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Introduction to Hydration & Tautomerism

The addition of water to alkynes requires a strong acid, generally sulphuric acid like with alkenes, and is facilitated through mercuric sulphate. Though, not similar to the additions to double bonds which give alcohol products, an addition of water to alkynes gives ketone products (apart from the acetylene that yields acetaldehyde). The explanation for this deviation lies in enol-keto tautomerization, demonstrated by the equation shown below. Enol is the initial product from the addition of water to an alkyne, and this instantly rearranges to the more stable keto tautomer.

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Tautomers are considered as quickly interconverted constitutional isomers, generally distinguished by a distinct bonding location for a labile hydrogen atom (colored red here) and a differently located double bond. The equilibrium among tautomers is not only fast under normal conditions but it frequently strongly favors one of the isomers (acetone, for an instance, is 99.999% keto tautomer). Yet in such type of one-sided equilibria, evidence for the existence of the minor tautomer comes from the chemical behaviour of the compound. Tautomeric equilibria are catalyzed through traces of bases or acids that are usually exist in most chemical samples. The three instances displayed below demonstrate these reactions for distinct substitutions of the triple-bond. The tautomerization step is specified by a red arrow in the equation. For terminal alkynes the addition of water follows the Markovnikov rule, like in the second instance below and the final result is a methyl ketone (apart from for acetylene, displayed in the first instance ). For internal alkynes (the triple-bond is inside a longer chain) the addition of water is not regioselective. Two isomeric ketones will be formed if the triple-bond is not symmetrically located (that is if R & R' in the third equation are not identical).

HC≡CH   +   H2O   +   HgSO4 & H2SO4   -->  [ H2C=CHOH ]   -->   H3C-CH=O

RC≡CH   +   H2O   +   HgSO4 & H2SO4   -->  [ RC(OH)=CH2 ]   -->   RC(=O)CH3

RC≡CR'   +   H2O   +   HgSO4 & H2SO4   -->  [ RHC=C(OH)R'   +   RC(OH)=CHR' ]   -->   RCH2-C(=O)R'   +   RC(=O)-CH2R'

Two features have a significant effect on the enol-keto tautomerizations illustrated here. The first is the potential energy variation among the tautomeric isomers. This issue concludes the position of the equilibrium state. The activation energy is the 2nd issue for the interconversion of one tautomer to the other. This issue concludes the rate of rearrangement. Because the potential energy or stability of a compound is in large part a function of its covalent bond energies, by considering the bonds that are changed in the rearrangement, we can estimate the relative energy of enol tautomers and keto. From the following picture, we come to know that only three major changes occur and the standard bond energies for those changes are given to the right of the equation. In bond energy, the keto tautomer has a 17.5 kcal/mole advantage, therefore its predominance at equilibrium is supposed.

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The rapidity by which enol-keto tautomerization occurs suggests that the activation energy for this method is low. We noticed that the rearrangement is base & acid catalyzed, and very careful experiments have displayed that interconversion of tautomers is much slower if such types of catalysts are absent. A remarkable illustration of the influence of activation energy on such type of transformations may be seen in the following hypothetical rearrangement. Now we have substituted a methyl group (colored maroon in the diagram) for the proton of a conventional tautomerism and the methyl shifts from oxygen to carbon just like the proton does in going to a ketone from an enol.

H2C=CH-O-CH3   -X->   CH3-CH2-CH=O

For this rearrangement, the potential energy change is even more beneficial than for enol-keto tautomerism, being estimated at over 25 kcal/mole from bond energy changes. In spite of this thermodynamic driving force, the enol ether explained above is entirely stable to base treatment, and goes through rapid acid-catalyzed hydrolysis with loss of methanol, rather than the rearrangement. The controlling variations in this case must be prohibitively high activation energy for the explained rearrangement, merged with lower energy alternative reaction paths.

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