It’s time to go back and re-think nucleophiles and electrophiles.
- Nucleophiles are species that donate a pair of electrons.
- Electrophiles are species that accept a pair of electrons.
- Reactions occur when a nucleophile donates a pair of electrons to an electrophile.
- The more electron rich the nucleophile, the more reactive it is.
- The more electron-poor the electrophile, the more reactive it is.
- And increasing the reactivity will increase the speed of the reaction.
Back in Org 1 you usually thought of nucleophiles as being species like HO(–), (–)CN, (–)SH, and electrophiles as being things like acids and alkyl halides.
In Org 2, more and more you will see how Pi- systems can be nucleophiles (and electrophiles).
The Diels Alder reaction is a perfect example.
- The diene is electron rich (the nucleophile)
- The dienophile is electron-poor (the electrophile).
- When your diene has substituents on it that give it electron density (pi-donors like oxygen or nitrogen), it is more electron rich – and a better nucleophile.
- When your dienophile has substituents on it that take away electron density (pi acceptors like carbonyl groups) it is more electron poor – and a better electrophile.
- Conversely, making the diene more electron-poor or making the dienophile more electron-rich will slow down the reaction.
Tomorrow: Let’s wrap up the key ideas of this week.
Thanks for reading! James
PS another way of putting this is, “the reaction rate is increased by either increasing the energy of the HOMO (highest occupied molecular orbital) of the diene, or by decreasing the energy of the LUMO (lowest unoccupied molecular orbital) of the dienophile.”