interpret the IR spectrum for benzoin, benzil and benzilic acid. draw out compound on the corresponding spectrum and indicate the specific absorbance ( not the ranges from correlation table)for each type of bond. focus especially on amy OH bonds . some of the hydrogen bonding hroups display sharper than normal absorbance . Explain Why.(Hint -draw the molecules carefully with lots of detail).
The Correct Answer and Explanation is :
Benzoin:
Benzoin (C6H5CHOHCO-C6H5) contains a hydroxyl group (-OH) and a carbonyl group (C=O). In the IR spectrum, look for:
- Hydroxyl group (OH): A broad, strong absorption typically appears around 3200-3550 cm^-1. This is due to the O-H stretch, and it may show broadening due to hydrogen bonding.
- Carbonyl group (C=O): A sharp, strong absorption will appear around 1700 cm^-1, characteristic of the carbonyl stretch.
The hydroxyl group in benzoin might show sharper absorbance due to hydrogen bonding, which can increase the intensity and sharpness of the O-H stretch compared to compounds with non-hydrogen-bonded hydroxyl groups.
Benzil:
Benzil (C6H5COCOC6H5) is a diketone with two carbonyl groups. In the IR spectrum, expect:
- Carbonyl groups (C=O): Two strong absorptions will appear around 1700 cm^-1 (one for each carbonyl). The exact location can vary slightly based on conjugation.
- There are no OH groups in benzil, so the spectrum will lack the broad O-H stretch.
Benzilic Acid:
Benzilic acid (C6H5C(OH)2COOH) contains both a hydroxyl group and a carboxylic acid group. In the IR spectrum:
- Carboxylic acid (COOH): The O-H stretch appears around 2500-3300 cm^-1 but is broad and overlaps with the C-H stretches. The C=O stretch for the carboxyl group typically appears around 1700 cm^-1.
- Hydroxyl group (OH): Like benzoin, this stretch is broad but can appear sharper if hydrogen bonding is involved.
Explanation of Sharp Absorbance:
Hydrogen bonding influences the O-H stretch. When the hydroxyl group is involved in hydrogen bonding (like in carboxylic acids or phenols), it broadens the absorption and increases its intensity. In some cases, this can cause the O-H stretch to appear sharper than typical non-hydrogen-bonded OH stretches. The exact effect can depend on the strength and extent of the hydrogen bonding in the sample.
Now, let me generate the molecular structures for benzoin, benzil, and benzilic acid to visualize them alongside the explanations.
Here are the molecular structures for benzoin, benzil, and benzilic acid. The hydroxyl (-OH), carboxyl (-COOH), and carbonyl (C=O) groups are clearly shown for each compound.
Key Points:
- Benzoin: Contains a hydroxyl (-OH) group and a carbonyl group. The hydroxyl group’s IR absorption can be sharp due to hydrogen bonding.
- Benzil: A diketone, so only the two carbonyl stretches are present.
- Benzilic acid: Has both a hydroxyl group and a carboxyl group. The IR spectrum for this compound will show broadening in the O-H region due to the carboxyl group’s hydrogen bonding.
Hydrogen bonding in these compounds, especially with the hydroxyl and carboxyl groups, is a critical factor that sharpens and intensifies the OH stretch absorption in the IR spectrum.