Study Notes on Amines and Diazonium Salts

  • Definition: Amines are an important class of organic compounds that are derived by replacing one or more hydrogen atoms of an ammonia molecule with alkyl or aryl group(s).

  • Occurrence in Nature:

    • Found in proteins, vitamins, alkaloids, and hormones.

  • Synthetic Applications:

    • Used in the production of polymers, dyes, and drugs.

  • Examples of Biologically Active Compounds:

    • Adrenaline and ephedrine: Both contain secondary amino groups and are utilized to increase blood pressure.

    • Novocain: A synthetic amino compound employed as an anesthetic in dentistry.

    • Benadryl: A well-known antihistaminic drug that contains a tertiary amino group.

    • Quaternary ammonium salts: Used as surfactants.

    • Diazonium salts: Serve as intermediates in the preparation of various aromatic compounds, including dyes.

  • Unit Overview: The unit focuses on amines and diazonium salts.

I. AMINES

  • Nature of Amines:

    • Amines are derivatives of ammonia formed by replacing hydrogen atoms with alkyl and/or aryl groups.

    • Structure: The nitrogen atom in amines is trivalent and contains an unshared pair of electrons.

    • Hybridization: Nitrogen in amines is sp³ hybridized, resulting in a pyramidal geometry.

    • Orbital Overlap: The three sp³ hybridized orbitals of nitrogen overlap with orbitals from hydrogen or carbon.

    • Pyramidal Structure: The angle C–N–E (where E can be C or H) is less than 109.5°, e.g., 108° in trimethylamine.

  • Classification of Amines:

    • Primary (1°) Amines: One hydrogen atom of ammonia replaced (e.g., RNH₂).

    • Secondary (2°) Amines: Two hydrogen atoms replaced (e.g., R-NHR’).

    • Tertiary (3°) Amines: Three hydrogen atoms replaced (e.g., R₃N).

    • Simple vs. Mixed Amines: Simple amines have identical alkyl groups, while mixed amines contain different groups.

  • Nomenclature:

    • Common system names aliphatic amines using the alkyl prefix with "amine" (e.g., methylamine).

    • For secondary/tertiary amines, prefixes "di" or "tri" are used when groups are the same.

    • IUPAC Nomenclature: Primary amines are termed as alkanamines by replacing 'e' of alkane with 'amine' (e.g., CH₃NH₂ is methanamine).

    • In cases with multiple amino groups, positions are numbered on carbon atoms bearing –NH₂, applying prefixes as needed (e.g., H₂N–CH₂–CH₂–NH₂ is ethane-1,2-diamine).

    • For secondary and tertiary amines, substituents on nitrogen are labeled using locant N (e.g., CH₃NHCH₂CH₃ becomes N-methylethanamine).

  • Examples of Amines (Table 9.1):

    • Ethylamine: Ethanamine

    • n-Propylamine: Propan-1-amine

    • Isopropylamine: Propan-2-amine

    • Trimethylamine: N,N-Dimethylmethanamine

    • Aniline: Benzenamine

    • p-Bromoaniline: 4-Bromoaniline

II. PREPARATION OF AMINES

  • Methods:

    1. Reduction of Nitro Compounds:

      • Nitro compounds can be reduced to amines using hydrogen gas in the presence of nickel, palladium, or platinum, or through reduction with metals in acidic medium.

      • Nitroalkanes can also be reduced similarly, with iron scrap and HCl being preferred due to the hydrolysis of FeCl2.

    2. Ammonolysis of Alkyl Halides:

      • Alkyl or benzyl halides react with ammonia in ethanol, replacing the halogen with an amino group. This conversion is called ammonolysis.

      • The reaction is performed at high temperatures (373 K) and yields primary amines that can react further to form secondary and tertiary amines.

    3. Reduction of Nitriles:

      • Nitriles are converted to primary amines using lithium aluminium hydride (LiAlH4) or catalytic hydrogenation.

    4. Reduction of Amides:

      • Amides can be reduced with LiAlH4, leading to the formation of amines.

    5. Gabriel Phthalimide Synthesis:

      • This method is specifically for the preparation of primary amines. Phthalimide reacts with KOH and an alkyl halide, followed by alkaline hydrolysis to yield the primary amine.

      • Aromatic primary amines cannot be prepared this way due to the unreactivity of aryl halides against nucleophiles.

    6. Hofmann Bromamide Degradation Reaction:

      • An amide is treated with bromine in alkaline solution, resulting in the primary amine with one less carbon atom than the starting amide.

III. PHYSICAL PROPERTIES

  • State and Odor:

    • Lower aliphatic amines are typically gases with a fishy odor. Primary amines with three or more carbon atoms are liquids, while higher amines are solid.

    • Aniline and other arylamines are usually colorless but may get colored upon storage due to oxidation.

  • Solubility:

    • Lower aliphatic amines are soluble in water due to hydrogen bonding; solubility decreases with increasing molar mass.

    • Solubility patterns are influenced by the electronegativity of nitrogen and comparison with alcohols.

    • Primary and secondary amines participate in intermolecular hydrogen bonding, while tertiary amines do not, affecting their physical properties.

  • Boiling Points:

    • The order of boiling points for isomeric amines generally is:

    1. Primary Amines > Secondary Amines > Tertiary Amines

IV. CHEMICAL REACTIONS

  1. Basic Character: - Amines are basic and can react with acids to form ammonium salts. The basicity of amines is expressed in terms of $Kb$ and $pKb$.

    • The larger the value of $Kb$ or the smaller the value of $pKb$, the stronger the base.

    • Aliphatic amines are generally stronger bases than ammonia due to the +I effect of alkyl groups.

    • Aromatic amines are weaker than ammonia due to the electron-withdrawing nature of the aryl group.

V. DIAZONIUM SALTS

  • General Formula:

    • Diazonium salts have the general formula D R-N₂⁺X− where R is an aryl group and X is a counterion such as Cl−, Br−, HSO₄−, BF₄−, etc.

    • Named by adding “diazonium” to the name of the corresponding aromatic compound followed by the name of the anion (e.g., benzenediazonium chloride).

  • Preparation:

    • Formed through diazotization, where a primary aromatic amine reacts with nitrous acid at low temperatures (273-278 K).

    • The diazonium salt is unstable and is not usually stored; it is used immediately after preparation.

  • Reactions:

    • Divided into reactions involving substitution of nitrogen and retention of the diazo group (coupling reactions).

    • Fundamental for the synthesis of a variety of aromatic compounds through electrophilic substitution.

  • Importance in Synthesis:

    • They allow the preparation of substituted aromatic compounds that cannot be prepared via direct substitution.

    • Reactions involve the introduction of halides, hydroxyl, and other functional groups into the aromatic ring.