Tautomerism - Organic Chemistry

Tautomerism - Organic Chemistry

A very common form of tautomerism is that between a carbonyl compound containing an -H and its enol form.

In simple cases, the equilibrium lies well to the left. In certain cases, however, a larger amount of the enol form is present. There are three main types of the more stable enols:

  1. Molecules in which the enolic double bond is on conjugation with another double bond. For example, carboxylic esters have a much smaller enolic content than ketons. In molecules like acetoacetic ester, the enol is also stabilized by intramolecular hydrogen bonding.

  2. Molecules that contain two or three bulky aryl groups. For example, in case of 2, 2-dimesitylethenol, the keto content at equilibrium is only 5%. In this case steric hindrance destabilizes the keto form.

  3. Highly Fluorinated enols: The extent of enolization is greatly affected by solvant, concentratiom and temperature. Thus, acetoacetic ester has an enol content of 0.4% in water and 19.8% in toluene. In this case, water reduces the enol content by hydrogen bonding with the carbonyl, making this group less available for intramolecular bonding.

Other Proton-Shift Tautomerism

  1. Phenol-keto Tautomerism

    For most simple phenols, the equilibrium lies well to the side of the phenol, since only on that side is there aromaticity.

  2. Nitroso-Oxime Tautomerism

    This equilibrium lies far to the right, and as a rule nitroso compounds arestable only when there is no -hydrogen.

  3. Nitro-acinitro Tautomerism

    Primary and secondary nitroalkanes containing -H undergo tautomerism.

  4. Imine-enamine Tautomerism

    Enamines are normally stable only when there is no hydrogen on the nitrogen (such as R2C  =  CR    NR2). Otherwise, the imine from predominates.

Factors Influencing Reactivity

  1. Inductive Effect
    The electron pair forming the -bond is slightly displaced towards the more electronegative atom. This permanent state of polarization is called the inductive effect.

     l effect (i.e., electron-withdrawing effect) : NO2 > F > COOH > Cl > Br > l > OH > OR > C6H5

    + l effect (electron-releasing effect) : Me3C - > Me2CH - > MeCH2 - > CH3

  2. Electromeric Effect
    On the close approach of a reagent, the electronic system of an unsaturated molecule is deformed. When the reagent is removed, the electronic system reverts to the original state. This kind of polarizability of multiple bonds is known as electronic effect.

    Electromeric effect is temporary

  3. Mesomeric Effect
    The permanent polarization of a group conjugated with a -bond or a set of alternate -bonds is transmitted through the -electrons of the system resulting in a different distribution of electrons in the unsaturated chain. This is called mesomeric effect.

    + M effect possessing groups are : 

     M effect possessing groups are :  

  4. Hyperconjugation
    The ability of the -bond electrons of an -C  H bond to undergo conjugation with the adjacent -electrons is called hyperconjugation.

    The ortho-and para-directing effect of methyl group in toluene is attributed to electron release by hyperconjugation.

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