Identify the compound that is most likely to have NO significant absorption band for a multiple bond in the IR spectrum. A) oct-1-yne B) oct-1-ene C) ethylbenzene D) oct-4-yne

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Title: Infrared Spectroscopy and Absorption Bands

**Question:**

*Identify the compound that is most likely to have NO significant absorption band for a multiple bond in the IR spectrum.*

**Options:**

A) oct-1-yne  
B) oct-1-ene  
C) ethylbenzene  
D) oct-4-yne  

**Discussion:**

In infrared (IR) spectroscopy, compounds exhibit absorption bands corresponding to the vibrations of bonds within the molecules. Multiple bonds, such as double or triple bonds, typically show distinct absorption bands. However, when these bonds are symmetrically positioned or isolated within the molecule, these bands may become less prominent or invisible.

**Explanation:**

- **Oct-1-yne** (A) will show a distinct absorption for the terminal alkyne C≡C stretching.
- **Oct-1-ene** (B) will show absorption for the alkene C=C stretching.
- **Ethylbenzene** (C) might show aromatic stretching, but lacks isolated multiple bond absorptions that are usually as intense as those of alkenes and alkynes.
- **Oct-4-yne** (D) is a symmetrical internal alkyne, which often results in no significant absorption band for the C≡C stretch due to the symmetry cancelling out the dipole moment change.

**Correct Answer:** D) oct-4-yne

Oct-4-yne is the compound most likely to have no significant IR absorption for the multiple bond owing to its symmetrical structure.
Transcribed Image Text:Title: Infrared Spectroscopy and Absorption Bands **Question:** *Identify the compound that is most likely to have NO significant absorption band for a multiple bond in the IR spectrum.* **Options:** A) oct-1-yne B) oct-1-ene C) ethylbenzene D) oct-4-yne **Discussion:** In infrared (IR) spectroscopy, compounds exhibit absorption bands corresponding to the vibrations of bonds within the molecules. Multiple bonds, such as double or triple bonds, typically show distinct absorption bands. However, when these bonds are symmetrically positioned or isolated within the molecule, these bands may become less prominent or invisible. **Explanation:** - **Oct-1-yne** (A) will show a distinct absorption for the terminal alkyne C≡C stretching. - **Oct-1-ene** (B) will show absorption for the alkene C=C stretching. - **Ethylbenzene** (C) might show aromatic stretching, but lacks isolated multiple bond absorptions that are usually as intense as those of alkenes and alkynes. - **Oct-4-yne** (D) is a symmetrical internal alkyne, which often results in no significant absorption band for the C≡C stretch due to the symmetry cancelling out the dipole moment change. **Correct Answer:** D) oct-4-yne Oct-4-yne is the compound most likely to have no significant IR absorption for the multiple bond owing to its symmetrical structure.
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