Ultraviolet and Visible Spectroscopy Quiz
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Ultraviolet and Visible Spectroscopy Quiz

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Questions and Answers

How does the presence of impurities influence the UV spectrum of a compound?

  • It changes the color of the compound.
  • It has no observable effect on the spectrum.
  • It causes a systematic reduction in peak intensity.
  • It results in the appearance of additional peaks. (correct)

    • What is the effect of steric hindrance on the absorption wavelength of isomer A?

  • It decreases the absorption wavelength. (correct)
  • It has no effect on the absorption wavelength.
  • It increases the absorption wavelength.
  • It shifts the absorption wavelength to the same value.

    • Which of the following best describes the term 'bathochromic shift'?

  • A shift to higher energy and shorter wavelengths.
  • A permanent change in the structure of the molecule.
  • An increase in the intensity of absorption without wavelength change.
  • A shift to lower energy and longer wavelengths. (correct)

    • What is the effect of polar solvents on the π − π* transitions compared to n − π* transitions?

    <p>Polar solvents stabilize n − π* transitions more than π − π* transitions.</p> Signup and view all the answers

    Which of the following statements is true regarding electronic spectroscopy?

    <p>It provides information on the electronic structure of molecules.</p> Signup and view all the answers

    What is the term for the shift that results in absorption moving toward longer wavelengths?

    <p>Bathochromic shift</p> Signup and view all the answers

    Which statement correctly describes auxochromes?

    <p>They are only effective when conjugated with a chromophore.</p> Signup and view all the answers

    Which transition requires the least energy among the following?

    <p>n-π* transition</p> Signup and view all the answers

    How does conjugation affect the wavelength of absorption?

    <p>It shifts absorption towards longer wavelengths.</p> Signup and view all the answers

    In a polar solvent, π-π* transitions usually result in which type of spectral shift?

    <p>Bathochromic shift</p> Signup and view all the answers

    What effect does a hyperchromic shift have on absorption intensity?

    <p>It increases absorption intensity.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of chromophores?

    <p>They consist only of saturated bonds.</p> Signup and view all the answers

    What is the primary effect of auxochromes attached to chromophores?

    <p>They shift absorption to longer wavelengths.</p> Signup and view all the answers

    What does the HOMO-LUMO gap indicate about a molecule's conjugation?

    <p>The gap decreases as conjugation increases.</p> Signup and view all the answers

    Which statement correctly describes Absorbance in UV-visible spectroscopy?

    <p>Absorbance is the reciprocal of transmittance.</p> Signup and view all the answers

    In the context of electronic transitions, what does a σ−σ* transition involve?

    <p>Excitation from bonding sigma orbital to anti-bonding sigma orbital.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of Ultraviolet (UV) spectroscopy?

    <p>It operates in the 800-400 nm range.</p> Signup and view all the answers

    What does Beer-Lambert law express in UV-visible spectroscopy?

    <p>Absorbance is a function of concentration and path length.</p> Signup and view all the answers

    What type of transition is less energy-intensive than σ−σ* transition?

    <p>n−σ* Transition</p> Signup and view all the answers

    In the formula of absorbance, which variable represents the intensity of incident light?

    <p>Io</p> Signup and view all the answers

    Which of the following compounds is likely to undergo an n−π* transition?

    <p>Aldehydes</p> Signup and view all the answers

    What effect does a polar solvent have on n − π* transitions?

    <p>It causes a blue shift.</p> Signup and view all the answers

    Which of the following base values for conjugated dienes is correct?

    <p>214 nm for acyclic or heteroannular dienes</p> Signup and view all the answers

    What is the increment for adding a double bond that extends conjugation in α,β-unsaturated carbonyl compounds?

    <p>30 nm</p> Signup and view all the answers

    How does the presence of an -NR2 group affect the λmax for α,β-unsaturated carbonyl compounds?

    <p>It increases λmax by 95 nm.</p> Signup and view all the answers

    What is the effect of increasing the extent of conjugation in compounds?

    <p>It causes absorption to shift towards longer wavelengths.</p> Signup and view all the answers

    What is the base value for a 5-membered cyclic ketone according to Woodward-Fieser rules?

    <p>202 nm</p> Signup and view all the answers

    Which of the following polar groupings has the smallest increment for λmax?

    <p>-OCOCH3</p> Signup and view all the answers

    What determines whether compound A will appear at a longer wavelength than compound B in terms of conjugation?

