Which choice best explains why the molecules below can be distinguished by IR. H Molecule A O Molecule A has a peak just above 3000 cm-1 and Molecule B does not. Molecule A has a peak for an aldehyde around 2200 cm and Molecule B does not. -1 Molecule B has a carbonyl peak around 1750 cm1 and Molecule A does not. Molecule A and Molecule B can't be easily distinguished by IR. OOOO .CH3 Molecule B

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**Infrared Spectroscopy: Distinguishing Molecules**

### Which choice best explains why the molecules below can be distinguished by IR?

![Molecules](molecules.jpg)

- **Molecule A:** Contains a benzene ring with an aldehyde functional group.
- **Molecule B:** Contains a benzene ring with an acetyl group (a ketone substituted with a methyl group).

### Multiple Choice Explanation:

1. **Molecule A has a peak just above 3000 cm⁻¹ and Molecule B does not.**
2. **Molecule A has a peak for an aldehyde around 2200 cm⁻¹ and Molecule B does not.**
3. **Molecule B has a carbonyl peak around 1750 cm⁻¹ and Molecule A does not.**
4. **Molecule A and Molecule B can't be easily distinguished by IR.**

### Correct Answer: 
**Molecule B has a carbonyl peak around 1750 cm⁻¹ and Molecule A does not.**

#### Explanation:
Infrared (IR) spectroscopy is a powerful technique used to identify certain functional groups based on their characteristic absorption bands. Molecule A (which contains an aldehyde) and Molecule B (which contains a ketone) can be distinguished based on the specific absorption frequencies characteristic to these groups.

- **Aldehyde (Molecule A)**: Typically shows a distinctive absorption around 1725 cm⁻¹ corresponding to the C=O stretch of the aldehyde.
- **Ketone (Molecule B)**: Typically shows a strong absorption around 1705-1750 cm⁻¹ for the carbonyl (C=O) stretch.

Thus, **option 3** correctly identifies that Molecule B has a carbonyl peak around 1750 cm⁻¹, which Molecule A does not have, allowing easy distinction using IR spectroscopy.

### Note:
IR spectroscopy peaks are critical for identifying different functional groups in chemical molecules, making it an essential tool in molecular analysis in organic chemistry.
Transcribed Image Text:**Infrared Spectroscopy: Distinguishing Molecules** ### Which choice best explains why the molecules below can be distinguished by IR? ![Molecules](molecules.jpg) - **Molecule A:** Contains a benzene ring with an aldehyde functional group. - **Molecule B:** Contains a benzene ring with an acetyl group (a ketone substituted with a methyl group). ### Multiple Choice Explanation: 1. **Molecule A has a peak just above 3000 cm⁻¹ and Molecule B does not.** 2. **Molecule A has a peak for an aldehyde around 2200 cm⁻¹ and Molecule B does not.** 3. **Molecule B has a carbonyl peak around 1750 cm⁻¹ and Molecule A does not.** 4. **Molecule A and Molecule B can't be easily distinguished by IR.** ### Correct Answer: **Molecule B has a carbonyl peak around 1750 cm⁻¹ and Molecule A does not.** #### Explanation: Infrared (IR) spectroscopy is a powerful technique used to identify certain functional groups based on their characteristic absorption bands. Molecule A (which contains an aldehyde) and Molecule B (which contains a ketone) can be distinguished based on the specific absorption frequencies characteristic to these groups. - **Aldehyde (Molecule A)**: Typically shows a distinctive absorption around 1725 cm⁻¹ corresponding to the C=O stretch of the aldehyde. - **Ketone (Molecule B)**: Typically shows a strong absorption around 1705-1750 cm⁻¹ for the carbonyl (C=O) stretch. Thus, **option 3** correctly identifies that Molecule B has a carbonyl peak around 1750 cm⁻¹, which Molecule A does not have, allowing easy distinction using IR spectroscopy. ### Note: IR spectroscopy peaks are critical for identifying different functional groups in chemical molecules, making it an essential tool in molecular analysis in organic chemistry.
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