Organic Chemistry 1 · Bonding and formal charge
Trimethylamine bonds to borane: an amine–borane adduct
Conditions: BH3·THF
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The mechanism, step by step
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Nitrogen fills the empty p orbital of boron
Borane is a trigonal planar sp2 boron with only six valence electrons and an empty 2p orbital. Trimethylamine is a better donor than the THF oxygen it is supplied with, so the nitrogen lone pair displaces THF and forms a dative N–B bond.
The curved arrows
- The nitrogen lone pair moves toward the electron-poor boron and becomes the shared pair of the new N–B bond.
Boron after bonding
Count the electron domains on boron once the N–B bond has formed.
Trimethylamine–borane
Nitrogen has four bonds and no lone pair, so it carries +1; boron has four bonds and a full octet, so it carries −1, and the adduct is neutral overall. Amine–boranes are stable, easily handled solids, which is why they serve as bench-stable sources of BH3.
What you'll practice
Draw trimethylamine donating its lone pair to borane and describe how the hybridization of boron changes in the adduct.
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