Aldehyde Reactions: Imine Formation from Aldehyde, Ketone using RNH2

Imines can be formed from aldehydes and ketones using primary amines (RNH2):

If the starting structure contains a ketone functional group and a primary amine functional group, an intramolecular reaction will occur, forming a ring structure:

The reaction mechanism is depicted below:

In the first step, the lone pair of electrons from the ketone-oxygen pull a proton away from the acid (H2SO4 in this case), causing the electrons to return to the acid molecule, forming the conjugate base (HSO4-).

In the second step, the primary amine (RNH2) approaches the ketone bond and the free electrons from the N atom attack the ketone-carbon atom, causing the carbon-oxygen bond to break sending the electrons to the positively charged O atom.

In the third step, the lone pair electrons on the O atom attack another H2SO4 molecule, thereby deprotonating it and forming another conjugate base. Meanwhile, the conjugate base HSO4- deprotonates the nitrogen atom sending the lone pair electrons to the nitrogen atom.

In the fourth step, the nitrogen electrons attack the carbon atom, forming a double bond between the two atoms. The oxygen-carbon bond is broken, sending the electrons with the O atom as it leaves as an H2O molecule.

In the fifth step, the conjugate base HSO4- attacks the remaining proton on the amine nitrogen atom. This causes the hydrogen-nitrogen bond to break, sending those electrons back to the nitrogen atom.

There are many reagents that fall under the RNH2 umbrella but the R group is usually as small as a methyl group or as large as methylcyclohexane.