pls explain the dyad(diad) system

pls explain the dyad(diad) system


1 Answers

Aman Bansal
592 Points
11 years ago

Dear Mayank,


            (i) The type of isomerism in which a substance exist in two readily interconvertible different structures leading to dynamic equilibrium is known as tautomerism and the different forms are calledtautomers (or tautomerides).
            The term tautomerism (Greek: tauto = same; meros = parts) was used by Laar in 1885 to describe the phenomenon of a substance reacting chemically according to two possible structures.
            (ii) It is caused by the wandering of hydrogen atom between two polyvalent atoms. It is also known as Desmotropism (Desmos = bond and tropos = turn). If the hydrogen atom oscillates between two polyvalent atoms linked together, the system is a dyad and if the hydrogen atom travels from first to third atom in a chain, the system is a triad.
            (a) Dyad system : Hydrocyanic acid is an example of dyad system in which hydrogen atom oscillates between carbon and nitrogen atoms. H – C ≡ N  
            (b) Triad system
            Keto-enol system : Polyvalent atoms are oxygen and two carbon atoms.
            Examples :
            Keto-enol system
            Acetoacetic ester (Ethyl acetoacetate) :
Ethyl acetoacetate
         Acetoacetic ester gives certain reactions showing the presence of keto group (Reactions with HCN, H2NOH, H2NNHC6H5etc.)and certain reactions showing the presence of enolic group (Reactions with water and colour with Na, CH3COCl, NH3, PCl5, Br2 neutral F2Cl3etc.).
Enolisation is in order
            Acid catalysed conversion
            Acid catalysed
            Base catalysed conversion
             Base catalysed
            (c) Triad system containing nitrogen : Examples
            Nitrous acid exists in 2 forms
            Nitrous acid
            Nitro acinitro system
            Nitro acinitro                      
            (iii) Characteristics of tautomerism
            (a) Tautomerism (cationotropy) is caused by the oscillation of hydrogen atom between two polyvalent atoms present in the molecule. The change is accompanied by the necessary rearrangement of single and double bonds.
            (b) It is a reversible intramolecular change.
            (c) The tautomeric forms remain in dynamic equilibrium. Hence, their separation is a bit difficult. Although their separation can be done by special methods, yet they form a separate series of stable derivatives.
            (d) The two tautomeric forms differ in their stability. The less stable form is called the labile form. The relative proportion of two forms varies from compound to compound and also with temperature, solvent etc. The change of one form into another is also catalysed by acids and bases.
            (e) Tautomers are in dynamic equilibrium with each other and interconvertible ().
            (f) Two tautomers have different functional groups.
            (g) Tautomerism has no effect on bond length.
            (h) Tautomerism has no contribution in stabilising the molecule and does not lower its energy.
            (i) Tautomerism may occur in planar or nonplanar molecules.
            • Keto=enol tautomerism is exhibited only by such aldehydes and ketones which contain at least one α-hydrogen.
            For example
            CH3CHO, CH3CH2CHO, CH3COCH2COCH3 etc,.
            • Tautomerism is not possible in benzaldehyde (C6H5CHO), benzophenone (C6H5COC6H5), tri methyl acetaldehyde, (CH3)3C–CHO and chloral CCl3 – CHO as they do not have α–H.

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Aman Bansal

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