NITROGEN CONTAINING COMPOUNDS (1)

Nitrogen Containing Compounds Study Material

Amines

Definition

Amines are organic compounds derived from ammonia (NH3) by replacing one or more hydrogen atoms with alkyl or aryl groups. This results in compounds that play crucial roles in various biochemical processes and as intermediates in chemical syntheses.

Classification

  • Primary (1°) amine: Contains one alkyl group attached to the nitrogen atom.

  • Secondary (2°) amine: Contains two alkyl groups attached.

  • Tertiary (3°) amine: Contains three alkyl groups attached.

General Formula

The general formula for amines is CnH2n+3N, reflecting the number of carbon and nitrogen atoms present in the structure.

Preparation of Amines

  • Ammonolysis of Alkyl Halides: Involves the reaction of alkyl halides with ammonia, producing various amine types and quaternary ammonium salts, primarily yielding primary amines when excess ammonia is employed.

  • Ammonolysis of Alcohols: Similar to ammonolysis of alkyl halides, this method also yields a variety of amines.

  • Reduction Reactions:

    • Reduction with lithium aluminum hydride (LiAlH4) or sodium in ethanol (Na/C2H5OH) typically transforms amides or other compounds into amines.

    • Mendius Reaction: This method specifically refers to the reduction of nitriles to primary amines.

    • Hydrolysis of isocyanides and isocyanates can produce amines as well.

    • Hofmann's Bromamide Reaction: Involves converting an alkanamide into a 1° amine through treatment with bromine and an alkali.

    • Grignard Reagent Reaction: This reaction produces amines by reacting Grignard reagents with chloramines.

    • Gabriel Phthalimide Synthesis: A synthetic route that provides methods to obtain pure primary amines by reacting phthalimide with alkyl halides.

    • Curtius Reaction: This procedure transforms carboxylic acids into primary amines via a series of reactions involving azides.

    • Schmidt Reaction: This methodology produces primary amines by reacting isocyanates with hydrazoic acid.

Separation of Amines

Methods

  • Fractional Distillation: This method separates amines based on their boiling points, allowing for the isolation of different types of amines.

  • Hinsberg Method: Utilizes benzene sulphonyl chloride to differentiate between primary, secondary, and tertiary amines based on their solubility in potassium hydroxide (KOH).

  • Hofmann Method: This method employs ethyl oxalate to facilitate the separation of amines.

Physical Properties of Amines

  • The physical properties of amines vary significantly:

    • Hydrogen Bonding: 1° and 2° amines exhibit stronger hydrogen bonding compared to 3° amines, significantly affecting their boiling points.

    • Boiling Point Ranking: The boiling points for amines generally follow the trend: 1° > 2° > 3°.

    • Solubility: The solubility of amines in water decreases with increasing molecular weight due to the hydrophobic nature of larger alkyl groups.

Chemical Properties of Amines

  • Amines display basic character owing to the presence of a lone pair of electrons on the nitrogen atom, which can accept protons (H+).

  • Basic Strength Order in Aqueous Solution: The basic strength varies among amines as follows: (CH3)2NH > CH3NH2 > (CH3)3N > NH3.

  • Reactions:

    • Amines readily form salts when reacted with acids.

    • They react with alkyl halides to produce various amines, leading to nucleophilic substitution reactions.

    • Acetylation occurs with acyl chlorides to form amides.

    • Benzoylation (Schotten Baumann Reaction): A process where amines react with benzoyl chloride to yield benzamides.

    • Amines exhibit mild acidity when interacting with active metals.

    • They react with Tilden reagent for various tests and applications.

    • Schiff Bases Formation: Amines react with aldehydes or ketones, leading to the formation of Schiff bases, which are important intermediates in organic synthesis.

    • Carbylamine Reaction: A test specifically for primary amines that generates isocyanides upon reaction with chloroform and KOH.

    • Hofmann's Mustard Oil Test: Produces isothiocyanate upon treatment of amines with isothiocyanate solution.

    • Nitrous Acid Reaction: When amines react with nitrous acid (HNO2), nitrogen gas is evolved, a characteristic reaction for primary amines.

Points to Remember

  • Carcinogenicity: It is crucial to note that nitrosoamines are carcinogenic compounds and pose health risks.

  • Chirality: Amines can possess chiral nitrogen atoms; however, they do not form stable enantiomers due to rapid inversion of configuration.

  • Schiff's Bases: Primary amines when reacting with carbonyl compounds form Schiff bases, also known as anils.

Aniline (C6H5NH2)

Preparation Methods

  • Laboratory Method: Aniline can be synthesized from nitrobenzene using tin and hydrochloric acid (Sn/HCl).

  • Industrial Method: A similar reduction process is employed on a larger scale using various reducing agents, enhancing efficiency in aniline production.

  • From Phenol: Aniline can also be formed through a reaction between phenol and ammonia.

  • Hofmann's Bromamide Reaction: Converts benzamide directly into aniline, emphasizing its versatility.

  • Grignard Reagent Reaction: An alternative synthesis route where Grignard reagents react with chloramine.

Physical Properties of Aniline

  • Aniline is a colorless oily liquid but darkens to a brown color over time due to oxidation.

  • The boiling point is approximately 183°C, and it is heavier than water, leading to distinct physical behaviors in various solvents.

  • Aniline is toxic and has an unpleasant, pungent odor, necessitating careful handling during laboratory work.

Types of Reactions for Aniline

  • Basic Reaction: Aniline behaves as a weak base, forming salts with acids and indicating its basic properties.

  • Alkylation and Acylation Reactions: These reactions expand the diversity of derivatives obtainable from aniline, making it a key reagent in synthetic organic chemistry.

  • Carbylamine Reaction: Similar to other amines, aniline reacts to form isocyanides, useful in synthetic pathways.

  • Hofmann's Mustard Oil Reaction: This reaction results in the formation of isothiocyanates when aniline reacts with thiocyanate.

  • Reactions with Aldehydes: Aniline can create Schiff bases when treated with various aldehydes, which serve as intermediates in many chemical reactions.

  • Diazotization Reaction: This classic reaction involves converting aniline into diazonium salts, which are pivotal intermediates for the synthesis of azo compounds and other organic transformations.

Nitrobenzene (C6H5NO2)

Properties

  • Nitrobenzene is characterized by its light yellow oily appearance and a distinctive bitter almond smell; it is also steam volatile, denoting its tendency to evaporate.

  • The boiling point of nitrobenzene is about 211°C, and it is heavier than water, influencing its behavior in mixtures.

Chemical Reactions

  • Nitrobenzene participates in electrophilic substitution and nucleophilic substitution reactions, which are fundamental in synthetic organic chemistry.

  • Reduction of nitrobenzene with reagents like zinc and ammonium chloride results in the formation of hydroxylamine.

Usage of Nitrobenzene

  • Nitrobenzene is widely utilized as a solvent in various chemical reactions and is crucial in the manufacture of aniline and azo dyes, demonstrating its importance in both academic and industrial chemistry settings.