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Lecture #24 (Chapter 20)

Lecture #24 (Chapter 20) - Reactions of Amines The...

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Reactions of Amines The chemistry of amines is determined by the nonbonding electron pair on nitrogen. Amines are bases and nucleophiles. The amino group as a substituent on a benzene ring is a powerful activating group and ortho-para director in electrophilic aromatic substitution: NH 2 : E + NH 2 : E ortho and para N: + H + N: N-H + base + R-X N-R + + X - nucleophile
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Diazonium Ions: Reaction of Primary Amines with Nitrous Acid Nitrous acid, HNO 2 , is unstable and is prepared in the reaction mixture from a nitrite salt and a mineral acid such as HCl or HBr in the presence of the amine. overall chemistry RNH 2 + NaNO 2 + 2 HX H 2 O sodium nitrite R-N N: X - + diazonium salt unstable + NaX + 2 H 2 O A Mechanism for Diazotization of Primary Amines Na + :O N=O : : : : - sodium nitrite + H-OH 2 + H-O N=O : : : : + NaX nitrous acid : : The unstable nitrous acid is formed by protonation of the nitrite ion.
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formation of nitrosonium ion H-O N=O : : : : + H-OH 2 + : H-O N=O : : : H + : N=O : : : + + H 2 O nitrosonium ion nitrosation of amine R-NH 2 : + N=O : : : + R-N- N=O : : : H H + N-nitrosoamine dehydration of N-nitrosoamine A series of deprotonation and reprotonation steps leads to dehydration: R-N- N=O : : : H H + R-N=N-O-H H : : : + ~H + (-H 2 O) R-N=N : : + R-N N + : diazonium ion Alkyl diazonium ions are unstable . R-N N + : fast R + + :N N: The measurement of the released N 2 gas from diazotization of primary amines is used in some analytical procedures.
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The Chemistry of Aryl Diazonium Ions Aryl diazonium salts can be prepared and manipulated at 0 o C: ArNH 2 + NaNO 2 + 2 HX 0 o C, H 2 O sodium nitrite Ar -N N: X - + aryl diazonium salt stable below 5 o C + NaX + 2 H 2 O Note: Many dry aryl diazonium salts are explosive . Therefore, these compounds are typically not isolated. They are prepared and then used in the same solution in a number of replacement reactions .
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Replacement Reactions of Aryl Diazonium Salts The reaction of aryl diazonium salts with various reagents results in replacement of the diazonium group by other groups.
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