Question
Why is NF a weaker base than NH, even though both have a lone pair on nitrogen?
Solution — Step by Step
Basicity is the willingness of the lone pair to coordinate to a proton (or Lewis acid). The lone pair on N in NH is concentrated on N — H is less electronegative than N, so electron density flows toward N.
Fluorine is more electronegative than nitrogen. The N-F bond pulls electron density off N. The lone pair, while still on N, is less available to coordinate with a proton.
NH has a dipole moment of D (lone pair direction adds to N-H bond dipoles, all pointing roughly the same way). NF has a dipole moment of only D — the lone pair direction opposes the N-F bond dipoles, partially cancelling.
Lower lone-pair availability → lower basicity. NF’s nitrogen is far less willing to donate its lone pair than NH’s.
Final answer: F’s strong electron-withdrawing effect reduces electron density on N, weakening NF’s base strength.
Why This Works
Basicity depends on two things: how concentrated the lone pair is, and how stable the resulting protonated species is. Electron-withdrawing groups (F, Cl, NO) destabilise both — they pull electron density off the donor atom.
This is a JEE Main classic. The same logic explains why aliphatic amines (RNH) are stronger bases than aromatic amines (PhNH): the phenyl ring pulls electron density off N via resonance.
Alternative Method
Compare values directly. NH: . NF: essentially non-basic, . The numbers settle the argument.
For amine basicity comparisons, always assess: (1) inductive effect of substituents, (2) resonance withdrawal, (3) steric crowding. Electron donors (alkyl) increase basicity; electron acceptors (halogen, nitro, aryl) decrease it.
Common Mistake
Saying “NF has more electronegative atoms, so it is more polar”. Polarity isn’t the same as basicity. NH’s overall dipole is larger than NF’s because the lone pair direction reinforces the bond dipoles, not opposes.
Forgetting that basicity depends on availability, not just presence, of the lone pair. NF has a lone pair, but it is “trapped” by F’s pull. PCl, AsCl similarly are weaker Lewis bases than PH, AsH for the same reason.