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Solutions for Amines OCH3Cl N NH2 Br HC N NH2N C N Cl OCH3 H N Br NH2 OCH3 N C N Cl OCH3 H CH N Br NH2 OCH3 NH2 N Br N Cl GOOD – Cl– 13 The middle resonance form demonstrates how the negative charge is distributed onto the more electronegative nitrogen atom. 14 (a) GOOD – Br– C N NH2 N C NH2 H NH2 H N H NH2 N NH3 NH3 NH3 NH2 N C NH2 N NH2 NH2 NH2 N H Br H N (b) + + This is a benzyne-type mechanism. (For simplicity above, two steps of benzyne generation are shown as one step: first, a proton is abstracted by amide anion, followed by loss of bromide.) Amide ion is a strong enough base to remove a proton from 3-bromopyridine as it does from a halobenzene. Once a benzyne is generated (two possibilities), the amide ion reacts quickly, forming a mixture of products. Why does the 3-bromo follow this extreme mechanism while the 2-bromo reacts smoothly by the addition- elimination mechanism? Stability of the intermediate! Negative charge on the electronegative nitrogen makes for a more stable intermediate in the 2-bromo substitution. No such stabilization is possible in the 3- bromo case. stabilized by induction from nitrogen Amide ion will not attack at C-2 because the anion at C-3 is not as stable as the anion at C-2. two steps two steps 456