    <p>The presence of n − π* transitions.</p> Signup and view all the answers

    Study Notes

    Ultraviolet and Visible Spectroscopy

    • UV and visible spectroscopy, also known as electronic spectroscopy, measures the number of conjugated double bonds and aromatic conjugation within a molecule.
    • It involves the promotion of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
    • The HOMO-LUMO gap decreases as the conjugation increases.
    • The ultraviolet region corresponds to 400-200 nm, and the visible region to 800-400 nm.
    • UV-visible spectroscopy is based on Beer-Lambert's law: log (I₀/I) = ε.l.c or ε = A/c.l
      • I₀ = intensity of incident light
      • I = intensity of transmitted light
      • ε = molar absorptivity
      • l = path length of the sample
      • c = concentration of the sample
      • A = Absorbance

    Absorbance and Transmittance

    • Absorbance (A) is the reciprocal of transmittance.
    • Transmittance (T) is the fraction of incident light transmitted.

    Electronic Transitions

    • Electronic transitions occur when a molecule absorbs electromagnetic radiation in the UV-visible region, and its electrons are promoted from the ground state to the excited state, or from the bonding orbital to the anti-bonding orbital.

    Types of Electronic Transitions

    • σ - σ Transition*: Transition of an electron from a bonding sigma orbital (σ) to an anti-bonding sigma orbital (σ*). Examples include alkanes due to sigma bonds holding all atoms together.
    • n - σ & n - π Transition**: Transitions from a non-bonding molecular (n) orbital to an anti-bonding sigma orbital or anti-bonding pi orbital (π*). These transitions require less energy than σ - σ* transitions. Examples include alkyl halides, aldehydes, and ketones.
    • π - π Transition*: Typically observed in unsaturated molecules such as alkenes, alkynes, aromatics, and carbonyl compounds. This transition requires less energy compared to n - σ* transitions.
      • Energy order: n-π* < π-π* < n-σ* < σ-σ*

    Absorption, Intensity Shift, and UV Spectrum

    • Bathochromic shift (Red shift): Absorption shifts towards longer wavelengths (λmax).
    • Hypsochromic shift (Blue shift): Absorption shifts towards shorter wavelengths (λmax).
    • Hyperchromic shift: The intensity of the absorption maximum (εmax) increases.
    • Hypochromic shift: The intensity of the absorption maximum (εmax) decreases.

    Chromophores and Auxochromes

    • Chromophores: Covalently unsaturated groups that absorb electromagnetic radiation in the UV-visible region, giving the compound its color. Common examples are C=C, C≡C, benzene, and NO2.
    • Auxochromes: Saturated groups containing lone pairs of electrons. When attached to a chromophore, they cause the absorption to shift towards a longer wavelength. Common examples are -OH, -SH, and -NH2.

    Factors Affecting Absorbance and Intensity

    • Conjugation: As the number of double bonds (chromophores) in conjugation increases, the absorption shifts towards longer wavelengths.
    • Effect of solvent:
      • In polar solvents, π - π* transitions shift red due to dipole interactions lowering the energy of the excited state.
      • In polar solvents, n - π* transitions shift blue due to dipole interactions lowering the energy of the ground state.

    The Woodward-Fieser Rules

    • For Conjugated Dienes: A set of rules to predict the λmax (nm) for conjugated dienes, based on the base value and increments for different substituents and structures.
    • For α, β-Unsaturated Carbonyl Compounds: Similar rules predict the λmax (nm) for α, β-unsaturated carbonyl compounds based on the base value and increments for different substituents and structures.

    Applications of UV Spectroscopy

    • Extent of conjugation: Determines the degree of conjugation based on the wavelength of absorption.
    • Distinguishing conjugated and non-conjugated compounds: Conjugated compounds absorb at longer wavelengths than their non-conjugated counterparts.
    • Studying geometric isomerism: Steric hindrance can affect the UV spectrum, allowing for the distinction of geometric isomers in a compound.
    • Detecting impurities: Impurities can lead to additional peaks in the UV spectrum, providing information about the compound's purity.

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    UV Spectroscopy Lecture Notes PDF

    Description

    Test your understanding of ultraviolet and visible spectroscopy concepts, including HOMO-LUMO transitions, Beer-Lambert's law, and the significance of absorbance and transmittance. This quiz covers essential principles and applications in analytical chemistry. Challenge your knowledge and see how well you grasp these critical topics.

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