Lewis Bases
A Lewis base is an atomic or molecular species where the HOMO is highly localized. Typical Lewis bases are conventional amines such as ammonia and alkyl amines. Other common Lewis bases include pyridine and its derivatives. Some of the main classes of Lewis bases are
- amines of the formula NH3−xRx where R = alkyl or aryl. Related to these are pyridine and its derivatives.
- phosphines of the formula PR3−xAx, where R = alkyl, A = aryl.
- compounds of O, S, Se and Te in oxidation state 2, including water, ethers, ketones
The most common Lewis bases are anions. The strength of Lewis basicity correlates with the pKa of the parent acid: acids with high pKa's give good Lewis bases. As usual, a weaker acid has a stronger conjugate base.
- Examples of Lewis bases based on the general definition of electron pair donor include:
- simple anions, such as H– and F-.
- other lone-pair-containing species, such as H2O, NH3, HO–, and CH3–
- complex anions, such as sulfate
- electron rich π-system Lewis bases, such as ethyne, ethene, and benzene
The strength of Lewis bases have been evaluated for various Lewis acids, such as I2, SbCl5, and BF3.
| Heats of binding of various bases to BF3 | ||
|---|---|---|
| Lewis base | donor atom | Enthalpy of Complexation (kJ/mol) |
| Et3N | N | 135 |
| quinuclidine | N | 150 |
| pyridine | N | 128 |
| Acetonitrile | N | 60 |
| Et2O | O | 78.8 |
| THF | O | 90.4 |
| acetone | O | 76.0 |
| EtOAc | O | 75.5 |
| DMA | O | 112 |
| DMSO | O | 105 |
| Tetrahydrothiophene | S | 51.6 |
| PMe3 | P | 97.3 |
Read more about this topic: Lewis Acids And Bases
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