Chromophores, auxochromes and solvent effects shape the UV-Vis absorption behavior of drug molecules — a foundational topic for B. Pharm students. Chromophores (conjugated π-systems, carbonyls, nitro groups) determine electronic transitions (π→π*, n→π*) while auxochromes (–OH, –NH2, –OCH3, halogens) modify wavelength and intensity via resonance and inductive effects. Solvent polarity, hydrogen bonding and pH produce solvatochromic responses (bathochromic/hypsochromic and hyperchromic/hypochromic shifts) that influence spectral interpretation, assay accuracy and formulation. This concise but deeper set of MCQs covers mechanisms, examples, spectral shifts and practical implications for drug analysis. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is a chromophore in the context of UV-Vis absorption spectra?
- Conjugated π-system responsible for electronic absorption
- Any ionic group that gives color in solution
- A solvent molecule that shifts absorption bands
- A molecule that exclusively absorbs infrared radiation
Correct Answer: Conjugated π-system responsible for electronic absorption
Q2. What is the primary role of an auxochrome attached to a chromophore?
- To completely block absorption of light
- To change intensity and wavelength of absorption by resonance/inductive effects
- To convert electronic transitions into vibrational transitions
- To precipitate the chromophore out of solution
Correct Answer: To change intensity and wavelength of absorption by resonance/inductive effects
Q3. Which statement correctly contrasts π→π* and n→π* electronic transitions?
- π→π* transitions are usually lower intensity and occur at longer wavelengths than n→π*
- π→π* transitions are generally higher intensity and often occur at shorter wavelengths than n→π*
- n→π* transitions only occur in saturated hydrocarbons
- n→π* transitions always show greater molar absorptivity than π→π*
Correct Answer: π→π* transitions are generally higher intensity and often occur at shorter wavelengths than n→π*
Q4. What is a bathochromic shift?
- Shift of absorption to shorter wavelength (blue shift)
- Shift of absorption to longer wavelength (red shift)
- Decrease in absorption intensity without wavelength change
- Complete disappearance of a band
Correct Answer: Shift of absorption to longer wavelength (red shift)
Q5. What does a hyperchromic effect describe in an absorption spectrum?
- A decrease in molar absorptivity (absorbance intensity)
- An increase in molar absorptivity (absorbance intensity)
- A shift to shorter wavelength only in non-polar solvents
- A change in instrumental baseline
Correct Answer: An increase in molar absorptivity (absorbance intensity)
Q6. Which of the following groups is a classic auxochrome that donates electron density by resonance?
- –NO2
- –OH
- –CF3
- –COOH
Correct Answer: –OH
Q7. What is solvatochromism?
- Change in molecular weight of a solute with solvent
- Change in absorption maxima or intensity of a solute due to solvent polarity or specific solute–solvent interactions
- Conversion of UV light into visible light by solvent molecules
- Formation of precipitate when solvent changes
Correct Answer: Change in absorption maxima or intensity of a solute due to solvent polarity or specific solute–solvent interactions
Q8. Which of the following best explains why deprotonation of a phenol (to phenoxide) often produces a bathochromic shift?
- Deprotonation breaks all conjugation in the ring
- Phenoxide increases electron donation into the aromatic system, lowering the HOMO–LUMO gap
- Deprotonation converts the compound into an aliphatic hydrocarbon
- Phenoxide is insoluble and therefore shows a red shift
Correct Answer: Phenoxide increases electron donation into the aromatic system, lowering the HOMO–LUMO gap
Q9. Which functional group commonly acts as a chromophore due to its conjugated nitrogen–oxygen double bond?
- Ether (–O–)
- Nitro (–NO2)
- Alkyl (–CH3)
- Alcohol (–OH)
Correct Answer: Nitro (–NO2)
Q10. Increasing conjugation length in a molecule generally causes which spectral change?
- Hypsochromic shift to shorter wavelength
- No change in absorption maxima
- Bathochromic shift to longer wavelength
- Complete loss of UV absorption
Correct Answer: Bathochromic shift to longer wavelength
Q11. How does protonation of an aniline (–NH2) group typically affect its UV absorption?
- Protonation increases electron donation and causes a bathochromic shift
- Protonation removes lone-pair conjugation, causing a hypsochromic shift
- Protonation converts aniline to a nitro group
- Protonation has no effect on UV absorption
Correct Answer: Protonation removes lone-pair conjugation, causing a hypsochromic shift
Q12. Which law relates absorbance to concentration and path length in UV-Vis spectroscopy?
- Planck’s law
- Beer–Lambert law
- Raoult’s law
- Henry’s law
Correct Answer: Beer–Lambert law
Q13. Which property of a solvent most commonly causes nonspecific shifts in λmax (general solvatochromism)?
- Dielectric constant (polarity)
- Viscosity only
- Boiling point only
- Chromophore size
Correct Answer: Dielectric constant (polarity)
Q14. Why are n→π* bands usually weaker (lower ε) than π→π* bands?
- Because n→π* transitions are symmetry-allowed while π→π* are forbidden
- Because nonbonding electrons have poorer orbital overlap with π* orbitals, giving lower transition probability
- Because n→π* transitions always occur in the infrared region
- Because π→π* transitions require a catalyst
Correct Answer: Because nonbonding electrons have poorer orbital overlap with π* orbitals, giving lower transition probability
Q15. Which class of compounds typically exhibits strong π→π* absorption bands that move into the visible region with extensive conjugation?
