Chemically equivalent hydrogens

Inspection of the citrate structure shows a total of four chemically equivalent hydrogens, but only one of these—the pro-/J H atom of the pro-i arm of citrate—is abstracted by aeonitase, which is quite stereospecific. Formation of the double bond of aconitate following proton abstraction requires departure of hydroxide ion from the C-3 position. Hydroxide is a relatively poor leaving group, and its departure is facilitated in the aeonitase reaction by coordination with an iron atom in an iron-sulfur cluster. 

The benzenonium ion (62), the proposed intermediate for the formation of (63), has chemically equivalent hydrogens in the methylene group. Hence essentially equal amounts of 6-exo and 6-endo deuterium should appear in (68) if this mechanism is operative. 

Each peak represents chemically equivalent hydrogens ... 

Identify and count neighboring hydrogens that are not chemically equivalent. Use + 1 to figure the number of peaks created by splitting for the chemically equivalent hydrogens. 

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. 

No splitting is observed when chemically equivalent hydrogens are adjacent to each other, as with hydrogens "b" above. 

If all the hydrogen atoms in a compound are bonded to a common carbon atom and are chemically equivalent, then only one absorption would be observed in the NMR spectrum of the molecule. For example, the methane molecule, CH4, has four chemically equivalent hydrogen atoms and has only one peak in its NMR spectrum. 

Let us look more closely at two more examples, chloroethane and cyclohexane. Chloroethane should have two NMR peaks because it has the two sets of equivalent hydrogens cyclohexane has 12 chemically equivalent hydrogen nuclei and is expected to show only one absorption. However, are these expectations really justified Consider the possible conformations of these two molecules (Figure 10-14). 

Chemically equivalent hydrogens and carbons have the same chemical shift. Equivalence is best estabhshed by the application of symmetry operations, such as those using mirror planes and rotations. 

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