Question

For a doublet of doublet, how many neighbors does it have?

Answer 1

2

Explanation:

For a doublet of doublet you would see 4 peaks. When drawing the splitting tree this becomes obvious. So a splitting tree of a doublet of doublet would look like the following:
![https://useruploads.socratic.org/tYJUvD7zT0uS1ZGM18ww_doublet_of_doublet.jpg)
Here the proton what we look at in the molecule is the upper line. This proton has two neighbors with different coupling constants `(J)`.

So we start with the proton who has the highest `J`, which follows the `n+1` rule. Meaning we would see two peaks, shown between the arrow `J_(ba)`.

Now we take the proton with the lower `J`, which from each peak will make two doublets shown as `J_(bc)`.

This gives a signal with a doublet of doublets. Since we use `n+1` rule twice the amount of neighbors is 2.