Question

57. A C10H12O2 molecule shows IR absorptions at 3040, 2850, 2750, 1695, 1600, 1587, and 1270 cm. The ratio of carbon-bonded hydrogens in it's 'H-NMR spectrum is, 3:2:2:2:2: 1, with corresponding signal multiplicities of t, q, s, d, d & s. The corresponding chemical shifts are 8= 1.1, 3.5, 4.5, 7.3, 7.7, and 9.9. The proton decoupled 1C-NMR spectrum has only 8 signals. Predict the structure and match the multiplicity of each nonequivalent set of hydrogens. (You must draw bond-line structure ONLY and match the multiplicities on the structure for full credit)

Answer 1

• Total Hydrogens attached to Carbon are 3+2+2+2+2+1=12. Thus, no hydrogen attached to Oxygen.
• 2 Hydrogens are doublet with same chemical shift of 7.7 ppm and 2 other Hydrogens having same chemical shift of 7.3 ppm. This suggests a benzene ring with 2 substituent at para to each other.
• IR peak corresponding to 3040 confirms this.
• 3H (triplet) at 1.1 ppm suggests a Methyl group adjoining to a -CH₂- group.
• IR Peak of 2850 and 2750 confirms this
• 2 Hydrogens (Quartet) also confirms this. These hydrogens belong to the -CH₂- group which was adjoining to the Methyl group.
• However, notice that these hydrogens are quartet. This means that on the other side of -CH₂- group., there is either a Heteroatom or the benzene ring itself.
• Moreover, the chemical shift of Hydrogen on -CH₂- group. is 3.5 ppm. This suggests presence of an Oxygen on the other side.
• Degree of unsaturation is 5 out of which 4 are accounted for by the benzene ring and 1 double bond may be there.
• One H, singlet at 9.9 ppm indicates an aldehydic group (All the doubles bond accounted for.)
• So, till now, we have figured out the adjoining figure

С. Ч4 с. ч. О. - се и с. Исо, - с ч бсм, - си - Саче О - Счо С ч, 0, – си о СИ, О

• The only thing remaining are a -CH₂ group and an Oxygen atom, which possibly constitutes an ether group. Further, the remaining Hydrogens on -CH₂- are a singlet and are chemically shifted by 4.5 ppm. This indicates presence of Oxygen adjoining to this -CH₂- as well.
• Finally, we place all the things together and find out that this is our structure.

(1.1,t) (3.5,4) (4.5, s) (7.3 or 7.7, d) (7.3 or 7.7, d) (7.3 or 7.7, d) (7.3 or 7.7, d) (9.9, s) H o

In the above figure, the value of Chemical shifts is only for your understanding. The question asks you only to mention the multiplicity i.e. s, d, t or q.