🧫Organic Chemistry II Unit 5 – Amines and Nitrogen-Containing Compounds

What Are Amines?

• Organic compounds that contain a nitrogen atom with a lone pair of electrons
• Derivatives of ammonia (NH3) where one or more hydrogen atoms have been replaced by an alkyl or aryl group
• Can be primary (RNH2), secondary (R2NH), or tertiary (R3N) depending on the number of alkyl or aryl groups attached to the nitrogen atom
• Play important roles in biological systems (neurotransmitters, hormones, and alkaloids)
• Used in various applications (dyes, pharmaceuticals, and polymers)
• Exhibit unique properties due to the presence of the nitrogen atom with a lone pair of electrons
• Can act as bases and nucleophiles in chemical reactions

Structure and Classification

• Amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom
  • Primary amines (RNH2): one alkyl or aryl group attached to the nitrogen atom
  • Secondary amines (R2NH): two alkyl or aryl groups attached to the nitrogen atom
  • Tertiary amines (R3N): three alkyl or aryl groups attached to the nitrogen atom
• Quaternary ammonium salts (R4N+) are formed when all four hydrogen atoms of ammonium ion (NH4+) are replaced by alkyl or aryl groups
• The nitrogen atom in amines is sp3 hybridized, resulting in a tetrahedral geometry
• The lone pair of electrons on the nitrogen atom contributes to the basic and nucleophilic properties of amines
• Amines can also be classified as aliphatic (alkyl groups attached to nitrogen) or aromatic (aryl groups attached to nitrogen)
• The structure of amines determines their physical and chemical properties, such as boiling point, solubility, and reactivity

Nomenclature

• Common names of amines are derived by naming the alkyl or aryl groups attached to the nitrogen atom followed by the suffix "-amine"
  • Example: methylamine (CH3NH2), dimethylamine ((CH3)2NH), trimethylamine ((CH3)3N)
• IUPAC nomenclature: the prefix "amino-" is added to the name of the parent alkane, and the position of the amino group is indicated by a number
  • Example: 2-aminopropane (CH3CH(NH2)CH3)
• For secondary and tertiary amines, the prefix "N-" is used to indicate the alkyl or aryl groups attached to the nitrogen atom
  • Example: N-methylpropanamine (CH3NHCH2CH2CH3)
• Aromatic amines are named by adding the suffix "-amine" to the name of the aromatic compound
  • Example: aniline (C6H5NH2)
• When multiple amino groups are present, the prefixes "di-," "tri-," etc., are used to indicate the number of amino groups
  • Example: ethylenediamine (H2NCH2CH2NH2)

Physical Properties

• Lower molecular weight amines are gases or liquids at room temperature, while higher molecular weight amines are solids
• Have characteristic ammonia-like odor
• Form hydrogen bonds with water, resulting in high solubility in water for lower molecular weight amines
• Solubility in water decreases as the size of the alkyl or aryl groups increases
• Higher boiling points compared to hydrocarbons of similar molecular weight due to hydrogen bonding
• Boiling points increase with increasing molecular weight and the number of hydrogen atoms available for hydrogen bonding
• Lower vapor pressure compared to hydrocarbons of similar molecular weight
• Can form intermolecular hydrogen bonds with other amines or with water

Basicity and Reactivity

• Amines are basic compounds due to the lone pair of electrons on the nitrogen atom
• Can accept a proton (H+) to form an ammonium ion (RNH3+)
• Basicity decreases in the order: primary > secondary > tertiary amines
  • Steric hindrance and inductive effects of alkyl groups reduce the basicity of secondary and tertiary amines
• Aromatic amines are generally less basic than aliphatic amines due to the delocalization of the lone pair of electrons into the aromatic ring
• The basicity of amines is affected by the solvent, with polar protic solvents (water, alcohols) reducing the basicity through hydrogen bonding
• Amines can act as nucleophiles in various chemical reactions, such as substitution and addition reactions
• The nucleophilicity of amines follows the order: primary > secondary > tertiary amines
  • Steric hindrance in secondary and tertiary amines reduces their nucleophilicity

