Tautomerism proton-shift

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism), and the chemical reactivity of the two isomers may be quite different It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions will not be controlled by the position of the facile equilibrium. 

Ring-chain tautomerism294 (as in sugars) consists largely of cyclic analogs of the previous examples. There are many other highly specialized cases of proton-shift tautomerism. 

Addition of a proton (e.g. p-TsOH, H2S04) or a Lewis acid leads to a more reactive electrophile. Nucleophilic attack of the alcohol gives a tetrahedral intermediate in which there are two equivalent hydroxyl groups. One of these hydroxyl groups is eliminated after a proton shift (tautomerism) to give water and the ester. 

There are many other highly specialized cases of proton-shift tautomerism, including an internal Michael reaction (see 15-24) in which 2-(2,2-dicyano-l-methylethenyl)benzoic acid (161) exists largely in the open chain form rather an its tautomer (162) in the sohd state, but in solution there is an increasing amount of 162 as the solvent becomes more polar. ... 

Under acidic conditions, the first step involves protonation of the imine nitrogen followed by tautomerization to form an ene-hydrazine intermediate (7). After the tautomerization, a [3,3]-sigmatropic rearrangement occurs, which provides intermediate 8. Rearomatization then occurs via a proton shift to form the imine 9 which cyclizes to form the 5-membered ring 10. Finally, loss of ammonia from 11 generates the indole nucleus in 12. 

The solid-solid transformation of 2-amino-3-hydroxy-6-phenylazopyridine, 57a, to 57b proceeds through two intermediate phases (119). X-Ray and IR studies of the former, low-temperature, and the latter, high-temperature phase show that they are the phenolazo and quinone hydrazone forms, respectively. This solid-state tautomerism can be accounted for by a cooperative intermolecular shift of protons across the various hydrogen bonds. However, because of the complexity of the hydrogen-bond network, the actual pathway of the proton shift has not been uniquely defined. 

The free energy of activation for the proton shift process in the interconversion between tautomeric forms 8 and 9 was measured by the Fors6n-Hoffman double resonance saturation transfer method and determined to be 16.2 kcal mof1 (Figure 7) <20040BC1227>. This value was an average over three different structures (R = Me, Et, Pr1). 

Due to the properties of the cx-hydrogen and carbonyl ketones and aldehydes exist at room temperature as enol tautomers. Tautomerization involves a proton shift, in this case from the a-carbon position to the carbonyl oxygen position. Both tautomers exist at room temperature, but the ketone or aldehyde tautomer is usually favored. Tautomerization is a reaction at equilibrium, not a resonance. (Remember, in resonance structures atoms don move and neither resonance structure actually exists.)... 

There are Other forms of tautomerization but keto-enof tautomerization is the most likely form to be tested on the MCrtr. fn Older to recognize other forms, simply watch for the proton shift in equilibrium. 

D is correct Tautomerization involves a proton shift where the double bond of the carbonyl shifts to the carbonyl/a-carbon bond when the carbonyl oxygen is protonated. You should memorize tautomer formatio and structure. 

C is correct. Tautomerization is an equilibrium represented by a proton shift. Ketones tautomerize to form enols where the carbonyl carbon from the ketone becoms part of an alkene by forming a double bond with a neighboring carbon. In choice C, the carbonyl carbon does not form part of the alkene. 

DNA bases can undergo proton shifts while forming various tautomers. Studies of tautomerism in the gas phase can show the relative stabihties of the individual tautomers. However, a bulk solvent and also a microhydrated environment can change the relative stabilities substantially. 

Another mechanism of formation starts from the bisamino derivative (q) to the tautomeric aldoses (k) and (m) by way of (p) and carbonium ion (o). The diamino compound may be formed from the D-fructosylamine by opening of the ring and addition of a second amine molecule. Compound (s), a possible Erickson product, may be formed by this same route. Whether the factor responsible is a proton shift or a proton exchange is still unknown, so that differentiation between sequence (f-e-e -k and m) or (q-p-o-m-e ) has not yet been made. 

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