TABLE OF CONTENTS The high reactivity of the isocyanate group with hydrogen active compounds can be explained by the following resonance structures [1-3]:
Electron density is higher at the oxygen atom, while the carbon atom has the lowest electron density. As an immediate consequence, the carbon atom has a positive charge, the oxygen atom a negative one and the nitrogen atom an intermediate negative charge. The reaction of isocyanates with hydrogen active compounds (HXR) is in fact an addition at the carbon - nitrogen double bond [1-25]:
The nucleophilic centre of the active hydrogen compounds (the oxygen atom of the hydroxyl groups or the nitrogen atoms in the case of amines), attacks the electrophilic carbon atom and the hydrogen adds to the nitrogen atom of the -NCO groups. Electron withdrawing groups increase the reactivity of the -NCO groups and on the contrary, the electron donating groups decrease the reactivity against hydrogen active compounds. Aromatic isocyanates (R = aryl) are more reactive than aliphatic isocyanates (R = alkyl). Steric hindrance at -NCO or HXR ? groups markedly reduces the reactivity.
The reaction between isocyanates and alcohols, the most important reaction involved in polyurethane synthesis, is an exothermic reaction and leads, as mentioned before, to production of urethanes [1-26]:
The reaction between isocyanates and water leads to production of gaseous carbon dioxide and an urea group. This reaction is a very convenient source of a gas necessary to generate the cellular...
