Nitrogen Containing Organic Compounds - Basic Nature of Amines

  • Amines are organic compounds that contain one or more nitrogen atoms bonded to carbon atoms.
  • They can be classified as primary, secondary, or tertiary amines, based on the number of alkyl groups attached to the nitrogen atom.
  • Amines are considered derivatives of ammonia (NH3), where one or more hydrogen atoms are replaced by alkyl groups.
  • The basicity of amines depends on the lone pair of electrons on the nitrogen atom.
  • Amines can act as both bases and nucleophiles in organic reactions.

Classification of Amines

  1. Primary amines:
    • Have one alkyl group attached to the nitrogen atom.
    • General formula: R-NH2 (R represents an alkyl group)
    • Examples: methylamine (CH3NH2), ethylamine (C2H5NH2)
  1. Secondary amines:
    • Have two alkyl groups attached to the nitrogen atom.
    • General formula: R2-NH (R represents alkyl groups)
    • Examples: dimethylamine (CH3)2NH, diethylamine (C2H5)2NH
  1. Tertiary amines:
    • Have three alkyl groups attached to the nitrogen atom.
    • General formula: R3N (R represents alkyl groups)
    • Examples: trimethylamine (CH3)3N, triethylamine (C2H5)3N

Basicity of Amines

  • Amines can behave as bases due to the presence of a lone pair of electrons on the nitrogen atom.
  • The lone pair of electrons can accept a proton (H+) from an acid, forming an ammonium salt.
  • The basicity of amines increases with the number of alkyl groups (tertiary amines being the strongest bases) due to increased electron-donating effects.
  • The basicity of amines is also affected by the electron-withdrawing or electron-donating nature of any substituents present on the alkyl groups.

Comparison with Ammonia

  • Amines are analogous to ammonia, where one or more hydrogen atoms are replaced by alkyl groups.
  • While ammonia (NH3) is a weak base, amines are generally stronger bases due to the alkyl groups providing electron-donating effects.
  • The presence of an electron-donating alkyl group increases the availability of lone pair electrons, enhancing the basicity.

Acid-Base Reactions of Amines

  • Amines can react with acids to form ammonium salts.
  • The lone pair of electrons on the nitrogen atom in the amine reacts with the proton from the acid.
  • The reaction is similar to the reaction of ammonia with an acid, resulting in the formation of a positively charged ammonium ion.
  • The salt formed is generally a solid crystalline compound.

Examples of Acid-Base Reactions

  • Reaction of methylamine (CH3NH2) with hydrochloric acid (HCl):
    • CH3NH2 + HCl → CH3NH3+Cl- (Methylammonium chloride)
  • Reaction of ethylamine (C2H5NH2) with sulfuric acid (H2SO4):
    • C2H5NH2 + H2SO4 → C2H5NH3+HSO4- (Ethylammonium bisulfate)

Physical Properties of Amines

  • Amines are mostly colorless liquids or solids with distinct odors.
  • Lower molecular weight amines have strong, pungent smells (e.g., ammonia-like smell).
  • Higher molecular weight amines can have fishy or foul odors.
  • Amines have higher boiling points than corresponding alkanes of similar molecular weights, due to intermolecular hydrogen bonding.

Solubility of Amines

  • Small, lower molecular weight amines are soluble in water due to their ability to form hydrogen bonds with water molecules.
  • However, as the size of alkyl groups attached to the nitrogen increases, the solubility in water decreases.
  • Amines with more than six carbon atoms are generally insoluble in water due to the hydrophobic nature of the longer alkyl chains.

Physical Properties of Amines (contd.)

  • Amines can exhibit isomerism:

    • Structural isomerism: Different arrangement of alkyl groups around the nitrogen atom.
    • Geometrical isomerism: Restricted rotation around the nitrogen-carbon bond in cyclic primary amines.

  • Amines can undergo oxidation reactions to form nitro compounds:

    • Oxidizing agents such as chromic acid (H2CrO4) or potassium dichromate (K2Cr2O7) can be used.
    • Example: Conversion of methylamine to methyl nitro compound.

  • Amines can be synthesized through various methods:

    • Reduction of nitro compounds: Nitro compounds can be reduced to primary amines using reducing agents like metal hydrides (e.g., LiAlH4).
    • Gabriel synthesis: Primary alkyl halides react with potassium phthalimide to form N-substituted phthalimide, which can be hydrolyzed to form primary amines.
    • Hoffmann rearrangement: Treatment of primary amides with bromine and sodium hydroxide leads to the formation of primary amines.

