Given the fact that 1, 3-butadiene has a UV absorption of 217nm, the absorption wavelength (in nm) for the… · ABC26OR0734
Subject: Organic Chemistry · Chapter: Organic Spectroscopy · Topic: Organic Spectroscopy – General · Exam: GATE 2000-2022 · Marks: 2 · Difficulty: Medium
Given the fact that 1, 3-butadiene has a UV absorption of 217nm, the absorption wavelength (in nm) for the conjugated system shown below is ____ 
(Use these absorption value for auxochromic groups: alkyl: +5; exo-cyclic double bond: +5; every additional conjugated $\mathrm{C}=\mathrm{C}:+30$ )
(Use these absorption value for auxochromic groups: alkyl: +5; exo-cyclic double bond: +5; every additional conjugated $\mathrm{C}=\mathrm{C}:+30$ )Answer
Answer (as printed): 282
The options are missing from the source scan for this question, so there is nothing here for this letter to point at. The letter is what the book printed.
Explanation
Answer: 282 — Woodward–Fieser rules, starting from the 1,3-diene base of 217 nm:
• One additional conjugated C=C (triene): +30
• Alkyl/ring-residue substituents on the conjugated carbons: one on the C bearing =CH₂, one on the ring-A C=C carbon, and two on the ring-fusion-side C=C carbon of the bicyclic unit. That is 4 × 5 = +20.
• Exocyclic C=C bonds: the =CH₂, the C=C exocyclic to ring A, and the C=C exocyclic to the bicyclic ring. That is 3 × 5 = +15.
λmax = 217 + 30 + 20 + 15 = 282 nm
Study loop for Organic Spectroscopy