Question

Calculate the \( \lambda_{\text{max}} \) of the following molecule:

• A. 283 nm
• B. 273 nm
• C. 234 nm
• D. 244 nm

Hint:

- Conjugated systems generally result in longer wavelength absorption maxima (higher \(\lambda _\text{max}\) values). - The larger the conjugation, the lower the energy required for electron transitions, resulting in higher \( \lambda_{\text{max}} \) values.

Answer 1

The Correct Option is B

Step 1: Identify the chromophore and base value.
The parent system is a homoannular (same-ring) conjugated diene. Woodward-Fieser rules assign this chromophore a fixed base value of \[ 253 \text{ nm} \]

Step 2: Count the substituent contributions.
Each alkyl group or ring-residue attached directly to the diene carbons adds \( 5 \) nm to the base value. This molecule carries four such ring-residue substituents on the diene system, so the total increment is
\[ 4 \times 5 = 20 \text{ nm} \]

Step 3: Add the increments to the base value.
\[ \lambda_{\text{max}} = 253 + 20 \]
\[ \lambda_{\text{max}} = 273 \text{ nm} \]

Step 4: Final Answer.
Adding the four ring-residue increments to the homoannular diene base gives the predicted absorption maximum.
\[ \boxed{\lambda_{\text{max}} = 273 \text{ nm}} \]