# How are reactivity and dipole moment related?

Recently, I came across a question: which of the two compounds has a greater reactivity; $\ce{NH_{3}}$ or $\ce{NF_{3}}$?

It is known that despite the same structure the dipole moment of $\ce{NH_{3}}$ is greater because of the electronegativity difference. So, I though that the reactivity of $\ce{NF_{3}}$ should be more as it is easy to break the bond due to the high electronegativity of fluorine but the answer turned out to be $\ce{NH_{3}}$ and I can't figure out why.

• Welcome to chemistry.SE! It seems you are very familiar to the workings of the site, but anyhow I leave the links for the help center and the tour. – M.A.R. Jan 1 '15 at 6:54
• @MARamezani Hi. Yes I am quite familiar to SE. Thanks for the edit. – Yashbhatt Jan 1 '15 at 7:01

The reactivity of amines is generally based on how nucleophilic they are. In turn, their nucleophilicity is dependent upon how much of the nitrogen lone pair of electrons really exists on the nitrogen atom (e.g. the lone pair electron density on nitrogen). In ammonia ($\ce{NH3}$), the lone pair electron density on nitrogen is very high making ammonia quite nucleophilic, quite reactive. In the case of nitrogen trifluoride ($\ce{NF3}$), the three extremely electronegative fluorine atoms inductively remove much of the electron density from nitrogen; therefore $\ce{NF3}$ is less reactive than ammonia in a nucleophilic sense.

Yashbhatt - interesting question, but I think that this question is too vague. Reactivity is a very broad term.

Are we talking about reactivity in the context of proton transfer? In that case NH3 will be more reactive than NF3 as a proton donor (Brønsted acid) since it actually has protons to donate.

In the context of Brønsted basicity, again, NH3 will likely be more reactive than NF3 because in NF3 the electron-withdrawing fluorines reduces affinity for positive charges.

In the context of nucleophilicity, a lot of factors come into play. In a protic solvent, it's hard to tell a priori whether NH3 is more or less nucleophilic than NF3. NH3 has a greater partial negative charge on its central nitrogen atom. But this may also mean greater hydrogen bonding and therefore reduced nucleophilicity versus NF3, which doesn't exhibit hydrogen bonding (due to its lack of hydrogens).

• You are right. But I was asked this question in a test plainly as which is more reactive and was given an explanation that NH3 is more reactive because it has a higher dipole moment and so it will strongly attract other molecules and something like that. – Yashbhatt Jan 1 '15 at 7:07
• @Yashbhatt I think that a higher DM does not necessarily translate to a higher reactivity. It's a vague rule of thumb. What are you guys studying right now? – Dissenter Jan 1 '15 at 7:09
• We were studying chemical bonding. VSEPR, VBT and MOT. This specific case was mentioned as to why despite the same structure, they have different DP's. There was no mention of reactivity in the book. – Yashbhatt Jan 1 '15 at 7:12
• What textbook are you using @Yashbhatt? – Dissenter Jan 1 '15 at 7:18
• It belongs to the Central Board in India(CBSE). – Yashbhatt Jan 1 '15 at 7:21