Reactions of Amines with Acids

  • Amines can undergo protonation when treated with acids:
    • The lone pair of electrons on the nitrogen atom forms a bond with a proton (H+) from the acid.
    • This results in the formation of an ammonium ion.
  • The reaction between amines and acids is generally exothermic.
  • The extent of protonation depends on the basicity of the amine and the acidity of the acid.
  • Examples:
    1. Reaction of ethylamine (C2H5NH2) with hydrochloric acid (HCl):
      • C2H5NH2 + HCl → C2H5NH3+Cl- (Ethylammonium chloride)
    2. Reaction of tert-butylamine [(CH3)3CNH2] with sulfuric acid (H2SO4):
      • (CH3)3CNH2 + H2SO4 → (CH3)3CNH3+HSO4- (t-Butylammonium bisulfate)

Basicity of Amines and pKa Values

  • The basicity of an amine can be quantified using the pKa value.
  • pKa is the negative logarithm of the acid dissociation constant (Ka) and indicates the strength of an acid or base.
  • Lower pKa values indicate stronger acids, while higher pKa values indicate weaker acids.
  • The pKa values of some common amines are as follows:
    • Ammonia (NH3): pKa = 9.25
    • Methylamine (CH3NH2): pKa = 10.64
    • Diethylamine (C2H5)2NH: pKa = 10.76
    • Triethylamine (C2H5)3N: pKa = 11.0

Factors Affecting Amine Basicity

  • Steric hindrance:
    • The bulkiness of alkyl groups attached to the nitrogen atom reduces the basicity.
    • Primary amines are generally more basic than secondary or tertiary amines due to less steric hindrance.
  • Electron-withdrawing groups:
    • Substituents that withdraw electron density from the nitrogen atom reduce the basicity.
    • Examples: Nitro groups (-NO2), carbonyl groups (>C=O).
  • Electron-donating groups:
    • Substituents that donate electron density to the nitrogen atom increase the basicity.
    • Examples: Alkyl groups (-R), amino groups (-NH2).
  • Resonance effects:
    • Aromatic amines, due to the presence of conjugation, have lower basicity than aliphatic amines.

Nomenclature of Amines

  • IUPAC (International Union of Pure and Applied Chemistry) nomenclature for amines:
    • Identify the longest carbon chain connected to the nitrogen atom and use the suffix -amine.
    • Number the carbon atoms in the chain, starting from the end closest to the nitrogen atom.
    • Prefixes like N-methyl, N-ethyl, etc. are used to indicate substituents on the nitrogen atom.
  • Common names of amines:
    • Use the alkyl groups attached to the nitrogen atom as prefixes, followed by the word “amine.”
    • For primary amines, the alkyl groups are listed in alphabetical order.
    • Examples: methylamine, ethylamine, N,N-dimethylamine.

Uses of Amines

  • Amines have various applications in industries and everyday life:
    • Pharmaceuticals: Amines are key building blocks in the synthesis of many drugs.
    • Dyes and pigments: Amines are used as intermediates in the production of dyes.
    • Rubber and plastics: Amines are used as accelerators and stabilizers in the polymer industry.
    • Fuels: Amines can be used as additives to improve the performance of gasoline and diesel fuels.
  • Amines also have biological significance, being important components of proteins, amino acids, and vitamins.

Quiz Time!

  • Identify the type of amine:
    1. CH3CH2NHCH3
    2. (CH3)2CHNH2
    3. (CH3)3N
  • Determine the pKa value:
    1. (CH3)2NH
    2. C2H5NH2
  • Which of the following groups decreases the basicity of an amine?
    1. Nitro group (-NO2)
    2. Methyl group (-CH3)
    3. Carbonyl group (>C=O)

Quiz Answers

  • Identify the type of amine:
    1. CH3CH2NHCH3: Secondary amine
    2. (CH3)2CHNH2: Tertiary amine
    3. (CH3)3N: Tertiary amine
  • Determine the pKa value:
    1. (CH3)2NH: pKa = 10.76
    2. C2H5NH2: pKa = 10.64
  • Which of the following groups decreases the basicity of an amine?
    1. Nitro group (-NO2)
    2. Carbonyl group (>C=O)

Summary

  • Amines are nitrogen-containing organic compounds.
  • They can be classified as primary, secondary, or tertiary amines based on the number of alkyl groups attached to the nitrogen atom.
  • Amines have a lone pair of electrons on the nitrogen atom, making them basic in nature.
  • The basicity of amines increases with the number of alkyl groups attached to the nitrogen atom.
  • Amines can react with acids to form ammonium salts.
  • Basicity of amines is influenced by steric hindrance, electron-withdrawing/donating groups, and resonance effects.

References

  1. McMurry, J. (2016). Organic Chemistry (9th ed.). Boston, MA: Cengage Learning.
  1. Solomons, T. W. G., Fryhle, C. B., & Snyder, S. A. (2020). Organic Chemistry (12th ed.). Hoboken, NJ: John Wiley & Sons, Inc.

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