Classify nitrogen groups and convert amines to protonated forms.
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Method and worked examples
Amines contain a nitrogen atom with a lone pair that can accept H+. A primary amine has one carbon group attached to nitrogen, a secondary amine two and a tertiary amine three. This classification counts groups on nitrogen, unlike alcohol classification, which counts neighbours of the OH-bearing carbon. Names such as butan−1−amine locate the amino group on the parent chain.
An amine acts as a base: RNH2+H2O ⇌ RNH3++OH−. Reaction with an acid forms a protonated ammonium species and its counter-ion. Adding base can regenerate the uncharged amine. The charged form is usually more water-soluble because of strong interactions with water. Protonation changes charge and solubility without removing the carbon skeleton.
Example 1
Write the protonated form of CH3CH2NH2.
Nitrogen accepts H+ using its lone pair: CH3CH2NH3+. The carbon skeleton remains unchanged and the species has charge +1.
Example 2
Classify (CH3)2NH and (CH3)3N.
The first has two carbon groups on nitrogen and is secondary. The second has three and is tertiary. Neither classification counts carbon neighbours on an adjacent carbon atom.
Watch out:Its nitrogen lone pair can accept H+; an existing N–H bond is not required for basic behaviour.
Amines, protonation and salts practice | Year 12 Chemistry | Ascent Arcade