Lesson 4 of 12 · 5 min
Physical properties of amines
NCERT §9.5
The store's triethylamine drum smells of fish, and a bottle of aniline on the shelf has gone brown. The chemist asks Ravi why the aniline looks old and why the three butylamine isomers boil at different temperatures.
The lesson in notes
In short
The lower aliphatic amines are gases with a fishy smell. Primary amines with three or more carbons are liquids, and higher ones are solids.
Aniline and other arylamines are usually colourless when pure but darken on storage as air oxidises them.
Lower aliphatic amines dissolve in water by hydrogen bonding with it. Solubility falls as the water-repelling alkyl part grows, and higher amines are essentially insoluble. Amines dissolve in alcohol, ether and benzene.
Nitrogen (electronegativity 3.0) is less electronegative than oxygen (3.5), so alcohols are more polar than amines and form stronger hydrogen bonds. Butan-1-ol is therefore more soluble in water than butan-1-amine.
Primary and secondary amines associate through N–H···N hydrogen bonds. Primary amines have two N–H hydrogens and associate more; tertiary amines have none and do not associate. Boiling points of isomers: primary > secondary > tertiary.
Boiling points (molar mass, K): n-C₄H₉NH₂ (73) 350.8; (C₂H₅)₂NH (73) 329.3; C₂H₅N(CH₃)₂ (73) 310.5; C₂H₅CH(CH₃)₂ (72) 300.8; n-C₄H₉OH (74) 390.3.