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Solutions for Aromatic Compounds
45 continued
CH3
OH
CH3CH3
OH
CH3
CH(CH3)2
C
CH3
H
OH
CH3
C
CH3
OH
H
CH3
Resonance stabilization of this benzylic cation includes forms 
with positive charge on three ring carbons and on oxygen (shown).
Several isomeric combinations are consistent with the spectra (although the single H giving δ 6.6 suggests 
that either Y or Z is the OH group—an OH on a benzene ring shields hydrogens ortho to it, moving them 
upfield).
thymol
The final question is how the molecule fragments in the mass spectrometer:
m/z 135
+ • CH3
CH3
OH
CH(CH3)2
OH
CH(CH3)2
CH3
CH(CH3)2
OH
CH3
These structures fit the data equally well.
other
resonance
forms
CH3 C
O
C CH3
O
C
CH3
OH
H
46
Mass spectrum: Molecular ion at 170; two prominent peaks are M − 15 (loss of methyl) and M − 43 (as we 
shall see, most likely the loss of acetyl, CH3CO).
Infrared spectrum: The two most significant peaks are at 1680 cm–1 (conjugated carbonyl) and 1600 cm–1 
(aromatic C=C).
NMR spectrum: A 3H singlet at δ 2.7 is methyl next to a carbonyl, shifted slightly downfield by an 
aromatic ring. The other signals are seven aromatic protons. The 1H at δ 8.7 is a deshielded proton next to 
a carbonyl. Since there is only one, the carbonyl can have only one neighboring hydrogen.
Conclusions:
+ m/z 127 including 7H ⇒ mass 120 for carbons ⇒ 10 C
The fragment C10H7 is almost certainly a naphthalene. The correct isomer (box) is indicated by the NMR.
This isomer is a less good 
answer as it would have 
two deshielded protons in 
the NMR, one of which 
would be a singlet (it's a 
doublet in the spectrum).
The structure of thymol is:
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