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14.12 a) This compound has a DU of 5. The two bands in the region of 2830- 2700 cm⁻¹ and the carbonyl absorption at 1706 cm⁻¹ indicate the presence of an aldehyde. The observed shift of the carbonyl band to a lower wavenumber suggests that it is a conjugated aldehyde. The absorptions in the region of 3100-3000 cm⁻¹ are due sp²-hybridized C-H bonds, and those at 3000-2850 cm⁻¹ are due to sp³-hybridized C-H bonds. The appearance of four bands in the 1600-1450 cm⁻¹ region and the strong band near 800 cm⁻¹ suggest the presence of an aromatic ring. (Check the NMR spectrum for confirmation. Note that an aromatic ring has DU = 4.) The spectrum shows presence of five different types of hydrogens in this compound. The singlet at 9.8 δ is typical of an aldehydic hydrogen. The four H's in the 8-7 δ region confirm the presence of an aromatic ring with two CH substituents. The pattern for these hydrogens, two rather symmetrical distorted doublets, is typical for para substitution. The hydrogens on adjacent positions couple to give a pair of doublets and the other pair of adjacent H's do the same. The 2 H's that appear as quartet at 2.7 δ must be coupled to the 3 H's appear as a triplet at 2.3 δ, indicating CH₂CH₃ the presence of an ethyl group. The compound is 4-ethylbenzaldehyde. b) This compound has a DU of 1. The carbonyl absorption at 1741 cm⁻¹ along with the strong absorptions in the 1300-1000 cm⁻¹ region suggest the presence of an ester group. The bands in the region of 3000-2850 cm⁻¹ are due to sp³-hybridized C-H bonds. The H-NMR spectrum shows the presence of four types of hydrogens and the integrals provide the actual number of hydrogens. The signal at 4.2 δ must be due to a CH₂ attached to the oxygen of the ester. This signal appears as four lines, so the CH₂ group must be next to a CH₃ group, the triplet at 1.3 δ (a group). The CH₂ protons of the two triplets (somewhat distorted) at 3.7 δ and 2.9 δ must be coupled ( a group). The IR, NMR, and DU data fit ethyl 3-bromopropanoate. O BrCH₂CH₂COCH₂CH₃ 216