Amines

• Structure & properties of amines

  • primary, secondary, tertiary amines determined by no. of substituents on N atom
  • has tetrahedral & pyramidal shapes (109.5°, 107°)
  • BP: -6.3°C -> has H bonds
  • aren't as strong as alcohol H bonds as dipole is weaker
  • solubility: most soluble in less polar solvents
  • 1° amines with C length <= 4 very soluble in water & alcohols
  • benzene doesn't form H bonds -> insoluble
  • reactivity: have lone e- pair which bonds with:
  • H+ ion when acting as a base
  • electron-deficient C atom when acting as a nucleophile
• Properties of amines as bases


  • amines can accept protons -> base (Bronsted-Lowry base)
  • acids form salts (ionic compounds)
  • amines with alkyl groups are more soluble
  • alkyl groups release electrons -> move to N atom -> more -ve charge (inductive effect) -> stronger base
  • aryl groups decrease solubility
  • aryl groups withdraw electrons from N group - orbital with lone pair overlaps delocalised ring -> N more positively charged -> weaker base

  A) tertiary amine

  B) secondary amine

  C) primary amine

  D) ammonia

  E) phenylamine

  F) diphenylamine

  G) triphenylamine

  
• Reactions of amines

  1. ammonia + halogenoalkane (nucleophilic substitution)
  2. 1° amine -> 2° amine -> 3° amine -> 4° ammonium salt
  3. seperated out by fractional distillation

  
• reactions of amines

  2. forming primary amines via reduction of nitriles

  purer product & C chain is 1C longer

  step 1: halogenoalkane + cyanide (nucleophilic substitution)

  conditions: aqueous ethanol

  RBr + CN- -> R-C≡N + Br-

  step 2: nitrile reduced to primary amine

  conditions: hydrogen with nickel catalyst

  R-C≡N + 2H2 -> R-CH2NH2
• reactions of amines

  3. forming phenylamine from nitrobenzene

  reduction of nitrile group, -NO2, to amine group, -NH2

  step 1: benzene undergoes nitration to form nitrobenzene

  step 2: nitrobenzene reduced to form phenylamine

  conditions: tin/HCl reducing agent at room temp

  C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O

  intermediate: C6H5NH3+Cl- + NaOH -> C6H5NH2 + H2O + NaOH

  NaOH added to free amine
• reactions of amines

  4. forming amides by reacting amines with acid chlorides/acid anhydrides via nucleophilic substitution

  good for forming polymers e.g. nylon

  
• Uses of amines

  • nylon, polyurethane, dyes & drugs
  • quaternary ammonium salt used for hair/fabric conditioners
  • +ve organic group + -ve charges on wet hair form a coating that stops static build up & smooths surface
  • called catatonic surfacants