🥼Organic Chemistry Unit 24 – Amines and Heterocycles

Introduction to Amines

• Amines are organic compounds that contain a nitrogen atom with a lone pair of electrons
• Amines can be considered derivatives of ammonia ($NH_3$) where one or more hydrogen atoms have been replaced by an alkyl or aryl group
• The nitrogen atom in amines is sp³ hybridized and has a trigonal pyramidal geometry
• Amines are classified as primary, secondary, or tertiary based on the number of alkyl or aryl groups attached to the nitrogen atom
• Amines are widely found in nature, including in amino acids, neurotransmitters (dopamine, serotonin), and alkaloids (morphine, nicotine)
• Amines have diverse applications in the pharmaceutical industry, agriculture, and materials science

Structure and Classification of Amines

• Primary amines have one alkyl or aryl group attached to the nitrogen atom and two hydrogen atoms (R-$NH_2$)
• Secondary amines have two alkyl or aryl groups attached to the nitrogen atom and one hydrogen atom (R₂-NH)
• Tertiary amines have three alkyl or aryl groups attached to the nitrogen atom (R₃-N)
• Quaternary ammonium salts have four alkyl or aryl groups attached to the nitrogen atom and a positive charge ($R_4N^+$)
  • Quaternary ammonium salts are not considered amines because they lack a lone pair of electrons on the nitrogen atom
• Cyclic amines, such as piperidine and pyrrolidine, have the nitrogen atom incorporated into a ring structure
• Aromatic amines, such as aniline, have the nitrogen atom directly attached to an aromatic ring
• The structure of amines determines their physical properties, reactivity, and basicity

Nomenclature of Amines

• Common names of simple amines often end with the suffix "-amine" preceded by the names of the attached alkyl or aryl groups (ethylamine, dipropylamine)
• IUPAC nomenclature for amines follows a systematic approach based on the longest carbon chain
• The suffix "-amine" is added to the name of the parent alkane, and the position of the amino group is indicated by a number (propan-1-amine)
• If the amino group is not the principal functional group, it is designated as an "amino" prefix (2-aminoethanol)
• For secondary and tertiary amines, the alkyl groups are listed as separate words in alphabetical order before the word "amine" (ethylmethylamine, triethylamine)
• Cyclic amines are named by adding the suffix "-ine" to the name of the corresponding cycloalkane (piperidine, pyrrolidine)
• Aromatic amines are named as derivatives of aniline, with substituents on the aromatic ring indicated by numbers (4-chloroaniline)

Physical Properties of Amines

• Lower molecular weight amines are gases or low-boiling liquids at room temperature, while higher molecular weight amines are liquids or solids
• Amines have a characteristic ammonia-like odor, which becomes less pronounced as the molecular weight increases
• Primary and secondary amines can form hydrogen bonds with water and other protic solvents, making them more soluble than their corresponding alkanes
  • The solubility of amines in water decreases as the size of the alkyl groups increases
• Tertiary amines are less soluble in water compared to primary and secondary amines due to the absence of N-H hydrogen bonds
• Amines have higher boiling points than their corresponding alkanes due to the presence of intermolecular hydrogen bonding
• The boiling points of amines increase with increasing molecular weight and the number of N-H bonds
• Amines are generally less dense than water, with densities ranging from 0.6 to 0.8 g/mL

Basicity and Reactivity of Amines

• Amines are basic compounds due to the lone pair of electrons on the nitrogen atom, which can accept a proton (H⁺)
• The basicity of amines is generally lower than that of inorganic bases like sodium hydroxide (NaOH) but higher than that of water
• The basicity of amines depends on the electron-donating or electron-withdrawing nature of the attached groups
  • Alkyl groups are electron-donating and increase the basicity of amines
  • Aryl groups are electron-withdrawing and decrease the basicity of amines
• The basicity of amines also depends on the hybridization of the nitrogen atom and the presence of resonance stabilization
  • sp³ hybridized amines (aliphatic amines) are more basic than sp² hybridized amines (aromatic amines) due to greater s-character of the lone pair
• Amines react with acids to form ammonium salts, which are soluble in water and have higher melting points than the parent amines
• Amines can act as nucleophiles in substitution and addition reactions due to the lone pair of electrons on the nitrogen atom
• The reactivity of amines as nucleophiles decreases in the order primary > secondary > tertiary due to steric hindrance

Synthesis of Amines

• Amines can be synthesized through various methods, depending on the desired structure and substituents
• Reduction of nitriles (R-CN) with hydrogen gas (H₂) and a metal catalyst (Ni, Pt, or Pd) yields primary amines (R-$CH_2NH_2$)
• Reduction of amides (R-CONH₂) with lithium aluminum hydride (LiAlH₄) produces primary amines (R-$CH_2NH_2$)
• Reductive amination of aldehydes or ketones with ammonia or primary amines in the presence of a reducing agent (NaBH₄ or H₂/catalyst) yields primary, secondary, or tertiary amines
• Alkylation of ammonia or primary/secondary amines with alkyl halides (R-X) leads to the formation of primary, secondary, or tertiary amines
  • This method may result in a mixture of products due to multiple alkylations
• Gabriel synthesis involves the reaction of phthalimide with an alkyl halide, followed by hydrazinolysis to yield a primary amine
• Hofmann rearrangement of amides with bromine and sodium hydroxide produces primary amines with one less carbon atom than the starting amide
• Curtius rearrangement of acyl azides with heat or catalysts yields primary amines via isocyanate intermediates

