When we talk about equivalent protons, they are 2 types of equivalence, chemical and magnetic equivalences.

Chemically equivalent protons:

         These 3 protons are chemically equivalent, therefore, they can be reacted with the same propability.

Magnetically equivalent protons:

        Protons that are chemically equivalent may be magnetically or non-magnetically equivalent.
        Equivalent protons having the same magnetic environment will give the same 1H-NMR signal and show no magnetic interaction in the spectrum.
        Non-magnetic equivalent protons give different 1H-NMR signals and can show magnetic interaction in the spectrum.

 

Consider CH3-CH2-X, there are 2 sets of protons (CH3 and CH2). Are the protons in CH3 group magnetically equivalent?

Consider the 3 conformations below, H1 and H3 are equivalent but not H2.

In (I), H1 and H3 are magnetically equivalent, in (II), H2 and H3 are magnetically equivalent, and in (III), H1 and H2 are magnetically equivalent. However, when it rotates all are eqivalent. The 3 1Hs are siad to be magnetically equivalent.

The examples of compounds with one set of magnetically equivalent protons

Propanone
2,3-Dimethyl-2-butene
Benzene

 

The examples of molecules with two or more sets of magnetically equivalent protons

(Z)-1-Chloropropene (3 sets)
1,4-Dimethylbenzene (2 sets)

C2H4Cl2 has 2 constitutional isomers.

              1,2-Dichloroethane has one set of equivalent protons
              1,1-Dichloroethane has two sets of equivalent protons

These two compounds give different NMR spectra which are distinguishable.

 

The stereochemistry can affect the environment and can be classified as homotopic, enantiopic, or diastereotopic group.

For achiral molecules ;

          1. Homotopic groups are atoms or groups on an atom that do not give a chiral molecule when one of the groups is replaced by another group
                    For example, the protons of the –CH2 group of propane (CH3CH2CH3) are magnetically equivalent which is called homotopic.

Consider 3 conformers of propane.

H1 and H2 are magnetically equivalent in all conformers.

                    Homotopic atoms or groups have identical chemical shifts under all conditions.

 

          2. Enantiotopic groups are atoms or groups on an atom that give a chiral molecule when one of the atoms or groups is replaced by another group.
                    For example, the protons of the –CH2 group of ethanol (CH3CH2OH) are magnetically equivalent which is called enantiotopic.

Consider 3 conformers of ethanol.

                    H1 and H2 are magnetically equivalent in all conformers. When one of the atom or group is replaced by another group, a new stereocenter is created and a set of enanteomers results.

                    Enantiotopic groups have identical chemical shifts in normal conditions but different chemical shift in chiral environments.

 

For chiral molecules ;

         3. Diastereotopic groups are atoms or groups on an atom that is bonded to two nonidentical groups, one of which contains a stereocenter (or chiral carbon).  When one of the atom or group is replaced by another group, a new stereocenter is created and a set of diastereomers results.

Consider 3 conformers of 2-but anol.

                    In these 3 conformations, H1and H2 are non-magnetically equivalent. When H1or H2 is replaced by another group, a new stereocenter is created, therefore, the protons of the –CH2 group of 2-butanol (CH3CH2CH(OH)CH3) are non-magnetically equivalent which is called diastereotopic.

                    Diastereotopic groups have different chemical shifts under all conditions.