A group of protons is magnetically equivalent when they not only having the same chemical shift out also have identical spin spin coupling to each individual nucleus outside the group.
CHEMICAL EQUIVALENCE AND NONEQUIVALENCE:-
In ethyl bromine, the CH₃ protons and CH₂ protons are completely different types of chemical environment and are obviously chemically nonequvialent. The question or chemical equivalence is not so simple to answer in other cases. Consider, for example, the protons shown in colour in the following example.
In each examples, the protons look as if they should be equivalent because they have the same connectivity relationship to other groups in the molecules. For example in compound C each of the coloured hydrogen is connecting to carbon that's connected to a chlorine and to a (CH₃CHCl) group. We can say that this protons are connectively equivalent. The colored hydrogen in each of the above molecules are connectively equivalent, and the CH₃− and CH₂− protons in ethyle bromide connectively nonequvialent.
Connectively nonequvialent group are always chemically nonequvialent. The converse of it's statement is, however, actually not true. Some connectively equivalent groups are chemically equivalent : but others not. Whether two connectively equivalent groups are chemically equivalent depending on the their stereochemical relationship. The stereochemical relationship between two connectively equivalent groups becomes very easy to see if we make a simple substitution test : the substitute each connectively equivalent groups i turn with a fictitious group.(G) The stereochemical relationship of the resulting molecules then determines the stereochemical relationship of the group (G)
In compound A let us substitute each hydrogen in turns with a group G.
Each of this "new" molecules can be super imposed on the other atom. When the substitution test giving identical molecules, the connectively equivalent groups are said to be homotopic. Homotopic groups are chemically equivalent and indistinguishable. They having the same chemical shift in nmr spectrum.
Substitution of the each connectively equivalent hydrogen in compound B giving enantiomers.
When substitution test gives enantiomers, the connectively equivalent groups are said to be enantiotopic. Enantiotopic groups are chemically nonequvialent in a chiral environment but they chemically equivalent in an achiral environment. This the protons in B would have different chemical shift in a chiral. The optically active solvent. But optically inactive solvents they appears at singlet.
Finally, substitution or each connectively equivalent hydrogen in C gives diastereoisomers.
When the substitution test giving diastereoisomer, the connectively equivalent groups are said to be diastereotopic. Diastereotopic groups are chemically nonequvialent under all conditions. They have different types of chemical shifts and split each other.
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