Page 29 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.4 pK VALUES: WHAT MAKES FOR A GOOD LEAVING GROUP?
a 9
Let us take a couple of concrete examples.

SeCl + 2 (CH ) SiBr → SeBr + 2 (CH ) SiCl (1.4)
3 3
2
2
3 3
PCl + AsF → PF + AsCl 3 (1.5)
3
3
3
In reaction 1.4, Se is a softer Lewis acid than Si, and bromide is a softer Lewis base than
chloride. It makes sense therefore that Se and Br should link up, as should Si and Cl. In
the second example (reaction 1.5), As is a softer Lewis acid center than P, and chloride is a
softer Lewis base than fluoride. These ligand exchanges are thus consistent with the HSAB
principle.

pK VALUES: WHAT MAKES FOR A GOOD LEAVING GROUP?
1.4 a
Compared with the multitude of factors affecting nucleophilicity, the efficacy of a leaving
group is much more easily predictable. In short, a weaker Brønsted base makes a better
leaving group. We can simply look up the pK of the conjugate acid of a leaving group
a
to arrive at a good idea of its leaving ability. Table 1.4, a short pK table, will serve our
a
purposes very well.
Observe that the best leaving groups are conjugate bases of the strongest acids. Thus,
iodide and bromide are excellent and popular leaving groups in organic chemistry. The
worst leaving groups are very strong bases, such as amide, hydride, and alkyl anions.
−
Hydroxide and alkoxide (RO ) are also poor leaving groups in organic chemistry. The
Williamson ether synthesis mentioned above (reaction 1.1) illustrates this last point well.
Like all elementary reactions, the reaction is in principle reversible, but the reverse reaction,
− −
I displacing a CH O anion, does not occur for all intents and purposes.
3
A couple of additional observations are worth making, again with specific reference to
organic chemistry.

Common Leaving Groups and the pK Values of Their
TABLE 1.4 a
Conjugate Acids
Leaving Group Conjugated Acid pK
a
Good I – HI −10
Br – HBr −9
Cl – HCl −8
HSO 4 − H SO 4 −3
2
p-CH –C H –SO 3 – p-CH –C H –SO H −3
4
3
3
4
6
3
6
H O H O + −1.7
3
2
F – HF 3.2
CH COO – CH COOH 4.74
3
3
NH 3 NH 4 + 9.25
HO – H O 15.74
2
CH O – CH OH 15.2
3
3
NH 2 – NH 3 38
H – H 2 42
Bad CH –CH –CH –CH – CH –CH –CH –CH 50
3 2 2 2 3 2 2 3