Equivalent hydrogen atoms

As a second example of molecular symmetry, consider the ammonia molecule. It has three symmetrically equivalent hydrogen atoms, but it is not... 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

The lines in the spectrum of Fig. 1 are, even under optimum experimental conditions, quite broad. This is due to small unresolved hyperfine interaction with the eighteen equivalent hydrogen atoms. Either the width of the lines, or nmr measurements (La Mar et al., 1973) can reveal the magnitude of this y-hydrogen splitting (ca. 0.15 G—dependent on temperature and solvent). 

Because proteins are made up of L-amino acids, they exhibit chirality in their structures, lacking planes or points of symmetry. Proteins also can exhibit chirality in their interactions with other chiral molecules as well as prochiral centers in other molecules. This latter point is beautifully illustrated by fumarase s catalysis of the dehydration of L-malate, a molecule containing two seemingly equivalent hydrogen atoms ... 

Coupling follows the n + 1 rule. According to this rule, a peak splits into n + 1 peaks due to neighboring hydrogen atoms, where n is the number of equivalent hydrogen atoms. The amount of splitting is expressed by the coupling constant (J). 

In the ammonium ion (NH4+), for example, each of the four equivalent hydrogen atoms has 2 valence electrons in its covalent bond to nitrogen, and the nitrogen atom has 8 valence electrons, two from each of its four N-H bonds ... 

The two products are taken over the sets of equivalent hydrogen atoms in transition state and reactants, respectively, and t and r represent the numbers of equivalent hydrogen atoms in each set. 

The case of phosphoric acid was selected by Salomaa et al. (1964a) to exemplify a protolytic equilibrium in which both the acid and base form involved contain more than a single equivalent hydrogen atom. The appropriate form of the general equation (32) is thus... 

From the constitutional formulae given below, first of all draw formulae for all the possible configurational isomers and then deduce how many sets of chemically equivalent hydrogen atoms are present in the represented compounds. 

The chemically equivalent hydrogen atoms are those with the same numbers in the following formulae. 

At the active site of beef heart mitochondrial succinate dehydrogenase (EC 1.3.99.1), the FAD is covalently linked by C-8a to nitrogen of a histidine residue (Fig. 2b) [20]. In the catalytic reaction, removal of a proton from C-3 of the succinate may be followed by attack of the 3-carbanion on N-5 of the FAD to form an intermediate adduct, which breaks down with loss of a proton from C-2 of the succinate, giving fumarate and a reduced FAD moiety [21]. This mechanism is not certain, but it is established that the succinate loses two non-equivalent hydrogen atoms by a trans elimination (Fig. 4) [22], In other enzymes, different types of covalent attachment of the FAD are known [23]. 

Fig. 4. In the succinate dehydrogenase reaction, iwo non-equivalent hydrogen atoms are removed by tram elimination.

Mercury salts can react directly with hydrocarbons exchanging hydrogen for mercury. This reaction is an electrophilic substitution (equation 5) and hence can take place with arenes, cyclopentadienyls, terminal aUcynes, and also with aliphatic hydrocarbons that contain activated carbon-hydrogen bonds (e.g. carbonyl or nitrile compoimds). When the hydrocarbon contains several equivalent hydrogen atoms, polymercuration is often observed. 

Comparing the form of equation (29) with the chemical equilibrium (11) to which it refers, we see that it is a product of factors of the form (1 — n + n< ), raised to a power corresponding to the number of equivalent hydrogen atoms in each set of equivalent hydrogen atoms in the species concerned. The exponent is positive for reactant species (p) and negative (r) for the products of reaction, i,e. for any equilibrium... 

Dev., 17(1978)266. The latter report a value of -1.9 but did not account for the number of equivalent hydrogen atoms. We have taken the values from this reference... 

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