why a primary amine R-NH2, will not react with Br2 but will react when a single bromine atom is attached to a hydrocarbon chain e.g. bromomethane (to make CH3-NH-R )
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    (Original post by MathsNerd1)
    The bromine water would be decolourised and a white precipitate will be formed?
    Correct
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    Do we need to know how the chromatogram is produced in each case (TLC and GC) for the exam?
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    (Original post by Zzzyax)
    can someone explain why a primary amine R-NH2, will not react with Br2 but will react when a single bromine atom is attached to a hydrocarbon chain e.g. bromomethane (to make CH3-NH-R )
    primary second and teritiary amines are NOT in the specification.
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    (Original post by _HabibaH_)
    Hi, how did you work out that atom economy? And are there any other AS equations that I should know? Thank you in advance
    Atom economy=Mr of desired product(in this case C6H5NO2)/Mr of all products(add Mr of H2O to C6H5NO2) *100

    I thought percentage yield might be useful,but that was more related to F325,where they were more likely to ask that.

    My teacher made us write ALL the reactions we learnt from AS to A2,coz it's useful when doing the reaction pathways questions.
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    Can someone please tell me how to describe nucleophilic substitution and addition, in my revision notes it say:
    Nucleophilic addition: the C=O bond is polar, the nucleophile attacks the electron deficient carbon forming a new covalent bond and that an addition reaction occurs across the double bond. Is this right? because it doesn't seem like a very scientific explanation.
    And what would the description of nucleophilic sub be?
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    Stretch and challenge for all:

    How many carbon atoms are in the molecule below?
    How many chiral centres are in the molecule below?
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    (Original post by k54g00)
    Satsriakal Ji!

    Substances adsorb onto a SOLID stationary phase, but are separated by RELATIVE SOLUBILITY on a liquid stationary phase.

    TLC- Solid- Adsorption
    GC- Liquid- Rel. Solubility.

    But, if i'm not mistaken, can't GC also have a solid inert support, thus separating by adsorption?
    Nice little Satsriakal there! I see you recognised my name! Haha .
    Thank you! It's been cleared up now .
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    (Original post by thers)
    Correct
    Well I actually know something then Anymore? Sorry if I'm taking up your time but this is really helping me out
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    (Original post by thers)
    Correct
    give me hard questions please
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    can somebody dell me on the januart 2011 paper for question 5 ii how are you meant to estimate the Rf value ?
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    (Original post by mrmccarl)
    limitation of tlc:
    similar compounds have similar Rf values

    Unknown compounds would no reference of Rf values.

    difficulty in finding a solvent which can separate all components.

    limitation of gc:
    similar retention time

    no all components separate fully

    unknown compounds would have no reference.



    for retention time all we basically need to know is the definition which is the time the component spends in the column or even simpler...injection to detection
    Legend
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    brady explains is amazing, is there one for drawing structures complex ones
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    (Original post by Joey952)
    can somebody dell me on the januart 2011 paper for question 5 ii how are you meant to estimate the Rf value ?
    distance travelled by solute divided by distance over solvent front!
    Should be 2.something...
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    (Original post by Zzzyax)
    can someone explain why a primary amine R-NH2, will not react with Br2 but will react when a single bromine atom is attached to a hydrocarbon chain e.g. bromomethane (to make CH3-NH-R )

    NH2 cannot be oxidised , cannot react with Br2 or NaOH.
    nitrogen can have 4 bonds if it is positively charged
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    (Original post by Varsh05)
    Atom economy=Mr of desired product(in this case C6H5NO2)/Mr of all products(add Mr of H2O to C6H5NO2) *100

    I thought percentage yield might be useful,but that was more related to F325,where they were more likely to ask that.

    My teacher made us write ALL the reactions we learnt from AS to A2,coz it's useful when doing the reaction pathways questions.
    Thank you sooooo much you absolute babe of a candidate!

    Yes it's coming back to me now.
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    (Original post by kitkat95)
    Can someone please tell me how to describe nucleophilic substitution and addition, in my revision notes it say:
    Nucleophilic addition: the C=O bond is polar, the nucleophile attacks the electron deficient carbon forming a new covalent bond and that an addition reaction occurs across the double bond. Is this right? because it doesn't seem like a very scientific explanation.
    And what would the description of nucleophilic sub be?
    It's better to draw the mechanism than to describe in words.
    Mechanisms are more likely to gain you most marks,if not all.
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    (Original post by thers)
    Stretch and challenge for all:

    How many carbon atoms are in the molecule below?
    How many chiral centres are in the molecule below?
    16 carbon

    4 chiral
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    (Original post by thers)
    Stretch and challenge for all:

    How many carbon atoms are in the molecule below?
    How many chiral centres are in the molecule below?
    15 carbons

    3 chiral centres?
 
 
 
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