# Spins spin coupling on proton NMR for an aldehyde?

On a proton NMR, does the hydrogen in the CHO group cause coupling, or get coupled itself? Specifically, I a considering 2-methylpropanal. I cannot decide whether the H in the aldehyde group would be split into a doublet or remain as a singlet, and I am unsure as to whether the single proton on carbon number two would be a heptet (from the 6 adjacent protons), an octet ( from the 6 protons plus the proton in the aldehyde group), or if it would be split into some combination of a heptet of doublets, or a doublet of heptets...

Any advice on dealing with spin spin coupling in proton NMR would be much appreciated!

## 1 Answer

I cannot decide whether the H in the aldehyde group would be split into a doublet or remain as a singlet […]

What would be the argument against $^3J$ coupling between these protons? The aldehyde proton will appear as a doublet.

Don't forget that coupling works both ways! $\ce{(CH3)2C\color\red{H}-}$ will (at least) split to a doublet too. Coupling with this proton will also split the signal for the methyl protons to a doublet.

What does this mean for the proton at C-2?

Formally, a doublet (from coupling with the aldehyde proton) of a septet (from coupling with six magnetically equivalent methyl protons) is to be expected.

In really, you will observe a smaller number of lines! I suggest to try this with a pen on squared paper to figure out what happens when the two coupling constants are identical or one is twice as big as the other.