Define Co-ordination number.(b) Write note on Reimer-Tieman reaction.(c) Write structural formula for methyl vinyl ether.

Answer:

(a) Coordination number: The number of ligand donor atoms directly bonded to the central metal atom or ion in a complex is called its coordination number, e.g. 6 in [Fe(CN)₆]^4– and 6 in [Co(en)₃]³⁺ (each en is bidentate).

(b) Reimer–Tiemann reaction: When phenol is heated with chloroform and aqueous sodium hydroxide at about 340 K, and the product is then acidified, an –CHO group is introduced at the ortho position, giving salicylaldehyde (2-hydroxybenzaldehyde):

C₆H₅OH + CHCl₃ + 4NaOH → (via the o-hydroxybenzal chloride intermediate, hydrolysed) → o-NaO–C₆H₄–CHO (sodium salicylaldehyde) + 3NaCl + 3H₂O; then H⁺ gives salicylaldehyde.

The reactive species is dichlorocarbene (:CCl₂), formed from CHCl₃ and OH^–, which attacks the activated ortho position of the phenoxide ion; the –CHCl₂ group formed is hydrolysed to –CHO. A small amount of the para isomer is also formed.

(c) Methyl vinyl ether: CH₃–O–CH=CH₂ (IUPAC: methoxyethene).