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    (Original post by shengoc)
    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.
    But If you have two molecules, an electrophile and a nucleophile and they combine together to form a new molecule, how do you know which one is the attacking species?
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    (Original post by xarcul)
    But If you have two molecules, an electrophile and a nucleophile and they combine together to form a new molecule, how do you know which one is the attacking species?
    it is quite a good question really, but i would say the nucleophile is always the attacking species, for it the one rich in electron density, if you think about it, if you have H+ as an electrophile, would it attack a negatively charged species, on what basis, H+ doesn't have any electrons at all!

    but yeah, based on what i have just said, the alkene reaction with bromine, could be categorised as nucleophilic addition too, but i am just relying on my chemical intuition, based on reading books and seeing examples, they are categorised as electrophilic addition.

    you might have to wait for EVS or charco to explain this.
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    (Original post by shengoc)
    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.
    Slightly off-topic, I suppose.. could you explain to me what the difference between a lone pair is and, for example, a radical electron? What makes one of them bonding, but the other non-bonding?
    How come you speak of a 'pair' of electrons? Are they two electrons in the same.. shell? Energy level? Surely, they're not two electrons 'close to each other', with coulomb interaction preventing such a thing from happening?

    I've ALWAYS had only a vague idea of what it really is, and I just can't seem to rid myself of this vagueness. I usually don't have this sort of thing.. :P but yeah, with this thing, I do.
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    (Original post by phen)
    Slightly off-topic, I suppose.. could you explain to me what the difference between a lone pair is and, for example, a radical electron? What makes one of them bonding, but the other non-bonding?
    How come you speak of a 'pair' of electrons? Are they two electrons in the same.. shell? Energy level? Surely, they're not two electrons 'close to each other', with coulomb interaction preventing such a thing from happening?

    I've ALWAYS had only a vague idea of what it really is, and I just can't seem to rid myself of this vagueness. I usually don't have this sort of thing.. :P but yeah, with this thing, I do.
    lone pair involves two electrons, radical involves an electron only, usually. However, sometimes, if you have two electrons in radical, ie like carbenes(look up wiki), they can be triplet(ie both spin up) or singlet(1 spin up, 1 spin down).

    energy of triplet is lower than singlet, this is by hund's first rule to maximise spin multiplicity( it is 3 for triplet and 1 for singlet).
    Handwaving argument: electrons repel less when alligned parallel (triplet) compared to when they are paired up(singlet)

    For lone pair, these are two electrons in the same orbital. Before bonding, atomic orbitals need to reorganize(hybridize) to form molecular orbitals, it is the electrons in those MO's that are either bonding, non bonding or antibonding. The lone pair comes in the non bonding category.
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    This is just a matter of nomenclature really. You are quite right insofar as one could call any ionic reaction 'nucleophilic' or 'electrophilic' - both components are required! As a matter of course, however, we would determine the nature of the attack as nucleo- or electrophilic depending upon the "less important" species: e.g. a Friedel-Crafts reaction on benzene is generally called an electrophilic attack, as we consider benzene the main species, even though we could well view benzene as a nucleophile attacking a carbocation/acylium ion.
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    what cpchem says...

    AND I would also add that it's a matter of perspective.

    If you are dealing with an alkene and examining its possible reactions it becomes the focus of your attention. You then describe any mechanism from the point of view of the reagent that is causing the change to your organic species.

    In the majority of cases there is no problem, as the reagent is an inorganic compound/mixture and the focus is clearly on an organic species.

    The boundaries become a little more blurred when both the reagent and the 'substrate' are organic.

    However, once again it becomes a matter of perspective.

    If the reagent is an 'accepted' reagent/mixture, then there is no real problem (for example the ethoxide ion), but if the reagent itself is an organic molecule/species then you may be at liberty to describe the mechanism from the point of view of either species.

    In terms of the issue of 'attack'. This is fundamentally semantics with no real meaning. In reality the molecules collide and this causes the change, a rearragement which occurs as it is thermodynamically favourable.

    Mechanisms seek to describe what probably/possibly happens to the electrons in the course of the collision to allow for the rearrangement/breaking/reforming of bonds etc.

    OK, so we understand that it is the electrons that are responsible for the bonding, so they must also be involved in the making and breaking of bonds/attachments.

    It is important ot communicate ideas between researchers/students/interested parties, etc so we arrive at an agreement/ convention:

    A reaction mechanism is described according to:

    (i) the nature of the reagent that causes the change (in the molecule on which we are focussing) and which initiates the reaction (nucleophile, electrophile, free radical)
    (ii) the overall effect of the change (i.e. substitution, elimination, addition, rearrangement etc)
    (iii) Where competing mechanisms are possible, some indication of perhaps the molecularity of the RDS or another means of differentiation (sN1, sN2 etc).

    Chemistry is very rarely black or white and a certain amount of flexibility of thought makes for far greater understanding of the underlying concepts.
 
 
 
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