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Organic Chemistry
Table of Contents
Unit 14 Overview: Conjugated Systems and UV Spectroscopy
14.1 Stability of Conjugated Dienes: Molecular Orbital Theory
14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations
14.3 Kinetic versus Thermodynamic Control of Reactions
14.4 The Diels–Alder Cycloaddition Reaction
14.5 Characteristics of the Diels–Alder Reaction
14.6 Diene Polymers: Natural and Synthetic Rubbers
14.7 Ultraviolet Spectroscopy
14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation
14.9 Conjugation, Color, and the Chemistry of Vision

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14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation

Citation:

Conjugation in molecules has a profound impact on UV absorption. By alternating single and multiple bonds, conjugated systems reduce the energy gap between molecular orbitals, causing absorption at longer wavelengths. This effect is crucial in understanding the colors of compounds and their spectroscopic properties.

UV spectroscopy relies on these conjugation effects to analyze and identify molecules. As conjugation increases, absorption shifts to longer wavelengths, allowing us to predict UV-active compounds and quantify their concentrations using the Beer-Lambert law and molar absorptivity.

Effects of Conjugation on UV Absorption

Conjugation effects on UV absorption

  • Conjugation alternates single and multiple bonds (double or triple bonds) in a molecule
    • 1,3-butadiene ($CH_2=CH-CH=CH_2$) is a conjugated system
  • Conjugation reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
    • Smaller energy gap results in absorption of longer wavelength (lower energy) UV light
  • Increasing conjugation extent shifts UV absorption wavelength to longer wavelengths (bathochromic shift or red shift)
    • 1,3-butadiene absorbs at a longer wavelength than ethene due to increased conjugation
  • Extended conjugation in linear polyenes or aromatic systems leads to absorption at even longer wavelengths
    • $\beta$-carotene with 11 conjugated double bonds absorbs in the visible region giving it an orange color
  • The group of atoms responsible for light absorption in a molecule is called a chromophore

UV absorptions of conjugated systems

  • Enones (conjugated ketones) exhibit UV absorption around 215-250 nm due to the $\pi \rightarrow \pi^*$ transition of the conjugated system
    • 3-buten-2-one (methyl vinyl ketone) absorbs at around 224 nm
  • Aromatic rings like benzene show characteristic absorption around 260-280 nm due to the $\pi \rightarrow \pi^*$ transition of the conjugated system
    • This absorption is often referred to as the E2 band
    • Benzene absorbs at around 254 nm
  • Substituted aromatic rings may show additional absorption bands or shifts in absorption wavelength depending on the substituent nature
    • Electron-donating groups (-OH, -NH2) cause a red shift in absorption
    • Electron-withdrawing groups (-NO2, -COOH) cause a blue shift in absorption
  • Solvent effects can influence the absorption wavelength and intensity of UV-active compounds

Predicting UV-active compounds

  • Compounds with conjugated systems like dienes, trienes, and aromatic rings are likely to absorb in the 200-400 nm range
    • 1,3-butadiene, 1,3,5-hexatriene, and benzene
  • Compounds with extended conjugation such as polyenes and polycyclic aromatic hydrocarbons (PAHs) may absorb at even longer wavelengths within this range
    • $\beta$-carotene and anthracene
  • Compounds with conjugated carbonyl groups like enones and conjugated aldehydes will also absorb in this range
    • 3-buten-2-one and cinnamaldehyde
  • Compounds lacking conjugation such as simple alkenes or isolated carbonyl groups are less likely to absorb in the 200-400 nm range
    • Ethene and acetone absorb below 200 nm

Quantitative aspects of UV spectroscopy

  • The Beer-Lambert law relates the absorbance of a solution to the concentration of the absorbing species
  • Molar absorptivity (also known as extinction coefficient) is a measure of how strongly a chemical species absorbs light at a given wavelength

Key Terms to Review (20)

