What makes a good leaving group?

And why do some perfectly plausible reactions just not happen?

Acyl chlorides readily react with hydroxide ions to form a carboxylic acid …

and yet the reverse reaction between chloride ions and a carboxylic acid simply does not happen …

The only difference between the two reactions is that in the first we break a C-Cl bond in the step from the intermediate to the products and in the second reaction we need to break a C-O bond.

We can see this more clearly on an energy profile diagram for the reaction between an acyl chloride and hydroxide ions.

The products are of lower energy, and hence more stable, than the reactants i.e. the relative energies of CH₃COCl and OH^– are higher than those of CH₃COOH and Cl^–.

If the reverse reaction (reaction 2) was to occur, the Cl^– would attack ethanoic acid and the same intermediate is formed from which the reaction has two choices – break the C-O bond (∼450 kJ mol^-1) or break the C-Cl bond (∼350 kJ mol^-1). It is far more likely that the weaker bond breaks and Cl^– leaves since this pathway has the lower activation energy. The intermediate reverts back to the starting materials, chloride ions and ethanoic acid, and the reaction does not proceed.

A good leaving group depends on the strength of the bond between the leaving group and the atom it its bonded to, but it also depends on the stability leaving group once it has left.

• the relative order of the halide ions as leaving groups reflects the strength of the C-X bond. C-I is the weakest bond and therefore the easiest to break. I^– is a better leaving group than F^–.
• fluorine, oxygen, nitrogen and carbon are all in period 2 of the periodic table. The effective nuclear charge felt by the valence (outer shell) electrons in fluorine is stronger than in oxygen (which is stronger than in nitrogen etc.) which means that the fluoride ion, F^–, is of lower energy and more stable than OH^–. CH₃^– is the least stable ion and a very poor leaving group.
• H₂O is a better leaving group than OH^– because it leaves as a neutral molecule. Similarly, N₂ is an excellent leaving group because it is a stable neutral molecule with a very strong N⫢N bond – as a result it is pretty unlikely to try to insert itself back into the molecule!
• the positively charged leaving groups in R-N₂⁺ and R-OH₂⁺ cause electron density to be pulled away from the alkyl (R) group they are bonded to leading to a weaker C-N or C-O bond which is more easily broken – another reason why N₂ and H₂O are good leaving groups.