imine
/ IM-een /
Replace the oxygen of a carbonyl with a nitrogen, and you get an imine: a carbon-nitrogen double bond, C=N, where the nitrogen also carries a hydrogen or a carbon group. Imines made from a primary amine and an aldehyde or ketone are also called Schiff bases. They are how nature joins carbonyls to amines, and they appear in vision, in metabolism, and in countless syntheses.
Formation is a condensation: a primary amine (R'-NH2) adds to the carbonyl as a nucleophile, giving a tetrahedral intermediate called a carbinolamine (or hemiaminal), which then loses water to form the C=N double bond. The whole sequence is exquisitely pH-dependent. Too acidic, and the amine gets protonated to -NH3+ and can no longer attack; too basic, and the dehydration step (which needs acid to protonate the leaving OH) slows down. The sweet spot is mildly acidic, around pH 4 to 6, where enough free amine and enough acid coexist.
Imines are reversible and easily hydrolysed back to the carbonyl and amine by water, which is why they are central to reductive amination (trap the imine, then reduce the C=N to a stable amine) and to enzyme chemistry (a Schiff base anchors retinal in rhodopsin, and pyridoxal phosphate uses imines to shuttle amino groups). A close cousin forms with secondary amines, which cannot make a C=N and instead give an enamine.
In your eye, the aldehyde of retinal forms a Schiff base (imine) with a lysine amino group in the protein opsin; light flipping the shape of that retinal triggers vision.
An imine (C=N) forms when an amine adds to a carbonyl and then loses water.
The pH window matters: imine formation is fastest near pH 4-6, not in strong acid (which kills the nucleophilic amine) nor in strong base (which stalls the dehydration). It is a tug-of-war between two opposing needs.