Reactions of Amines

• Amines react with acids to form water-soluble ammonium salts (R-$NH_3^+X^-$)
• Amines undergo N-alkylation with alkyl halides (R-X) to form secondary, tertiary, or quaternary ammonium salts
• Primary aromatic amines (anilines) react with nitrous acid (HONO) to form diazonium salts, which are important intermediates in the synthesis of azo dyes and other substituted aromatic compounds
• Primary and secondary amines react with acyl chlorides (R-COCl) or acid anhydrides ((RCO)₂O) to form amides (R-CONHR' or R-CONR'R'')
• Amines can act as bases and nucleophiles in various organic reactions, such as the Mannich reaction, Schiff base formation, and Michael addition
• Oxidation of primary amines with strong oxidizing agents (H₂O₂, KMnO₄) yields nitroso compounds (R-NO), while milder oxidation produces imines (R-CH=NR')
• Hofmann elimination of quaternary ammonium salts with strong bases (NaOH, KOH) produces alkenes and tertiary amines
• Cope elimination of N-oxides with heat generates alkenes and hydroxylamines

Introduction to Heterocycles

• Heterocycles are cyclic compounds containing at least one heteroatom (non-carbon atom) in the ring structure
• Common heteroatoms in heterocycles include nitrogen (N), oxygen (O), and sulfur (S)
• Heterocycles can be aromatic or non-aromatic, depending on their electronic structure and the number of π electrons in the ring
• Aromatic heterocycles, such as pyridine, furan, and thiophene, follow Hückel's rule of aromaticity (4n+2 π electrons)
• Non-aromatic heterocycles, such as piperidine, tetrahydrofuran, and tetrahydrothiophene, do not meet the criteria for aromaticity
• Heterocycles are widely found in nature, including in nucleic acids (pyrimidines and purines), vitamins (thiamine, biotin), and alkaloids (quinine, morphine)
• Heterocyclic compounds have diverse applications in the pharmaceutical industry, agriculture, and materials science

Common Heterocyclic Compounds

• Pyridine is a six-membered aromatic heterocycle with one nitrogen atom, isoelectronic with benzene
• Pyrrole is a five-membered aromatic heterocycle with one nitrogen atom and a hydrogen atom attached to the nitrogen
• Furan is a five-membered aromatic heterocycle with one oxygen atom
• Thiophene is a five-membered aromatic heterocycle with one sulfur atom
• Imidazole is a five-membered aromatic heterocycle with two nitrogen atoms at positions 1 and 3
• Pyrimidine is a six-membered aromatic heterocycle with two nitrogen atoms at positions 1 and 3
• Indole is a bicyclic aromatic heterocycle consisting of a benzene ring fused to a pyrrole ring
• Quinoline is a bicyclic aromatic heterocycle consisting of a benzene ring fused to a pyridine ring
• Isoquinoline is a bicyclic aromatic heterocycle, an isomer of quinoline, with the nitrogen atom at position 2

Synthesis and Reactions of Heterocycles

• Heterocycles can be synthesized through various methods, such as cyclocondensation reactions, cycloadditions, and ring-closing metathesis
• Pyridines can be synthesized via the Hantzsch pyridine synthesis, which involves the condensation of an aldehyde, two equivalents of a β-ketoester, and ammonia
• Pyrroles can be prepared through the Paal-Knorr synthesis, which involves the condensation of a 1,4-dicarbonyl compound with ammonia or a primary amine
• Furans can be synthesized via the Paal-Knorr furan synthesis, which is similar to the pyrrole synthesis but uses an α-haloketone instead of a 1,4-dicarbonyl compound
• Thiophenes can be prepared through the Paal-Knorr thiophene synthesis, which involves the reaction of a 1,4-dicarbonyl compound with phosphorus pentasulfide (P₂S₅)
• Imidazoles can be synthesized via the Debus-Radziszewski imidazole synthesis, which involves the condensation of a 1,2-dicarbonyl compound, an aldehyde, and ammonia
• Heterocycles undergo various reactions, such as electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal-catalyzed cross-coupling reactions
• Pyridines and quinolines undergo electrophilic aromatic substitution reactions at the 3-position due to the electron-withdrawing effect of the nitrogen atom
• Pyrroles, furans, and thiophenes undergo electrophilic aromatic substitution reactions at the 2- and 5-positions due to the electron-donating effect of the heteroatom
• Heterocycles can also participate in cycloaddition reactions, such as the Diels-Alder reaction, where they can act as either the diene or the dienophile

Applications in Pharmaceuticals and Industry

• Heterocyclic compounds are of great importance in the pharmaceutical industry, with many drugs containing heterocyclic moieties
• Pyridine-based drugs include isoniazid (anti-tuberculosis), nicotinamide (vitamin B3), and pyridoxine (vitamin B6)
• Pyrrole-based drugs include atorvastatin (cholesterol-lowering), ketorolac (anti-inflammatory), and sunitinib (anticancer)
• Furan-based drugs include ranitidine (antacid), nitrofurantoin (antibiotic), and methoxsalen (psoriasis treatment)
• Thiophene-based drugs include olanzapine (antipsychotic), raloxifene (osteoporosis treatment), and tiagabine (anticonvulsant)
• Imidazole-based drugs include cimetidine (antacid), metronidazole (antibiotic), and ondansetron (antiemetic)
• Indole-based drugs include indomethacin (anti-inflammatory), sumatriptan (migraine treatment), and tryptophan (essential amino acid)
• Quinoline-based drugs include quinine (antimalarial), ciprofloxacin (antibiotic), and camptothecin (anticancer)
• Heterocyclic compounds are also used in the development of organic light-emitting diodes (OLEDs), semiconductors, and liquid crystals
• Heterocycles are used as building blocks for the synthesis of various polymers, such as polypyrroles, polythiophenes, and polyanilines, which have applications in energy storage, sensors, and electronic devices
• In the agricultural industry, heterocyclic compounds are used as pesticides, herbicides, and fungicides to protect crops and improve yields