Organic Chemistry
Spectroscopy is a technique that involves using light to investigate compounds and learn more about their structure. Different compounds absorb different wavelengths of light, providing insight into their properties and molecular structure.
IR spectroscopy, which utilizes infrared light, helps identify the types of bonds present in a molecule. It operates on the principle that certain frequencies of light match the vibration frequency of molecular bonds, causing the light to be absorbed. For example, N-H and O-H bonds display broad peaks beyond 3300 wavenumbers, while C-O double bonds show sharp peaks around 1700 wavenumbers. The fingerprint region at low wavenumbers is unique for each compound, making it an important characteristic to consider.
UV-vis spectroscopy employs ultraviolet and visible light frequencies to excite electrons within a compound, prompting them to jump to a higher energy level. Based on the specific frequency of light absorbed, you can deduce information about the compound's structure. Most organic compounds appear white as they absorb higher-energy UV light. However, some compounds with long conjugated chains of pi bonds can absorb lower-energy visible light, resulting in a unique color. This is observed in beta-carotene, which appears orange due to its highly conjugated molecular structure.
Lesson Outline
<ul> <li>Introduction to Spectroscopy <ul> <li>Using light to probe compounds and learn about their structure</li> <li>Measuring how much of each wavelength of light is absorbed or transmitted</li> </ul> </li> <li>Infrared (IR) Spectroscopy <ul> <li>Uses infrared light, a type of low-energy light with a wavelength below that of red light</li> <li>IR light frequency can match the frequency of bond vibrations in a molecule, absorbing that wavelength of light</li> <li>IR spectrometer measures which frequencies of IR light are absorbed</li> <li>Strong bonds and light atoms create high vibration frequencies, while weak bonds and heavy atoms create slower vibrations</li> <li>IR spectrum appearance and features <ul> <li>Y axis measures percent transmittance, indicating how much light traveled through the compound</li> <li>X axis is measured in wavenumbers, which are directly proportional to frequency</li> </ul> </li> <li>Characteristic IR peaks <ul> <li>N-H and O-H bonds give broad peaks around 3300 wavenumbers</li> <li>C-double-bond-O bonds give sharp peaks around 1700 wavenumbers</li> <li>Fingerprint region: Peaks and valleys below 1000 wavenumbers, unique to each compound</li> </ul> </li> </ul> </li> <li>Ultraviolet-Visible (UV-Vis) Spectroscopy <ul> <li>UV and visible light have enough energy to excite an electron to a higher energy level</li> <li>UV-Vis spectrometer measures the frequency of light absorbed, providing clues on the compound's structure</li> <li>Color of organic compounds <ul> <li>Most organic compounds are white, reflecting all visible wavelengths of light</li> <li>Colored organic compounds, like beta-carotene, absorb visible light</li> <li>Color appearance depends on the light absorbed and reflected, based on the color wheel</li> </ul> </li> <li>Conjugated chains and light absorption <ul> <li>Long conjugated chains of alternating single and double bonds result in closely spaced molecular orbitals</li> <li>Electrons have small jumps in energy level when excited by light absorption</li> </ul> </li> </ul> </li> </ul>
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FAQs
Infrared spectroscopy is focused on longer wavelengths, typically within the range of 2.5 to 25 micrometers, and is primarily used to analyze molecular vibrations and rotations in organic compounds. Ultraviolet spectroscopy, on the other hand, operates within the shorter wavelengths of 10 to 400 nanometers and is used to study electronic transitions in molecules, specifically in conjugated chains and molecular bond interactions.
UV-visible spectroscopy is a technique that measures the absorption of ultraviolet and visible light by a sample, typically in liquid form. A spectrophotometer is used to shine light at different wavelengths through the sample, and the amount of light absorbed is recorded. This absorption pattern across wavelengths can provide information about the sample's molecular structure, concentration, and purity. By analyzing the peaks in the absorbance spectrum, one can gain insight into electronic transitions occurring within the molecule and identify functional groups or conjugated systems present in the compound.
Light absorption is crucial in spectroscopy because it provides insight into a molecule's energy levels and electronic transitions. When a molecule absorbs light, it experiences an increase in energy, and the electrons within the compound may transition to higher energy levels. Analyzing the absorption patterns of different wavelengths allows scientists to identify specific functional groups, conjugated systems, and molecular bond interactions within the compounds. This information is invaluable for understanding the structure and properties of organic molecules, as well as identifying unknown compounds and studying chemical reactions.
Conjugated chains are a series of alternating single and double bonds in a molecule, which results in overlapping p-orbitals that form a delocalized electron system. These delocalized electrons can move more easily between energy levels as they absorb light. The presence of conjugated chains within a molecule generally leads to a lower energy transition and a shift in the absorption spectrum towards longer wavelengths, also known as a bathochromic shift. This unique absorption pattern can be used to identify molecules with conjugated systems and provide information about their molecular structure and chemical properties.
In infrared spectroscopy, molecular bonds play a significant role in determining which wavelengths of light will be absorbed by a compound. Each type of bond has a characteristic vibration frequency directly related to the atomic masses of the bonded atoms and the bond strength. When an infrared photon with a wavelength that matches the vibrational frequency of a specific bond is absorbed, the bond's vibrational energy increases, creating a peak in the absorption spectrum. By analyzing these peaks, scientists can identify the types of molecular bonds present in the compound and gain insight into the compound's structure and the types of functional groups within the molecule.
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