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Mechanism

http://www.chemguide.co.uk/mechanisms/nucsub/ammonia.html#top

In the mechanism between NH3 and a haloalkane an intermediate product is
R-NH3+ where the + is on the nitrogen.
Why does nitrogen have a +

(the same question applies for water with haloalkane, why is + in H2O on H when R-H2O intermediate forms)

Reply 1

Kyri

Recall that a covalent bond can be considered a sharing of electrons. The NH3 nitrogen used to have two electrons in its lone pair and was neutral, but it formally only has one electron in the formed N-C bond (because the other one now belongs to the carbon). A loss of one electron from a previously neutral species leads to a singly positive species. This is why you get a + on the nitrogen and it's the same thing with oxygen.

The nitrogen becomes neutral again when it loses a proton and one of the electrons which belonged to the H in the N-H bond formally transfers to the nitrogen.

Reply 2

shengoc

Original post by RCB

because N here is tetravalent; ie 4 covalent bonds

N forms 3 bonds usually with 1 lone pair, and that gives it full octet count of 8 around N - what we want for second row elements.