Question

When you have a doublet of doublets how many neighboring Hydrogens will there be?

Answer 1

There is one neighbouring hydrogen in a distinct chemical environment.

Explanation:

In the first order spectrum, for which I will add an illustration later, the given absorption is split into a doublet by coupling to the first proton, and this doublet is split into another doublet by coupling to the SECOND chemically non-equivalent proton. Sometimes these doublets overlap to give what seems to be an apparent triplet. Such so-called multiplets can be resolved by doing the experiment on a higher field instrument.

Answer 2

There will be two neighbouring hydrogens.

Explanation:

A doublet of doublets (dd) occurs when a hydrogen atom is coupled to two non-equivalent hydrogens.

An example is the NMR spectrum of methyl acrylate.

(From Chemistry LibreTexts)

Each of the vinyl protons `"H"_text(a), "H"_text(b)` and `"H"_text(c)` is a dd.

Let's examine the `"H"_text(c)` signal at 6.21 ppm.

There are four separate peaks because `"H"_text(c)` is coupled to both `"H"_text(a)` and `"H"_text(b)`, but with different coupling constants for each.

The result is a doublet of doublets.

The splitting diagram shows that `"H"_text(c)` is split into a doublet by `"H"_text(a)` with a large coupling constant `J_text(trans)`.

These peaks are each split into doublets by coupling with `"H"_text(a)` with a much smaller coupling constant `J_text(gem)` = 1.5 Hz.