Chem 225 Notes Page 128. Chapter 23: Amines

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1 Chem 225 Notes Page 128 Chapter 23: Amines I. Introduction A. Classification of Amines Amines are classified by the number of carbon groups attached to the nitrogen: ammonia primary (1 o ) secondary (2 o ) tertiary (3 o ) quaternary ammonium ion B. IUPAC Nomenclature 1) Find the longest carbon chain which contains the carbon attached to the NR 3 group. 2) Count the number of carbons in this chain and determine the parent stem name. 3) Add the suffix -anamine. 1 carbon group 2 carbon groups 3 carbon groups 4 carbon groups 4) Number the carbon chain starting from the end closest to the NR 3 group. For cyclic compounds, the carbon with the NR 3 group is carbon 1. 5) Indicate the position of the nitrogen atom on the parent chain by adding the number, separated by dashes, in between the stem name and the -amine suffix (ie. stem-#- amine). 6) Add substituent names, with appropriate numbers, in front of the amine name. 7) Add the name of any other alkyl groups on the nitrogen in front, with an N- prefix. 8) For complicated carbon chains, name the NH 2 group as an amino substituent.

2 Chem 225 Notes Ch 23: Amines Page 129 C. Common Names Name all the carbon groups attached to nitrogen as alkyl groups, then add amine. D. Special Common Names (MEMORIZE) E. Nomenclature: Ammonium Salts Nitrogen with four groups attached has a positive formal charge. These compounds are called ammonium ions, and they usually exist as salts with a corresponding anion. To name ammonium compounds: 1) Name all of the carbon groups attached to the nitrogen as alkyl groups. 2) Add the name ammonium at the end. When the compound is a salt with an anion, add the name of the anion after the name of the ammonium ion.

3 Chem 225 Notes Ch 23: Amines Page 130 II. Properties of Amines A. Lone Pair Inversion at Nitrogen If the nitrogen of an amine has three different groups attached (plus the lone pair), it is a stereocenter. However, nitrogen stereocenters do not hold their configuration. They racemize because the lone pair can invert through the nitrogen: B. Basicity of Amines, Anilines, and Amides Carbon groups donate electrons to the nitrogen, which increases the electron density of the lone pair and increases the basicity of the amine: Conj. Acid pka Amine Base NH 4 EtNH NH EtNH 2 Et 3 NH Et 3 N Et 2 NH Et 2 NH However, the ammonium ion is stabilized by solvation. Tertiary ammonium ions are sterically hindered, which reduces solvation and makes them less stable. Therefore, tertiary amines are less basic than secondary amines.

4 Chem 225 Notes Ch 23: Amines Page 131 In aromatic amines (anilines) and amides, the nitrogen lone pair participates in resonance: Resonance reduces the electron density of the lone pair on nitrogen, so anilines and amides are less basic than amines: Conj. Acid pka Base 0 PhNH PhNH 2 NH NH 3 RNH 3 R 3 NH R 2 NH RNH 2 R 3 N R 2 NH

5 Chem 225 Notes Ch 23: Amines Page 132 III.Synthesis of Amines A. S N 2 Alkylation Observed reaction: Ammonia Problem: Polyalkylation Ammonia can deprotonate the ammonium salt of the product. The product amine can then react by S N 2 again: One solution is to use a large excess of ammonia, so that the product can t compete. The two reactions below also are used to avoid polyalkylation Observed reaction: Azide Ion Observed reaction: Gabriel Synthesis

6 Chem 225 Notes Ch 23: Amines Page 133 Mechanism of Gabriel Synthesis B. Reduction of Aromatic Nitro Groups Observed Reaction: C. Reductive Amination Observed Reaction: Mechanism:

7 Chem 225 Notes Ch 23: Amines Page 134 Example: D. Reduction of Amides and Nitriles Observed Reactions:

8 Chem 225 Notes Ch 23: Amines Page 135 IV.Reactions of Amines A. Reaction with Nitrous Acid Observed Reaction (1 o Amines): Observed Reaction (2 o amines): Alkyl diazonium ions are unstable and lose nitrogen (N 2 ) to form carbocations. N-Nitrosoamines are carcinogens. Aromatic diazonium (arenediazonium) ions are more stable, and can be used to put groups on the aromatic ring (see next reaction).

9 Chem 225 Notes Ch 23: Amines Page 136 B. Arenediazonium Replacement Reactions 1) Hydroxylation N N dil. H OH 2 SO 4, 2) Fluorination N N HBF F 4 3) Sandmeyer Reaction: Chlorination N N Cl CuCl 4) Sandmeyer Reaction: Bromination N N Br CuBr 5) Sandmeyer Reaction: Cyanation N N C N CuCN 6) Iodination N N I KI 7) Hydrogen Replacement N N H H 3 PO 2 8) Diazonium Coupling N N N N EDG EDG

10 Chem 225 Notes Ch 23: Amines Page 137 C. Hofmann Elimination Observed Reaction: Mechanism: Regioselectivity: The less substituted alkene is preferred (Hofmann rule):

Ammonia Primary Secondary Tertiary Quarternary Ammonium Ion

1 Chapter 19: Amines I. Introduction: Classification: NH 3 RNH 2 R 2 NH R 3 N R 4 N + Ammonia Primary Secondary Tertiary Quarternary Ammonium Ion Amines are a very common functional group in organic chemistry,