Organic Chemistry 2 · Acyl substitution and derivative choice
First urethane link from HDI and ethylene glycol
Conditions: DBTDL (cat.)
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The mechanism, step by step
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A glycol OH adds to the isocyanate carbon
The central carbon of N=C=O sits between two electronegative atoms and is strongly electrophilic. A hydroxyl oxygen of ethylene glycol attacks it and the C=N π pair moves onto nitrogen; dibutyltin dilaurate (DBTDL, not drawn) speeds this step by coordinating the reactants.
The curved arrows
- The hydroxyl oxygen lone pair forms the new O–C bond to the isocyanate carbon.
- The C=N π electrons move onto nitrogen, giving a nitrogen anion.
A second glycol protonates nitrogen
The nitrogen anion is strongly basic and takes an O–H proton from a second ethylene glycol molecule. The protons cannot hop directly from oxygen to nitrogen, so the glycol acts as a shuttle.
The curved arrows
- The nitrogen lone pair bonds to the glycol O–H proton.
- The O–H electrons stay on oxygen, giving an alkoxide.
The alkoxide removes the oxonium proton
The glycol alkoxide removes the proton on the positively charged carbamate oxygen. This neutralises the urethane and returns the shuttle glycol unchanged.
The curved arrows
- The alkoxide lone pair bonds to the oxonium proton.
- The O–H electrons stay on the carbamate oxygen.
2-Hydroxyethyl N-(6-isocyanatohexyl)carbamate
The first urethane (carbamate) link forms, leaving a free NCO at one end and a free OH at the other. Repeating the same addition at both ends builds the polyurethane chain with no by-product.
What you'll practice
Form the first carbamate link of a polyurethane: ethylene glycol adds to one isocyanate of hexamethylene diisocyanate, then a second glycol relays the proton to nitrogen.
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