Question

Determine the structure of the compound that has the IR and ¹H NMR spectra shown. It has the molecular formula: C₁₁H₁₄O₂

Bigger pic here---imgur.com/XF5Xs.jpg

Please provide a detailed explanation as best as you can or your thinking process so I can follow along... thanks!

Answer 1

Always calculate DBE first:
[(C₁₁H₂₄)-(C₁₁H₁₄)]/2= 5DBE

Most likely that means you've got benzene using up 4 of the DBE, therefore that leaves you with one more

IR) band at 1700cm^-1 indicates a carbonyl (the fifth DBE), since there's no trough from 3400cm^-1 to 2400cm^-1, it's not an acid. However, there's a peak at 2750cm^-1, so that indicates that you've got an aldehyde.

Since the carbonyl peak for an aldehyde is usually around 1725cm^-1 and on this IR, it's at 1700cm^-1, the aldehyde must be conjugated. All the double bond equivalents have been used up, so that means that the aldehyde is connected to the aromatic ring.

Now look for the substitution pattern of the ring...

From the IR spectrum, the peak at ~825cm^-1 indicates that it's a para-disubstituted ring. This is confirmed by the nmr spectrum where there's two doublets (characteristic of a para substitution) in the aromatic region. Also in the IR spectrum is a peak at ~10ppm, which is the aldehyde proton.

So far, 7 carbons, 5 hydrogens, and one oxygen have been accounted for... That leaves C₄H₉O to figure out.

NMR) It's usually easiest to start with the methyl group in the spectrum to figure out arrangement, just because it's almost always around 1ppm. The methyl group here is a triplet, so it must be next to a CH₂.

Looking at the relative integrations of the protons, the other three peaks must all be from CH₂ groups. Normal CH₂ groups have peaks around 1.5ppm (that would account for two of them), but one of the peaks is at around 4ppm. Since one of the oxygens has not yet been accounted for, one of the CH₂ groups must be attached to it (confirmed by a shift to ~4ppm)

So the oxygen can be connected to either another CH₂ group, or attached to the ring.

It's probably easiest at this point to draw out both possibilities and check the couplings...

1)

2)

3)

Only option 3 has the same coupling patterns as the nmr spectrum, so that must be the correct structure. Checking the formula, C₁₁H₁₄O₂, that means everything is accounted for.