UV/IR Interpretation: Woodward–Fieser rules for dienes and enones MCQs With Answer

Introduction

UV/IR Interpretation: Woodward–Fieser rules for dienes and enones is an essential topic for M.Pharm students studying spectral analysis. This blog focuses on applying Woodward–Fieser empirical rules to predict UV-visible λmax values of conjugated 1,3-dienes and α,β‑unsaturated carbonyl compounds (enones). You will find clear explanations of base values, typical substituent increments (alkyl, exocyclic, extra conjugation), and common exceptions, followed by exam-oriented multiple-choice questions. The MCQs test conceptual understanding, routine calculations, and practical interpretation of spectra—useful for coursework, practical exams, and research involving UV spectroscopy of pharmaceutical intermediates and natural products.

Q1. What is the Woodward–Fieser base value (approximate λmax) for an acyclic conjugated 1,3-diene?

• 214 nm
• 253 nm
• 215 nm
• 300 nm

Correct Answer: 214 nm

Q2. According to Woodward–Fieser rules, what is the typical base λmax for a homoannular (both double bonds in the same ring) conjugated diene?

• 214 nm
• 253 nm
• 215 nm
• 190 nm

Correct Answer: 253 nm

Q3. For conjugated 1,3-dienes, what increment (approx.) does each alkyl substituent or ring residue on the conjugated system add to the base value?

• +5 nm
• +10 nm
• +30 nm
• −5 nm

Correct Answer: +5 nm

Q4. In Woodward–Fieser rules for dienes, how much is typically added for each exocyclic double bond?

• +5 nm
• +10 nm
• +30 nm
• −10 nm

Correct Answer: +5 nm

Q5. What is the commonly used Woodward–Fieser base value for a simple α,β‑unsaturated ketone (enone) chromophore?

• 214 nm
• 215 nm
• 253 nm
• 300 nm

Correct Answer: 215 nm

Q6. For α,β‑unsaturated carbonyls (enones), what is the increment assigned for each alkyl or aryl substituent on the conjugated double bond (α or β position)?

• +5 nm
• +10 nm
• +20 nm
• +30 nm

Correct Answer: +10 nm

Q7. According to the rules, how much is added to the enone base λmax for an exocyclic double bond adjoining the conjugated system?

• +5 nm
• +10 nm
• +30 nm
• 0 nm (no change)

Correct Answer: +5 nm

Q8. Woodward–Fieser assigns a significant increment when an enone is extended by another conjugated double bond. What is the typical increment for such additional conjugation?

• +5 nm
• +10 nm
• +30 nm
• −10 nm

Correct Answer: +30 nm

Q9. What general effect do electron-donating substituents (e.g., alkyl or alkoxy) attached to a conjugated diene typically have on λmax according to Woodward–Fieser reasoning?

• They cause a bathochromic (red) shift — increase λmax
• They cause a hypsochromic (blue) shift — decrease λmax
• They eliminate the UV band completely
• No effect at all

Correct Answer: They cause a bathochromic (red) shift — increase λmax

Q10. How well do Woodward–Fieser rules predict λmax for cross‑conjugated dienes (non‑linear conjugation)?

• They are highly accurate for cross‑conjugation
• They are less reliable and may give anomalous results for cross‑conjugated systems
• They predict IR spectra rather than UV for cross‑conjugated dienes
• They give exactly the same values as for linear dienes

Correct Answer: They are less reliable and may give anomalous results for cross‑conjugated systems

Q11. Predict the λmax (approx.) for 2,3‑dimethyl‑1,3‑butadiene using Woodward–Fieser: base for acyclic diene = 214 nm; each alkyl substituent = +5 nm.

• 224 nm
• 214 nm
• 234 nm
• 253 nm

Correct Answer: 224 nm

Q12. Estimate the λmax for a homoannular cyclic diene with one alkyl substituent (use base 253 nm and alkyl increment +5 nm).

• 258 nm
• 253 nm
• 268 nm
• 214 nm

Correct Answer: 258 nm

Q13. For an α,β‑unsaturated ketone with one α‑alkyl and one β‑alkyl substituent, predict λmax (base 215 nm; +10 nm per alkyl on the double bond).

• 215 nm
• 225 nm
• 235 nm
• 255 nm

Correct Answer: 235 nm

Q14. If an enone is conjugated with an additional C=C (extended conjugation), what is the expected approximate change in λmax according to Woodward–Fieser?

• Small decrease (~−5 nm)
• Large bathochromic increase (~+30 nm)
• No change
• Complete disappearance of the band

Correct Answer: Large bathochromic increase (~+30 nm)

Q15. Which single substitution pattern produces the largest bathochromic shift for an enone under Woodward–Fieser increments?

• An exocyclic double bond (+5 nm)
• An alkyl substituent on the double bond (+10 nm)
• An additional conjugated C=C (extended conjugation) (+30 nm)
• Adding a methyl group far from the conjugation (no change)

Correct Answer: An additional conjugated C=C (extended conjugation) (+30 nm)

Q16. Which of the following is NOT a parameter explicitly used in the standard Woodward–Fieser empirical tables for predicting λmax?

• Number of conjugated double bonds and whether they are homoannular
• Alkyl or ring substituent increments
• Exocyclic double‑bond increments
• Molecular weight of the molecule

Correct Answer: Molecular weight of the molecule

Q17. A conjugated diene exhibits an experimental λmax around 260 nm. Which structural type is most consistent with this value using Woodward–Fieser approximations?

• An acyclic unsubstituted diene (base 214 nm)
• A homoannular cyclic diene with one alkyl substituent (≈258 nm)
• A simple enone (215 nm)
• A cross‑conjugated diene with no substituents (≈190 nm)

Correct Answer: A homoannular cyclic diene with one alkyl substituent (≈258 nm)

Q18. Which electronic transition is mainly responsible for the intense UV band whose λmax is predicted by Woodward–Fieser for enones?

• n→σ*
• π→π*
• n→π*
• σ→σ*

Correct Answer: π→π*

Q19. Which statement best describes the practical scope of Woodward–Fieser rules?

• They are empirical rules to predict IR absorption frequencies of carbonyls
• They predict exact λmax values for all conjugated systems with no error
• They provide approximate λmax values for UV spectra of conjugated dienes and enones based on substituent increments
• They are only applicable to saturated hydrocarbons

Correct Answer: They provide approximate λmax values for UV spectra of conjugated dienes and enones based on substituent increments

Q20. According to practical interpretation beyond the simple Woodward–Fieser table, which factor typically causes a hypsochromic (blue) shift by reducing effective conjugation in a diene or enone?

• Introduction of an additional conjugated double bond
• Steric twisting that disrupts planarity and conjugation
• Adding an alkyl substituent on the conjugated system
• Extending conjugation with another π system

Correct Answer: Steric twisting that disrupts planarity and conjugation

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