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    I don't understand the naming of organic mechanisms by either 'electrophilic' or 'nucleophilic'. Take for example, the reaction between ethene and Br2, this is known as electrophilic addition. However, the Br2 is the electrophile and the ethene is the nucleophile, so who's to say that the ethene isn't the attacking species, making it a nucleophilic mechanism?

    My chemistry teacher didn't have an answer for this, so can anyone shed some light?
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    I think... when it's organic mechanisms, the organic molecule that is the subject of the mechanism (ie. ethene in your reaction above) is the one being attacked, and hence the other molecule (quite possibly inorganic) is the one attacking so the 'attack' would be electrophilic or nucleophilic depending on that...
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    (Original post by xarcul)
    ..
    Hey, off-topic.. I know someone who got a straight out rejection from Glasgow for Veterinary Medicine too!
    Grats on the Cambridge offer, though.
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    Hmmm not the simplest thing to explain :p:

    In the example you gave you asked why can't it be nucleophilic instead of electrophilic? Because it is the electophile that is adding to the double bond, the nucleophile isn't adding to anything as such. A nucleophilic addition is the addition of a nucleophile (which isn't really possible with ethene) but is possible with something like a ketone (the ketone acts as the electrophile).
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    (Original post by xarcul)
    I don't understand the naming of organic mechanisms by either 'electrophilic' or 'nucleophilic'. Take for example, the reaction between ethene and Br2, this is known as electrophilic addition. However, the Br2 is the electrophile and the ethene is the nucleophile, so who's to say that the ethene isn't the attacking species, making it a nucleophilic mechanism?

    My chemistry teacher didn't have an answer for this, so can anyone shed some light?
    No, the reaction of Ethene and Bromine is Electrophillic Addition. Simply because, Bromine acts as the Electrophille, as the molecule has different polarity, and it attacks the Ethene Double bond.. Which brakes, forming a Carbocation, and the Bromine Molecule left with the lone pair of electrons forms another bond to the Ethene, forming a 1,2 dibromoethene molecule. I can't see how the Ethene could be assumed as the attacking species, as it does not have a difference in polarity.
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    (Original post by Haz Shah)
    No, the reaction of Ethene and Bromine is Electrophillic Addition. Simply because, Bromine acts as the Electrophille, as the molecule has different polarity, and it attacks the Ethene Double bond.. Which brakes, forming a Carbocation, and the Bromine Molecule left with the lone pair of electrons forms another bond to the Ethene, forming a 1,2 dibromoethene molecule. I can't see how the Ethene could be assumed as the attacking species, as it does not have a difference in polarity.
    Bromine is non-polar normally - it's the increased electron density of the double bond that is inducing a dipole
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    (Original post by EierVonSatan)
    Bromine is non-polar normally - it's the increased electron density of the double bond that is inducing a dipole
    to form a induced dipole, which is a temporary dipole, induced by the double bond of ethene yea
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    (Original post by Haz Shah)
    No, the reaction of Ethene and Bromine is Electrophillic Addition. Simply because, Bromine acts as the Electrophille, as the molecule has different polarity, and it attacks the Ethene Double bond.. Which brakes, forming a Carbocation, and the Bromine Molecule left with the lone pair of electrons forms another bond to the Ethene, forming a 1,2 dibromoethene molecule. I can't see how the Ethene could be assumed as the attacking species, as it does not have a difference in polarity.
    I did say that it was called electrophillic addition! (spelt with only one 'l' incidentally). As to which is the attacking species, surely it is always a case of positive charge is attracted to negative and vice versa, so the force of attraction is mutual; ie, one does not attack the other but instead they are attracted to eachother.
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    (Original post by xarcul)
    I did say that it was called electrophillic addition! (spelt with only one 'l' incidentally). As to which is the attacking species, surely it is always a case of positive charge is attracted to negative and vice versa, so the force of attraction is mutual; ie, one does not attack the other but instead they are attracted to eachother.
    You can think of them as attraction of charges, but don't think too much that way, as most organic reactions involve covalent bond formation instead of ionic characterised reactions.

