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Why does Nitrogen gain a positive charge in this mechanism ?

In this nucleophilic substitution :Screenshot 2020-03-30 at 17.44.30.png
Reply 1
Because the nitrogen has formed 4 bonds, when it "normally" only forms three. The positive charge comes from a H+ ion which forms a dative covalent bond from the nitrogen, making it's "fourth bond", in which the positive charge is shared over the whole part of the compound. The RNH3+ part of the molecule is deprotonated to form RNH2 and a H+ ion.
(edited 4 years ago)
Reply 2
Original post by Deggs_14
Because the nitrogen has formed 4 bonds, when it "normally" only forms three. The positive charge comes from a H+ ion which forms a dative covalent bond from the nitrogen, making it's "fourth bond", in which the positive charge is shared over the whole part of the compound. The RNH3+ part of the molecule is deprotonated to form RNH2 and a H+ ion.

At what point does it form a dative covalent bond with a H+ ion ? It was already bonded to 3 Hs in NH3 and then forms a fourth bond with its lone pair with the Carbocation.
Reply 3
Original post by lhh2003
At what point does it form a dative covalent bond with a H+ ion ? It was already bonded to 3 Hs in NH3 and then forms a fourth bond with its lone pair with the Carbocation.


It doesn’t, you’re correct in saying the dative covalent bone forming is the nucleophilic attack of the carbon from the nitrogen in ammonia. A H atom is lost to a form H+ ion, which in essence is another molecule of NH3 reacting as a base by accepting a proton forming NH4+.
Reply 4
I'm also v confused w/ this. because isn't the N atom more electronegative than the other atoms its bonded to so if anything it should have a partial -ve charge.

Original post by lhh2003
In this nucleophilic substitution :Screenshot 2020-03-30 at 17.44.30.png
Original post by Anis_M
I'm also v confused w/ this. because isn't the N atom more electronegative than the other atoms its bonded to so if anything it should have a partial -ve charge.


Same I feel that people have answered why nitrogen forms a positive charge on the NH4+ but why does the nitrogen atom itself when attached to the alkane form a positive charge?
Original post by Lebkuchen
Same I feel that people have answered why nitrogen forms a positive charge on the NH4+ but why does the nitrogen atom itself when attached to the alkane form a positive charge?

Hey. Think of it like this, a dative covalent bond is when both electrons come from 1 atom. In this case, heterolytic fission (bond breaking of C-Br) occurred bc of partial charges. The Br took both electrons in the bond and thus formed a negative ion (anion). The N has donated both of its electrons (lone pair of electrons) to C or carbocation. That’s why N has a positive charge. It loses a H+ to get back to normal neutral N as the N-H bond breaks via heterolytic fission also and N has its lone pair back. Hope that helps :smile:
Original post by Muhammad Razak
Hey. Think of it like this, a dative covalent bond is when both electrons come from 1 atom. In this case, heterolytic fission (bond breaking of C-Br) occurred bc of partial charges. The Br took both electrons in the bond and thus formed a negative ion (anion). The N has donated both of its electrons (lone pair of electrons) to C or carbocation. That’s why N has a positive charge. It loses a H+ to get back to normal neutral N as the N-H bond breaks via heterolytic fission also and N has its lone pair back. Hope that helps :smile:

wait lowkey I think I understand it as nitrogen forms the dative covalent bonds it donates a pair thus getting the positive charge and the positive charge is cancelled out when nitrogen gains back a pair of electrons not used in forming bonds which in essence is like losing a proton as it's protons - electrons = 0 the compound is now neutral
Original post by Anis_M
I'm also v confused w/ this. because isn't the N atom more electronegative than the other atoms its bonded to so if anything it should have a partial -ve charge.

This has little to do with electronegativity and more to do with electrons. Before the N attacks it is neutral and has a lone pair of electrons (those 2 black dots). When it forms a bond with something else it effectively gives up one of those electrons, and since it’s lost an electron the N is overall now positively charged.

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