Molar absorptivity (ε): Molar absorptivity, denoted as ε, is a measure of how well a chemical species absorbs light at a given wavelength, expressed in units of L mol^-1 cm^-1. It quantifies the capacity of molecules, particularly conjugated compounds, to absorb ultraviolet (UV) or visible light, which is fundamental in understanding their electronic structure and behavior.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Solvent Effects: Solvent effects refer to the influence that the surrounding solvent environment can have on the behavior and properties of chemical reactions, molecules, and spectroscopic measurements. The nature and polarity of the solvent can significantly impact the energetics, kinetics, and outcomes of various organic chemistry processes.
Beer-Lambert Law: The Beer-Lambert law, also known as Beer's law, is a fundamental relationship in spectroscopy that describes the attenuation of light as it passes through a medium. It establishes a direct correlation between the concentration of an absorbing species in a solution and the amount of light absorbed by that solution.
Ultraviolet Spectroscopy: Ultraviolet spectroscopy is a technique used to analyze the absorption of ultraviolet (UV) light by molecules, providing information about their electronic structure and the presence of chromophores, which are functional groups that can absorb UV radiation. This analytical method is widely used in various fields, including organic chemistry, biochemistry, and materials science, to identify and characterize organic compounds.
Chromophore: A chromophore is a functional group or conjugated system within a molecule that is responsible for the molecule's color. It is the part of a molecule that absorbs specific wavelengths of light, leading to the observed color of the molecule.
λmax: λmax, or the wavelength of maximum absorption, is a key concept in ultraviolet (UV) spectroscopy that describes the specific wavelength at which a molecule absorbs light most strongly. This term is crucial in understanding the interpretation of UV spectra and the effects of conjugation on the absorption properties of organic compounds.
Extinction Coefficient: The extinction coefficient is a measure of the ability of a chemical species to absorb light at a given wavelength. It is a fundamental parameter that quantifies the strength of the interaction between light and matter, and it is a crucial factor in interpreting ultraviolet (UV) and visible light spectra.
Red Shift: Red shift refers to the phenomenon where the wavelength of electromagnetic radiation, such as light, is shifted towards longer, redder wavelengths. This occurs when the source of the radiation is moving away from the observer, causing a decrease in the observed frequency of the radiation.
Polyenes: Polyenes are organic compounds that contain multiple carbon-carbon double bonds arranged in a conjugated system. These types of compounds are particularly important in the context of interpreting ultraviolet spectra, the Wittig reaction, and the molecular orbitals of conjugated pi systems.
$ ext{pi} ightarrow ext{pi}^*$ transition: The $ ext{pi} ightarrow ext{pi}^*$ transition is an electronic transition that occurs in conjugated organic molecules. It involves the excitation of an electron from a bonding $ ext{pi}$ orbital to an antibonding $ ext{pi}^*$ orbital, which can lead to the absorption of ultraviolet (UV) or visible light.
E2 Band: The E2 band is a characteristic absorption band observed in the ultraviolet (UV) spectrum of conjugated organic molecules. It arises due to the electronic transitions between the highest occupied molecular orbital (HOMO) and the second lowest unoccupied molecular orbital (LUMO+1) of the conjugated system.
Trienes: Trienes are organic compounds that contain three carbon-carbon double bonds within a conjugated system. They are an important class of molecules that exhibit unique spectroscopic properties due to the effect of conjugation on their electronic structure.
Polycyclic Aromatic Hydrocarbons: Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds composed of multiple aromatic rings fused together. They are a significant topic in the context of interpreting ultraviolet spectra and understanding the effect of conjugation on the absorption properties of molecules.
Dienes: Dienes are organic compounds that contain two carbon-carbon double bonds. They are an important class of conjugated systems, which have significant implications for the interpretation of ultraviolet (UV) spectra.
Bathochromic Shift: A bathochromic shift, also known as a red shift, is a phenomenon in which the absorption or emission spectrum of a molecule is shifted to longer wavelengths (lower energy) compared to a reference compound. This shift is typically observed in ultraviolet and visible light spectroscopy and is closely related to the concepts of conjugation, aromaticity, and the chemistry of vision.
Molar Absorptivity: Molar absorptivity, also known as molar extinction coefficient, is a measure of the ability of a molecule to absorb light at a specific wavelength. It is a fundamental parameter in ultraviolet (UV) spectroscopy, which is used to quantify the concentration of a substance in a sample by measuring the amount of light it absorbs.
$\beta$-carotene: $\beta$-carotene is a naturally occurring, lipid-soluble pigment that belongs to the carotenoid family. It is responsible for the orange-red color in many fruits and vegetables, and it plays a crucial role in the interpretation of ultraviolet spectra due to its highly conjugated structure.
Conjugated System: A conjugated system refers to a series of alternating single and double bonds within a molecule, typically in a linear or cyclic arrangement. This arrangement allows for the delocalization of electrons, which has important implications for the molecule's stability, reactivity, and spectroscopic properties.
Enones: Enones are a class of organic compounds containing a carbonyl group (C=O) conjugated with a carbon-carbon double bond. They are characterized by the presence of this extended $\pi$-conjugated system, which gives them unique chemical and physical properties.