    Positive charge don't attack the negative ones, it is the rich electron density(ie Nucleophile HOMO) that attacks the poorer electron density atom(ie electrophile LUMO). Energetically, this has to be favourable, if it is not(ie HOMO-LUMO gap is too big, such reactions would be unfavourable, just simple energetics)

    One more thing, Br2 is non polar and neutral, it is the rich electron density of alkene that polarise the Br2 slightly, and this "tilts" the reaction forwards.
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    (Original post by xarcul)
    I did say that it was called electrophillic addition! (spelt with only one 'l' incidentally). As to which is the attacking species, surely it is always a case of positive charge is attracted to negative and vice versa, so the force of attraction is mutual; ie, one does not attack the other but instead they are attracted to eachother.
    The force of attraction is not mutual. If you look closely, the Bromine atom attacking the Ethene double bond is Positive..
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    (Original post by Haz Shah)
    The force of attraction is not mutual. If you look closely, the Bromine atom attacking the Ethene double bond is Positive..
    it is the rich electron density of alkene that polarises the Br-Br bond, this initiates electrophilic attack by rich electron density of alkene(Nucleophile) onto the delta positive Br(E+), with Br- as a leaving group. Br- then comes in as a nucleophile and attacks the intermediate formed from the first step, and hence you get 1,2 dibromo(that is if it is concentrated Br2, not Br2 in water).
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    (Original post by Haz Shah)
    The force of attraction is not mutual. If you look closely, the Bromine atom attacking the Ethene double bond is Positive..
    I can't see what you're getting at here I'm afraid...

    Surely its agrees with the basic scientific principle; equal and opposite forces - both attract the other with exactly the same force.
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    (Original post by shengoc)
    it is the rich electron density of alkene that polarises the Br-Br bond, this initiates electrophilic attack by rich electron density of alkene(Nucleophile) onto the delta positive Br(E+), with Br- as a leaving group. Br- then comes in as a nucleophile and attacks the intermediate formed from the first step, and hence you get 1,2 dibromo(that is if it is concentrated Br2, not Br2 in water).
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    (Original post by Haz Shah)
    This is the typical reaction mechanism for dibromination of alkene. Sometimes, you may see the intermediate in cyclic formed, ie the Br is connected to both the carbon with a positive charge on the Br, but this is just a canonical form of it, so it is fine.
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    (Original post by shengoc)
    This is the typical reaction mechanism for dibromination of alkene. Sometimes, you may see the intermediate in cyclic formed, ie the Br is connected to both the carbon with a positive charge on the Br, but this is just a canonical form of it, so it is fine.
    Okay, so now you're both happy with the actually mechanism, do you have any sort of an answer to my question? :p: Thanks!
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    Also, note that there is a Lone Pair of electrons on the Br- ion in the Carbocation stage.
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    (Original post by xarcul)
    I can't see what you're getting at here I'm afraid...

    Surely its agrees with the basic scientific principle; equal and opposite forces - both attract the other with exactly the same force.
    i think what you are saying here is regarding newton's third law, but that is only applicable in the classical sense, ie big mass, like motion of cars, humans, etc.

    but at atomic or molecular scale, classical newtonian mechanics fail, we need to use quantum theory for approximation to atomic/molecular orbitals and this is the idea behind HUMO-LUMO, but there is no need to concern too much physical chem into organic chem at a level. So, your statement was correct, but not applicable in organic reactions due to their small mass, however, opposite dipoles obviously favour each other more, and promote better interaction between them.
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    (Original post by shengoc)
    i think what you are saying here is regarding newton's third law, but that is only applicable in the classical sense, ie big mass, like motion of cars, humans, etc.

    but at atomic or molecular scale, classical newtonian mechanics fail, we need to use quantum theory for approximation to atomic/molecular orbitals and this is the idea behind HUMO-LUMO, but there is no need to concern too much physical chem into organic chem at a level. So, your statement was correct, but not applicable in organic reactions due to their small mass, however, opposite dipoles obviously favour each other more, and promote better interaction between them.
    I see what you're saying. So how would this relate to whether the mechanism is called electrophilic or nucleophilic?
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    And when I say, "I see what you're saying", I mean I trust that you know far more about this than me! :P
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    (Original post by xarcul)
    I see what you're saying. So how would this relate to whether the mechanism is called electrophilic or nucleophilic?
    Most of organic reactions involve both an electrophile and a nucleophile. The one with richer electron density is the nucleophile. The one poorer in density is electrophile. They may or may not be charged.

    ie OH- is a nucleophile. lone pair on NH3 is a nucleophile.

    the reaction is called electrophilic addition because it is the addition of an electrophile(Br+) onto double bond, but this is initiated due to the presence of alkene.

    electrophilic substitution is more common in aromatic systems(benzene) where addition across the double bond is much less favoured.

    nucleophilic substitution reaction can be found in most sn1 or sn2 reactions.

    nucleophilic addition reactions are usually involved in addition-elimination mechanism involving most carnonyls(ketones/aldehydes,etc), ie the C is slightly positive, attacked by Nu-, form tetrahedral intermediate, then a leaving group usually halides leave, giving a new product.
 
 
 
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