Physical Properties, Reactivity, and Basic Character of Amines

Intermolecular Hydrogen Bonding and Physical Properties

• Extent of Intermolecular Hydrogen Bonding:

  • The degree of intermolecular hydrogen bonding in amines follows the relative order:   \text{Primary} > \text{Secondary} > \text{Tertiary}

  • Primary amines ($\text{R-NH}_2$) possess two hydrogen atoms attached to the nitrogen atom, enabling them to form extensive intermolecular hydrogen bonds between the hydrogen atom of one molecule and the nitrogen atom of another.

  • Secondary amines have only one hydrogen atom attached to nitrogen, forming fewer hydrogen bonds.

  • Tertiary amines ($\text{R}_3\text{N}$) have no hydrogen atoms bonded directly to nitrogen and cannot form intermolecular hydrogen bonds with each other.

• Comparison of Boiling Points (Table 9.2):

  • Compound $n\text{-C}_4\text{H}_9\text{NH}_2$ (Primary amine):

  • Molar mass: $73$

  • Boiling point: $350.8\,K$

  • Compound $(\text{C}_2\text{H}_5)_2\text{NH}$ (Secondary amine):

  • Molar mass: $73$

  • Boiling point: $329.3\,K$

  • Compound $\text{C}_2\text{H}_5\text{N}(\text{CH}_3)_2$ (Tertiary amine):

  • Molar mass: $73$

  • Boiling point: $310.5\,K$

  • Compound $\text{C}_2\text{H}_5\text{CH}(\text{CH}_3)_2$ (Alkane / Isopentane):

  • Molar mass: $72$

  • Boiling point: $300.8\,K$

  • Compound $n\text{-C}_4\text{H}_9\text{OH}$ (Alcohol / 1-Butanol):

  • Molar mass: $74$

  • Boiling point: $390.3\,K$

• Boiling Point Trends and Observations:

  • Alcohols have significantly higher boiling points ($390.3\,K$) compared to amines of similar molar mass due to the higher electronegativity of oxygen compared to nitrogen, which creates stronger hydrogen bonding.

  • Among isomeric amines of molar mass $73$, primary amines exhibit the highest boiling point ($350.8\,K$) due to maximum hydrogen bonding, followed by secondary amines ($329.3\,K$), and tertiary amines ($310.5\,K$).

  • All amines have higher boiling points than alkanes of similar molar mass ($300.8\,K$ for molar mass $72$) due to polar interactions and hydrogen bonding capability.

Chemical Reactivity and Nucleophilic Character of Amines

• Factors Deciding the Reactivity of Amines:

  • Electronegativity Difference: The electronegativity difference between nitrogen and hydrogen atoms creates polar $\text{N-H}$ bonds.

  • Unshared Electron Pair: Nitrogen contains an unshared pair of electrons (lone pair), making the nitrogen atom electron-rich.

  • Number of Hydrogen Atoms on Nitrogen: The number of hydrogen atoms attached directly to nitrogen determines the specific chemical pathway and course of reaction, causing primary ($-\text{NH}_2$), secondary ($-\text{NH}-$), and tertiary ($\equiv\text{N}$) amines to differ significantly in many reactions.

• Nucleophilic Behavior:

  • Amines behave as nucleophiles in chemical reactions due to the presence of the unshared pair of electrons on the nitrogen atom.

Basic Character and Salt Formation Reactions

• Basic Nature of Amines:

  • Amines are basic in nature and readily react with acids to form salts.

• General Reaction with Acids:

  • Amines react with hydrogen halides or acids ($\text{HX}$) to produce alkylammonium salts:   $\text{R-NH}_2 + \text{HX} \rightarrow \text{R-NH}_3^+\text{X}^-$

• Reaction of Aniline to Form Anilinium Chloride:

  • Aniline ($\text{C}_6\text{H}_5\text{NH}_2$) reacts with hydrochloric acid ($\text{HCl}$) to yield anilinium chloride ($\text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^-$):   $\text{C}_6\text{H}_5\text{NH}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^-$