The expected bond angle in an idealized tetrahedral geometry is , but in trimethylamine, steric fact — Amines Chemistry Question

💡 Solution & Explanation

In trimethylamine, the nitrogen is $sp^3$ hybridized, which gives an ideal tetrahedral angle of 109.5°. While a lone pair usually compresses this angle (as in ammonia at $107^\circ$), the presence of three bulky methyl groups introduces steric repulsion, pushing the $C-N-C$ bond angle back open to approximately $108^\circ$.