B.21 How would each of the following pairs of compounds differ in their IR spectra? a. b. C. d. and OH OCH 3 and -OCH3 and and OCH3 OCH₂ EN

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**B.21 How would each of the following pairs of compounds differ in their IR spectra?**

**a.**
- Cyclopentene and 1-butyne

**b.**
- Hydroxyacetone (acetol) and Methyl acetate

**c.**
- 1,2-Dimethoxycyclohexane and Hexyl acetate

**d.**
- 1-Butyne and Heptanenitrile

### Explanation of Differences in IR Spectra:

- **a.** Cyclopentene and 1-butyne would differ primarily in their C-H stretch regions. Cyclopentene, with its alkene group, will show a C=C stretch around 1650 cm⁻¹, and an alkene C-H stretch just above 3000 cm⁻¹. In contrast, 1-butyne will show a strong C≡C stretch near 2100-2200 cm⁻¹ and a terminal alkyne C-H stretch typically around 3300 cm⁻¹.
  
- **b.** Hydroxyacetone, an alcohol, will have a broad O-H stretch around 3200-3550 cm⁻¹, while methyl acetate will show a C=O stretch near 1750 cm⁻¹ typical of esters. The absence of an O-H stretch in methyl acetate will be a key distinguishing feature.

- **c.** 1,2-Dimethoxycyclohexane has ether linkages, which will show C-O stretches typically around 1000-1300 cm⁻¹. Hexyl acetate, as an ester, will prominently present a C=O stretch near 1750 cm⁻¹. The absence of carbonyl stretch in 1,2-dimethoxycyclohexane differentiates it from hexyl acetate.

- **d.** 1-Butyne and heptanenitrile differ as 1-butyne contains a terminal alkyne, showing a C≡C stretch around 2100-2200 cm⁻¹ and a sharp C-H stretch around 3300 cm⁻¹. Heptanenitrile will display a C≡N stretch, typically around 2250 cm⁻¹. The presence of the nitrile stretch and the absence of terminal alkyne C-H stretch in heptanenitrile are distinguishing factors.
Transcribed Image Text:**B.21 How would each of the following pairs of compounds differ in their IR spectra?** **a.** - Cyclopentene and 1-butyne **b.** - Hydroxyacetone (acetol) and Methyl acetate **c.** - 1,2-Dimethoxycyclohexane and Hexyl acetate **d.** - 1-Butyne and Heptanenitrile ### Explanation of Differences in IR Spectra: - **a.** Cyclopentene and 1-butyne would differ primarily in their C-H stretch regions. Cyclopentene, with its alkene group, will show a C=C stretch around 1650 cm⁻¹, and an alkene C-H stretch just above 3000 cm⁻¹. In contrast, 1-butyne will show a strong C≡C stretch near 2100-2200 cm⁻¹ and a terminal alkyne C-H stretch typically around 3300 cm⁻¹. - **b.** Hydroxyacetone, an alcohol, will have a broad O-H stretch around 3200-3550 cm⁻¹, while methyl acetate will show a C=O stretch near 1750 cm⁻¹ typical of esters. The absence of an O-H stretch in methyl acetate will be a key distinguishing feature. - **c.** 1,2-Dimethoxycyclohexane has ether linkages, which will show C-O stretches typically around 1000-1300 cm⁻¹. Hexyl acetate, as an ester, will prominently present a C=O stretch near 1750 cm⁻¹. The absence of carbonyl stretch in 1,2-dimethoxycyclohexane differentiates it from hexyl acetate. - **d.** 1-Butyne and heptanenitrile differ as 1-butyne contains a terminal alkyne, showing a C≡C stretch around 2100-2200 cm⁻¹ and a sharp C-H stretch around 3300 cm⁻¹. Heptanenitrile will display a C≡N stretch, typically around 2250 cm⁻¹. The presence of the nitrile stretch and the absence of terminal alkyne C-H stretch in heptanenitrile are distinguishing factors.
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