Electro/nucleo-philes

[NiCKEL]

Do electro(nucleo)philes refer to just the atom or the whole molecule? I hear the three types of nucleophiles are lone pairs, sigma bonds, and pi bonds. Would an electrophile be like the H+ ion in HBr then, or just HBr? Is it wrong to call the long-pair-containing molecule the nucleophile?

[Msj Chem]

If HBr was to attack a double bond, then the positive end of the molecule could be referred to the as the electrophilic portion of the molecule. This is due to the difference in electronegativity between H and Br making the H end slightly positive. 

There's a good explanation here:

http://www.chem.ucla.edu/harding/tutorials/elec_nuc/elec_nuc.html

[guanemo]

Do electro(nucleo)philes refer to just the atom or the whole molecule? I hear the three types of nucleophiles are lone pairs, sigma bonds, and pi bonds. Would an electrophile be like the H+ ion in HBr then, or just HBr? Is it wrong to call the long-pair-containing molecule the nucleophile?

 

 

What is a nucleophile? It is an electron rich species. That could either be a molecule like H2O (the Oxygen contains a lone pair of electrons), a single atom ( like iodide I-,  a single atom from the diatomic iodine).Nucleophiles also act as ligand (chapter 13) because they are electron rich and are able to donate their electron pair via dative bonding. Nucleophiles could either be negative or slightly negative. 

 

Nucleophiles react via dative bonding which is when they donate an electron pair (not just one electron like with covalent bonding) to the nucleophile. So would it be fair to say that HBr is an electrophile/nucleophile combination? no. Look at the electronegativity values in the data table and determine weheter the nature of the bond is ionic or covalent (chapter 4) . HBr is a covalent bond, and to top that there is no dative bonding so it can't be an electrophile/nucleophile combination. Nucleophiles and electrophiles are what forms chilates (compelx structures with a transition metal and a ligand chapter 13), and its amazing how much you could learn about them. But if your teacher isn't good i would recommnd you to buy the Hodder's education iB chem book because it explains everything well. You will also get to know more about the amazing nature and chemistry of what makes up our world when you get to chapter 20 (the S2N and S1N reactions). They make nucleophiles and electrophiles seem much clearer. 

 

HBr as a molecule - without considering its specific elements or method of creation could act as an electrophile in many cases depending on the reaction involved and its a strong acid.

 

Enjoy and i hope it helps

Edited January 20, 2016 by guanemo

[WesPotts2k16]

I remain confused as well. It was described to me as either an electron rich region of density (nucleophile) or electron deficient region of density (electrophile). So would this be referring to the species as a whole as "nucleo/electro-philic" or to the areas surrounding the bonds on the species, for example, the bond area around the H+ in the HBr species? 

[kw0573]

I remain confused as well. It was described to me as either an electron rich region of density (nucleophile) or electron deficient region of density (electrophile). So would this be referring to the species as a whole as "nucleo/electro-philic" or to the areas surrounding the bonds on the species, for example, the bond area around the H+ in the HBr species? 

Electrophile refers to the entire species (molecule or whatnot). This is confirmed in Msj Chem's link, which says that nucleophile is a synonym (while highlighting different properties) to lewis base. It's the same way you do not refer to part of a molecule as the lewis base. This has been the general treatment of the term. Some teachers clarifies where in the molecule would a hydrogen be pulled towards, hence uses the term "nucleophilic portion" or the like, instead of "nucleophile" which describes the entire species. 

In many instances where Br^- is a nucleophile, it is dissociated from H⁺. This is often true because HBr completely dissociates in water, as guanemo pointed out. HBr rarely behaves as a nucleophile, because Br essentially gets 8 electrons (very very polar bond), so H₂Br⁺ is not favoured.

Ammonia (NH₃) is another nucleophile. Here N is the electronegative atom and it attracts a fourth H to form NH₄⁺. We wouldn't call N the nucleophile, because it's the whole species that reacts. 

That being said, you should never draw electron arrows originating from an H when NH₃ acts as the nucleophile ... you draw it from N. This is why many teachers emphasize nucleophilic portions/regions, instead of just calling ammonia a nucleophile. Yet, IB will not ask you Br^-, HBr, which one is the nucleophile.