Question

What are equivalent hydrogens?

Answer 1

Equivalent hydrogens are `H`-atoms that are completely interchangeable as to their role in the molecule.

One simple example are the `H`'s in methane `CH_4`

An example of the opposite is methanol `CH_3OH`
While 3 of the `H`'s are directly attached to the one `C` the 4th is attached to an `O`-atom:

(pictures from wikipedia)
So you can say that the hydrogens in the `CH_3`-group are equivalent, but the one in the `OH`-group is different.

Equivalent Hydrogens have to do with stereochemistry or even HNMR. When you have a molecule like 1-bromopropane, the first carbon has two hydrogens on it, each of which are not equivalent from a stereochemistry perspective.

If you were to replace one Hydrogen with a Deuterium, for example, the molecule would then be chiral at the first carbon and be capable of having an enantiomer. Place the Deuterium where the other hydrogen is and you get the other enantiomer. They Hydrogens are then considered "enantiotopic" and not equivalent.

If you did the same for the second carbon, you would also get two different enantiomers, so these Hydrogrens are also non equivalent and called "enantiotopic." The third carbon with three Hydrogens on it, if you replace any one Hydrogen with a Deuterium you still don't have a chiral carbon, so we call those Hydrogens "homotopic" or equivalent.

In the case of HNMR, the two Hydrogens on carbon 1 are equivalent because they give the same signal on the spectrum. The Hydrogens at carbon 2 are equivalent to each other. The same is true for carbon 3. So, on HNMR you have only three unique Hydrogens.