Química Orgânica 1 · Caminhos de substituição
Methylate ammonia and free the amine
Condições: Iodomethane with a large excess of ammonia in ethanol. Follow the first methylation to methylammonium iodide, then proton transfer to a second ammonia giving methylamine. Each NH₂ or NH₃ formula includes the hydrogens not drawn separately. Further methylation toward di-, tri- and tetramethylammonium products is possible but not requested.
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Nitrogen attacks the methyl carbon
Ammonia is a neutral nucleophile. Methyl iodide is the least hindered SN2 substrate, and iodide is an excellent leaving group. The attacking nitrogen keeps all three of its hydrogens during this step, so it gains a fourth bond and a positive charge.
As setas curvas
- The lone pair on ammonia nitrogen bonds to the methyl carbon.
- The C–I pair leaves with iodine as iodide.
A second ammonia removes a proton
Methylammonium (pKa approximately 10.6) is a slightly weaker acid than ammonium (pKa approximately 9.2), so this transfer is an unfavorable equilibrium (K ≈ 0.04) that lies to the left; the large excess of ammonia still keeps a small amount of methylamine free.
Nota: Transfer the separately drawn N–H proton.
As setas curvas
- The lone pair on the second ammonia takes the separately drawn proton from the methylammonium ion.
- The N–H pair stays on the methylammonium nitrogen as its lone pair.
Methylamine and ammonium iodide
The methylamine nitrogen is back to three bonds and one lone pair: neutral methylamine, CH₃NH₂. The second ammonia became ammonium, which pairs with iodide. Methylamine is itself a nucleophile, a little stronger than ammonia, so it can compete for more iodomethane. Using a large excess of ammonia makes ammonia the most likely partner for each iodomethane molecule and keeps the mixture mostly at the primary amine stage.
O que você vai praticar
Separate the ammonium ion formed when neutral nitrogen does SN2 from the neutral amine released by a second ammonia, and explain why excess ammonia limits overalkylation.
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