Proton chemically equivalent

Molecules giving rise to one NMR absorption peak—all protons chemically equivalent... 

Protons are equivalent to one another and have the same chemical shift when they are m equivalent environments Often it is an easy matter to decide simply by mspec tion when protons are equivalent or not In more difficult cases mentally replacing a proton m a molecule by a test group can help We 11 illustrate the procedure for a sim pie case—the protons of propane To see if they have the same chemical shift replace one of the methyl protons at C 1 by chlorine then do the same thing for a proton at C 3 Both replacements give the same molecule 1 chloropropane Therefore the methyl protons at C 1 are equivalent to those at C 3... 

Chemical equivalence atomic nuclei in the same chemical environment are chemically equivalent and thus show the same ehemieal shift. The 2,2 - and 3,3 -protons of a 1,4-disubstituted benzene ring, for example, are ehemically equivalent because of molecular symmetry. 

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. 

Rule 1 Chemically equivalent protons do not show spin-spin splitting. The equivalent protons may be on the same carbon or on different carbons, but their signals don t split. 

Three C-H protons are chemically equivalent no splitting occurs. 

Any spin system that contains fluorine substituents that are chemically equivalent, but not magnetically equivalent is, by definition, second order. Such spectra can appear deceptively simple, or more commonly they can be amazingly complex. The fluorine and proton spectra of the simple, symmetrical compound, 1,1-difluoroethene exemplify the latter situation (Figures 2.5 and 2.6). 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

So we can say that a particular set of protons are chemically equivalent if they remain in exactly similar environment. The case of 2 bromo propene affords another interesting example ... 

So protons with the same chemical shift are called equivalent protons. Non equivalent protons have different chemical shifts expressed by 8 (Delta) pr x (Tau) scales. Since majority of organic compounds have protons resonating at lower fields than the protons of tetramethylsilane, 8 for chemical shift for TMS has been given a value zero, a scale has been given in which most proton resonances are of the same sign and any protons or set of protons absorbing at a field lower than TMS is given a positive value for 8. 

This is due to spin-spin coupling. It should be noted that chemically equivalent protons do not show spin-spin coupling due to interactions among themselves. 

N-methylation of aniline is an interesting reaction from the products selectivity point of view, especially N-methylaniline (NMA), due to the presence of chemically equivalent protons attached to the nitrogen atom. The high nucleophilicity of the amine nitrogen results in the formation of mixture of N-methyl and N, N-dimethylated products. Ko et al [90]... 

Magnetic equivalence. A group of protons is magnetically equivalent when they not only have the same chemical shift (chemical equivalence) but also have identical spin-spin coupling to each individual nucleus outside the group. 

Rule 7 Spin systems that contain groups of chemically equivalent protons that are not magnetically equivalent cannot be analysed by first-order methods. 

It s important to know the chemical shift of each chemically equivalent set of hydrogen atoms. A proton adjacent to an atom that has a high electronegativity has a lower electron density than a proton adjacent to an atom with a low electronegativity. Therefore, a proton adjacent to an oxygen atom, for example, comes into resonance at a higher frequency than a proton adjacent to, for example, a carbon atom (lower electronegativity). 

The E and Z configurations were assigned by determining the Jhh coupling constant between the two chemically equivalent olefinic protons from the satellites in the H P NMR spectrum, and has been confirmed by X-ray analysis of both isomers [19],... 

SAMPLE SOLUTION (a) All the protons of 1,2-dichloroethane (CICH2CH2CI) are chemically equivalent and have the same chemical shift. Protons that have the same chemical shift do not split each other s signal, and so the NMR spectrum of 1,2-dichloroethane consists of a single sharp peak. ... 

One way of checking whether two protons are in equivalent environments is to imagine that each is separately replaced with a different atom or group. If the product of replacing HA is identical with that obtained by replacing HB, then Ha and HB are chemically equivalent. If the two products are nonidentical, then HA and HB are nonequivalent. For example, replacement of Ha or Hb in 3, 4, and 5 by an atom X would give different products. Therefore, HA and HB are nonequivalent in 3, 4, and 5. 

Finally, chemically equivalent protons do not split each other s... 

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