The oxidative addition reaction may be described alternatively as an insertion of an atom or group E on a X-Y bond:
E + X-Y ⇄ X-E-Y
For example, the reaction Cl-Cl + CO ⇄ COCl2 can be considered as an insertion in the Cl-Cl bond. In this case the choice between oxidative addition or insertion is a simple matter of preference.
Consider now E reacting with any hydrogen compound (HY):
E + H-Y ⇄ H-E-Y
Although still an addition (the reverse is the elimination of HY), there is no change in the oxidation states. The reaction of some covalent oxides with water to give the oxoacids are examples of thus type of reaction. Thus, the hydration of CO 2 sub> is an insertion (although usually is described as a simple acid-base process).
O=C=O + H-OH ⇄ H-O-CO-OH
Cl2+ CO ⇄ OCCl2 Non concerted
Cl2+ CO ⇄ OCCl2 Concerted
SO3 + H2O ⇄ H2SO4 Gas phase
S3O9 + 3H2O ⇄ 3H2SO4Aqueous media
Carbonyl chloride is formed from CO and chlorine. The carbon atom formally changes from divalent to tetravalent. This reaction does not take place unless it is performed photochemically. It is a radical reaction that begins with the homolytic cleavage of Cl-Cl bond. The concerted mechanism (see CO + Cl2 concerted) is prohibited because it involves the interaction of the HOMO of CO (donor) with the LUMO of chlorine (the acceptor), resulting in a non-effective interaction.
The concerted mechanism for the reaction of CO with chlorine, is symmetry forbidden. It involves the interaction of the HOMO of CO (donor) with the LUMO of Cl2, (the acceptor), resulting in a non-effective interaction.
A water molecule acts as a nucleophile and attacks the sulfur atom followed by proton transfer.
A water molecule acts as a nucleophile and attacks one of the sulfur atoms followed by proton transfer. Further addition of water molecules generates three molecules of H2SO4
CO2 reacts with water and generates an equilibrium with carbonic acid. This is the general behavior for covalent oxides. p>
The mechanism involves nucleophilic addition of water to carbon atom of CO2 (where the LUMO is concentrated), followed by intramolecular transfer of protons (tautomeric equilibrium). The equilibrium lies far to the CO2. Some of these processes can be also described as water additions or oxide insertions.