Chapter 23 - Amines
Chapter 23 - Amines CHEM 225
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This 5 page Class Notes was uploaded by MelLem on Monday April 11, 2016. The Class Notes belongs to CHEM 225 at Simmons College taught by Professor gurney in Spring 2016. Since its upload, it has received 19 views. For similar materials see Organic chemistry 2 in Chemistry at Simmons College.
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Date Created: 04/11/16
Chapter 23 Amines Lecture 18 Chem 225 23.1 Introduction to Amines Classification of Amines: • Amines are derivatives of ammonia, in which one or more of the protons has been replaced with alkyl or aryl groups. • Amines are classified as primary, secondary, and tertiary, this is in regards to how many R groups are attached to the Nitorgen atom. • Amines are abundant in nature, naturally occurring amines isolated from plants are called alkaloids. • Many pharmaceuticals also contain amine moieties. Reactivity of Amines: • The nitrogen atom of an amine possess a lone pair that represents a region of high electron density. o The presence of this lone pair is responsible for most of the reactions exhibited by amines. o Specifically, the lone pair can function as a base or nucleophile. 23.2 Nomenclature of Amines Nomenclature of Primary Amines: • A primary amine is a compound containing an NH2 group connected to an alkyl group. • If the R group is generally simple, they tend to be named as alkyl amines. Ex: (Draw the Structures) Ethyl Amine Isopropyl Amine Cyclohexyl Amine • Primary amines that contain more complex alkyl groups are generally named as alkanamines, with the suffix –amine. • If there are other functional groups present (of higher priority), the amine is often named as a substituent (Amino) Ex: 4-aminobutanol Para-animobenzoic acid • Aromatic Amines are also called aryl amines – generally named as derivatives of aniline. Ex: Aniline Meta-chloroaniline Nomenclature of Secondary and Tertiary Amines: • Can also be named as alkyl amines or as alkanamines • Naming will depend on the complexity of the molecule Ex: (Draw the Structures) Ethyl methyl propyl amine Diethyl amine Complex structures are named as follows: (s) 2,2-dichloro-N-Ethyl-N-Methyl-3-hexanamine • The N represents that the substituent is attached to the Nitrogen rather than a carbon atom on the structure. 23.3 Properties of Amines Geometry: • The nitrogen atom of an amine is typically sp3 hybridized with the lone pair occupying an sp3 hybridized orbital. • All 4 orbitals are arranged in the shape that approximates a tetrahedron with bond angles of 108 degrees. • Amines containing three different alkyl groups are chiral compounds. o Compounds of this type are generally not optically active at room temperature because pyramidal inversion occurs rapidly creating racemic mixtures. Colligative Properties • Amines exhibit solubility trends that are similar to the trends exhibited by alcohols. o Amines with fewer than 5 carbons per amino group will typically be soluble in water. • Primary and secondary amines can form intermolecular Hydrogen bonds and typically have higher Boiling Points than alkanes but lower boiling points than alcohols. • The boiling points increase as a function of their capacity to form hydrogen bonds. Ex: Propane -42C Ethyl Amine 17C Ethanol 78C • Primary Amines will typically exhibit higher boiling points than tertiary amines. Ex: Propylamine 50C Ethyl methyl Amine 34C Trimethyl amine 3C Basicity of Amines: • One of the most important properties of amines is their basicity. • Amines are generally stronger bases than alcohols or ethers, and they can effectively be protonated by weak acids. Delocalization Effects: • The ammonium ions of most alkyl amines are characterized by a PKa value between 10- 11, but aryl amines are very different. • Ammonium ions of aryl amines are more acidic than the ammonium ions of alkyl amines. o Aryl amines are less basic. • The lone pair occupies a p orbital and is delocalized by the aromatic system. o The resonance stabilization is lost if the lone pair is protonated. • EDG – such as methoxy will slightly increase the basicity of aryl amines. • EWG – such as nitro will significantly decrease the basicity of aryl amines. Amines at Physiological pH: • An amine moiety also exists primarily as a charged ammonium ion at physiological pH. R3N + H + ---▯- R3NH + <-- 23.4 Preparation of Amines – A Review Preparing an amine from an alkyl halide: • Amines can be prepared from alkyl halides in a 2 step process in which the alkyl halide is converted to a nitrile, which then undergoes reduction. 1) Cyanide ion functions as a nucleophile, displacing the halide leaving group. 2) involves reduction of the resulting nitrile. This reduction can be accomplished with a strong reducing agent such as LAH. Preparing an Amine from carboxylic acids CH3 (CH2)4 COOH --- 1) SOCl2 / 2) H2O ▯ CH3(CH2)4 CONH2 --- 1) Xs LAH / 2) H2O ▯ CH3(CH2)5 NH2 1. The carboxylic acid is first converted into an amide, which is then reduced with a strong reducing agent such as LAH to give the amine. Preparing aniline and its derivatives from Benzene: Benzen + HNO3/ H2SO4 ▯ Nitro Benzene Nitrobenzene + H2/Pt or Fe, Zn, Sn, or SnCl2 / H30+ ▯ Aniline 23.5 Preparation of amines Via substitution Reactions Alkylation of Ammonia: • Ammonia is a very good nucleophile and will readily undergo alkylation when treated with alkyl halides. • As a primary amine is formed, it can further undergo alkylation to produce a secondary amine which can then undergo even further alkylation to produce a tertiary amine. • The tertiary amine is able to undergo even further alkylation to produce a quaternary ammonium salt. o If the quaternary ammonium salt is the desired product then an excess of the alkyl halide is used and ammonium is said to undergo exhaustive alkylation. • Monoalkylation is difficult to achieve because each successive alkylation renders the nitrogen atom more nucleophilic. The Azide Synthesis: • A better method for preparing primary amines than alkylation of ammonia because it avoids the formation of secondary and tertiary amines. CX + NAN3 ▯ CN3 CN3 + H2/Pt or 1) LAH / 2) H2O ▯ CNH2 23.12 Nitrogen Heterocycles • A heterocycle is a ring that contains atoms of more than one element. • Common organic heterocycles are comprised of carbon and either, nitrogen, oxygen or sulfur. Pyrrole and Imidazole: • Pyrrole- o 5 membered aromatic ring containing one Nitrogen. o The lone pair participates in its aromaticity. • Imidazole – o Similar to pyrrole, but has a second Nitrogen at the 3 position. o Hisamine, a imidazole is an important biological compound. Pyridine and Pyrimidine: • Pyridine – o 6 membered aromatic ring containing 1 Nitrogen atom o The lone pair of the nitrogen occupies an sp2 hybridized orbital, as a result pyridine is a stronger base than pyrrole. • Pyrimidine – o Similar to pyridine, but has an extra Nitrogen atom at the 3 position. o Pyrimidine is less basic than Pyridine due to the inductive effects of the second nitrogen.
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