HNMR Spectra of Ethanal from http://science.widener.edu/svb/nmr/known_jdx.html.
This is the complete spectrum of ethanal. The peak at 0.0 corresponds to the compound
added to the sample to define zero. Usually this compound is tetramethylsilane (TMS)
because the hydrogens on the methyl groups are so shielded they will be the most zero.
Therefore, the other two signals correspond to the two different types of protons present
in ethanol. The molecular formula for ethanal is CH3CHO. The three protons on the
carbon are all the same, so the other type of proton is the one connected to the C=O. The
larger signal around 1.50 indicates that there are more protons of that sort than there are
of the shorter signal just above 9.0.
The picture takes a closer look at the larger signal from the picture above. The location of
this signal (around 1.65 ppm) indicates that is it highly shielded. Of the two types of
protons in ethanal discussed above, the protons on the first carbon (the carbon not bonded
to the oxygen) are more shielded than the protons on the carbon double bonded to the
oxygen. This signifies that this signal correlates to the protons on the CH3- of ethanal.
This peak also has two peaks that fulfill the requirements for being a doublet because the
area is 1 for each peak. According to the N + 1 rule, this tells us that there is only one
proton on the carbon next to the carbon that has these protons. This is accurate and
confirms that this signal corresponds to the proton in CH3 because on the carbon adjacent
to this carbon there is only one proton. All of the protons on the –CH3 are identical and
identical protons do not split each other. Therefore there are only two peaks.
The picture takes a closer look at the larger signal from the picture above. The location of
this signal (around 9.33 ppm) indicates that the protons are much less shielded than the
protons of the previous signal. As determined before, this signal must have fewer protons
than the more shielded signal because it is shorter. Since there are only two types of
protons in ethanal and from the picture above it was determined that the more shielded
signal correlated to the protons on -CH3, this signal must correspond to the proton on the
-COH portion of the molecule. This proton is bonded to the carbon that is also double
bonded to oxygen, making the proton very deshielded. This tells us why the signal for
this proton is more downfield than the signal for the other protons. This signal has four
peaks, meaning that on the carbon next to the carbon this proton is on, there are three
protons. And, indeed, there are because the other portion of the molecule is –CH3. These
four peaks are a quartet because the area under them (1:3:3:1) matches with the expected
area for a quartet. This is accurate and confirms that this signal corresponds to the proton
in CH3 because on the carbon adjacent to this carbon there is only one proton.