- Simple alkanes
- Conjugated polyenes and azo dyes
- Inorganic salts with no conjugation
- Monosubstituted methane derivatives
Correct Answer: Conjugated polyenes and azo dyes
Q16. A carbonyl group (C=O) typically shows which type of electronic transition in the UV-Vis region?
- n→π* and π→π* transitions, with n→π* often lower intensity
- Only σ→σ* transitions
- Only d→d transitions
- Only charge-transfer transitions with metals
Correct Answer: n→π* and π→π* transitions, with n→π* often lower intensity
Q17. Which of the following is an electron-donating auxochrome by resonance?
- –NO2
- –NH2
- –CF3
- –COOH
Correct Answer: –NH2
Q18. What does a hypsochromic shift indicate?
- Absorption shift to longer wavelength (red shift)
- Absorption shift to shorter wavelength (blue shift)
- Increase in absorbance intensity without shift
- Complete quenching of fluorescence
Correct Answer: Absorption shift to shorter wavelength (blue shift)
Q19. Which experimental observation indicates a hyperchromic effect when comparing two spectra of the same compound?
- Lower absorbance at λmax in the later spectrum
- Higher absorbance at λmax in the later spectrum
- Shift of λmax but identical absorbance
- Complete loss of the absorption band
Correct Answer: Higher absorbance at λmax in the later spectrum
Q20. Deprotonation of a phenolic drug typically causes which spectral changes?
- Hypsochromic shift and hypochromic effect
- Bathochromic shift and hyperchromic effect
- No spectral change
- Conversion to an IR-active only molecule
Correct Answer: Bathochromic shift and hyperchromic effect
Q21. Which instrument parameter is used to quantify molar absorptivity (ε) experimentally?
- Retention time in chromatography
- Absorbance at λmax with known concentration and path length (Beer–Lambert law)
- pH of the solvent
- Boiling point of the sample
Correct Answer: Absorbance at λmax with known concentration and path length (Beer–Lambert law)
Q22. Specific solvent–solute interactions that involve hydrogen bonding and change λmax are classified as:
- Nonspecific dispersion interactions
- Specific solvation effects (hydrogen bonding)
- Instrumental artifacts
- Van der Waals exclusion
Correct Answer: Specific solvation effects (hydrogen bonding)
Q23. Why does extending conjugation reduce the HOMO–LUMO gap?
- Extended conjugation increases the energy of both orbitals equally, keeping the gap constant
- Extended conjugation delocalizes electrons, stabilizing the HOMO and lowering the LUMO energy difference, reducing the gap
- Conjugation converts HOMO to LUMO physically
- Conjugation removes all nonbonding electrons
Correct Answer: Extended conjugation delocalizes electrons, stabilizing the HOMO and lowering the LUMO energy difference, reducing the gap
Q24. Which transition type typically produces a weak, low-intensity absorption band in UV-Vis spectra?
- π→π*
- n→π*
- σ→σ*
- Metal-to-ligand charge transfer in organic solvents
Correct Answer: n→π*
Q25. Which structural feature in a drug is most likely to give a strong visible-region absorption when extended sufficiently?
- Aliphatic methyl chains
- Extended conjugated polyene or an azo linkage
- Isolated single carbonyl without conjugation
- Saturated cycloalkane ring
Correct Answer: Extended conjugated polyene or an azo linkage
Q26. What typically causes a hypochromic effect in an absorption spectrum?
- Increased conjugation of the chromophore
- Reduced transition probability due to loss of conjugation or strong solute–solvent interactions that dampen absorption intensity
- Formation of a more polar excited state only
- Increase in instrumental slit width
Correct Answer: Reduced transition probability due to loss of conjugation or strong solute–solvent interactions that dampen absorption intensity
Q27. Which factor generally does NOT change the wavelength of maximum absorption (λmax) for a simple chromophore under dilute conditions?
- Chemical substitution on the chromophore
- Solvent polarity in many cases
- Path length of the cuvette
- Specific hydrogen bonding interactions with solvent
Correct Answer: Path length of the cuvette
Q28. Positive solvatochromism refers to which trend when solvent polarity increases?
- λmax shifts to shorter wavelength (blue shift)
- λmax shifts to longer wavelength (red shift)
- No change in λmax
- Complete disappearance of absorption bands
Correct Answer: λmax shifts to longer wavelength (red shift)
Q29. How can one experimentally distinguish a π→π* band from an n→π* band?
- π→π* bands are lower in intensity (ε) than n→π* bands
- π→π* bands typically have much higher molar absorptivity (ε) than n→π* bands
- Only n→π* bands change with temperature
- Only π→π* bands are affected by pH
Correct Answer: π→π* bands typically have much higher molar absorptivity (ε) than n→π* bands
Q30. Why is understanding chromophores, auxochromes and solvent effects important in pharmaceutical analysis?
- To predict color but not relevant to quantitation
- To optimize UV methods, predict λmax shifts, improve assay accuracy and understand stability and impurity detection
- Only to choose organic solvents for extraction
- It is obsolete due to modern MS techniques
Correct Answer: To optimize UV methods, predict λmax shifts, improve assay accuracy and understand stability and impurity detection
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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