Synthesis of Amines

• Reduction of nitriles (RCN) using hydrogen gas and a metal catalyst (Ni, Pt, or Pd) yields primary amines
  • Example: CH3CN + 2H2 → CH3CH2NH2
• Reduction of amides (RCONH2) using LiAlH4 produces primary amines
  • Example: CH3CONH2 + LiAlH4 → CH3CH2NH2
• Reductive amination of aldehydes or ketones with ammonia or amines in the presence of a reducing agent (NaBH4 or H2/catalyst) yields primary, secondary, or tertiary amines
  • Example: CH3CHO + NH3 + NaBH4 → CH3CH2NH2
• Gabriel synthesis: phthalimide is alkylated with an alkyl halide, followed by hydrazinolysis to yield a primary amine
  • Example: C6H4(CO)2NH + CH3CH2Br → C6H4(CO)2NCH2CH3 → CH3CH2NH2
• Hofmann rearrangement: amides are treated with bromine and a strong base (NaOH) to produce primary amines with one less carbon atom
  • Example: CH3CH2CONH2 + Br2 + NaOH → CH3CH2NH2 + CO2
• Curtius rearrangement: acyl azides are heated to produce isocyanates, which can be hydrolyzed to yield primary amines
  • Example: CH3CH2CON3 → CH3CH2NCO → CH3CH2NH2

Reactions of Amines

• Alkylation: amines react with alkyl halides to form alkylated amines
  • Example: CH3NH2 + CH3CH2Br → CH3NHCH2CH3 + HBr
• Acylation: amines react with acyl halides or anhydrides to form amides
  • Example: CH3NH2 + CH3COCl → CH3NHCOCH3 + HCl
• Sulfonylation: amines react with sulfonyl chlorides to form sulfonamides
  • Example: CH3NH2 + CH3SO2Cl → CH3NHSO2CH3 + HCl
• Diazotization: primary aromatic amines react with nitrous acid (HNO2) to form diazonium salts
  • Example: C6H5NH2 + HNO2 → C6H5N2+ + 2H2O
• Coupling reactions: diazonium salts can undergo coupling reactions with aromatic compounds to form azo dyes
  • Example: C6H5N2+ + C6H5OH → C6H5N=NC6H4OH + H+
• Hofmann elimination: quaternary ammonium salts can be converted to alkenes by treatment with a strong base
  • Example: (CH3)3NCH2CH3+ + NaOH → CH2=CH2 + (CH3)3N + H2O
• Oxidation: amines can be oxidized to form nitroso compounds, nitro compounds, or imines, depending on the oxidizing agent and reaction conditions
  • Example: (CH3)2NH + H2O2 → (CH3)2N-OH → (CH3)2N=O

Other Nitrogen-Containing Compounds

• Nitriles (RCN): organic compounds containing a -C≡N functional group
  • Can be reduced to primary amines or hydrolyzed to carboxylic acids
• Amides (RCONH2): organic compounds containing a -CONH2 functional group
  • Can be reduced to amines or hydrolyzed to carboxylic acids and ammonia
• Nitro compounds (RNO2): organic compounds containing a -NO2 functional group
  • Can be reduced to amines or used in the synthesis of aromatic compounds
• Azo compounds (R-N=N-R): organic compounds containing an -N=N- functional group
  • Used as dyes and pigments
• Imines (R2C=NR): organic compounds containing a -C=N- functional group
  • Formed by the condensation of aldehydes or ketones with primary amines
• Enamines (R2C=CR-NR2): organic compounds containing a -C=C-NR2 functional group
  • Formed by the condensation of aldehydes or ketones with secondary amines
• Heterocyclic compounds: cyclic compounds containing one or more heteroatoms (nitrogen, oxygen, or sulfur) in the ring
  • Examples: pyridine, pyrrole, imidazole, and indole