What are the key differences between bases and nucleophiles? What factors influence whether something is a good base or good nucleophile or both?
1 Answer
A key point few students get is that basicity is thermodynamic (regarding energetic stability) and nucleophilicity is kinetic (regarding the speed or rate of the process).
In other words...
- a "better" base wants an
#"H"^(+)# more (as a Bronsted-Lowry base), and more energetically favors donating a lone pair of electrons to get it (as a Lewis base). - a "better" nucleophile can donate a lone pair of electrons more quickly.
Some factors that effect an increase in basicity are:
- More electron-donating groups (e.g. alkyl groups,
#"CH"_3-"CH"_2 - cdots # ) surrounding the central atom that has a lone pair of electrons, since the lone pair electron density becomes more concentrated.
For example, the second molecule here is the better base.
- Higher
#bb("pKa")# . A stronger base has a higher#"pKa"# , or lower#"pKb"# (but#"pKa"# is used more often in organic chemistry).
For example,
#"NH"_3# compared to#"H"_2"O"# is a better base, since its#"pKa"# is#~~ 36# compared to#15.7# , respectively (in organic chemistry).
Some factors that effect an increase in nucleophilicity are:
- Less steric hindrance.
For example, in the above image, although
#("CH"_3)_3"C"-"O":^(-)# is a better base, it is a worse nucleophile than#"CH"_3"CH"_2-"O":^(-)# because it's bulkier, slower, and less capable of donating electron density into cramped spaces.
- A solvent that does not increase the basicity of the nucleophile, e.g. an aprotic solvent (such as dimethylsulfoxide [DMSO], dimethyl ether, etc). In this consideration, we note that a high thermodynamic tendency to be a base can interfere with the ability to act as a good nucleophile.
For example, the first scenario here more effectively allows the nucleophile to act as a nucleophile, because the solvent does not donate a proton to the nucleophile.
