How can you know if there is a doublet of doublets by looking at a structure?

The point is that you do the experiment first; i.e. acquire the #""^1H# #NMR# spectrum, and then compare it to your conception of the structure. Are the structure and the spectrum consistent? If yes, good, if no, it's back to the drawing board.

No ONE method of analysis is sufficient for characterization. A new organic compound, before its existence can be accepted, must have (i) #""^1H# and #""^13C# #NMR# spectra, (ii) accurate combustion analysis, i.e. #%C,H, N# #"microanalysis"#, and (iii) accurate melting point or boiling points. Melting points are not so old-fashioned as you might think. As you learn in organic chemistry, the fastest way to identify something, is to get a general idea of what sort of compound you have, make a few derivatives, and then compare the melting points with the literature, which is extensive.

So, to your question, how do you know that there is a doublet of doublets? Look at the spectrum first, and then try to rationalize that spectrum with a proposed structure.

A doublet of doublets (dd) is a pattern of four lines of approximately equal intensity that results from coupling to two different protons.

I can think of two situations in which I would expect to see dd splitting patterns:

• vinyl groups
• 1,3,4-trisubstituted benzenes

A. Vinyl groups

Typical coupling constants in alkenes are #J_"trans" ≈ "16 Hz"#, #J_"cis" ≈ "10 Hz"#, and #J_"gem" ≈ "2 Hz"#.

Consider the #""^1"H"#-NMR spectrum of methyl acrylate:

(From organic spectroscopy international)

Each proton on the vinyl group is split into a doublet of doublets by its neighbours.

1,3,4-Trisubstituted benzenes

Typical coupling constants in substituted benzenes are #J_"ortho" ≈ "8 Hz"#, #J_"meta" ≈ "2 Hz"#, and #J_"para" ≈ "0 Hz"#.

Consider the #""^1"H"#-NMR spectrum of 3,4-dichlorobenzoyl chloride:

(From www.chem.wisc.edu)

The proton that has ortho and meta neighbours (on carbon 6) will be a doublet of doublets.

It appears at δ 7.95 (J = 8.5, 2.3 Hz).