Write the postulates of Werner’s theory of co-ordination complexes.(ii) What is the action of ethane –1, 2 – diol on acetone?

Answer:

(i) Postulates of Werner’s theory:

1. The central metal atom in a complex shows two types of valencies: primary valency and secondary valency.
2. Primary valency is ionisable; it corresponds to the oxidation state of the metal and is satisfied by negative ions.
3. Secondary valency is non-ionisable; it corresponds to the coordination number and is satisfied by ligands (negative ions or neutral molecules). It is fixed for a given metal.
4. Secondary valencies are directed in space in fixed directions, giving the complex a definite geometry (e.g. octahedral for coordination number 6); primary valencies are non-directional.

Example: in [Co(NH₃)₆]Cl₃, primary valency 3 (three Cl⁻ ions) and secondary valency 6 (six NH₃).

(ii) Acetone reacts with ethane-1,2-diol (ethylene glycol) in the presence of dry HCl gas to form a cyclic ketal, with loss of water:

(CH₃)₂C=O + HO–CH₂–CH₂–OH —(dry HCl)⇌ (CH₃)₂C(–O–CH₂–CH₂–O–) + H₂O

The product is the cyclic ketal 2,2-dimethyl-1,3-dioxolane (acetone ethylene ketal), a five-membered ring containing two oxygen atoms. It is used to protect the carbonyl group.