SME
Organic Chemistry II
Conjugated dienes, Diels–Alder and UV spectroscopy
Use this for “A heteroannular diene (base 214 nm) carries 4 ring residues and 1 exocyclic C=C. Predict its λmax.”
Chart
A(234) = 1
This is an example. Type your own values to start a new problem.
λmax (nm)
=
λ₀ (nm)
+
5
×
nR
+
5
×
nₑₓₒ
+
30
×
nₑₓₜ
Formulas
λmax
=
λ₀
+
5
×
nR
+
5
×
nₑₓₒ
+
30
×
nₑₓₜ
234 = 214 + 5 × 3 + 5 × 1 + 30 × 0
Assumptions
•
Woodward–Fieser rules for dienes in ethanol: empirical, good to about 5 nm.
•
The base value is 214 nm for an open-chain or heteroannular diene and 253 nm for a homoannular one (both C=C in one ring).
•
Count each alkyl group or ring residue on the diene’s carbons once; a C=C exocyclic to a ring adds 5 nm, each extra conjugated C=C 30 nm.
Step-by-step
WE KNOW
λ₀ = 214 nm, nR = 3, nₑₓₒ = 1, nₑₓₜ = 0
FIND
predicted λmax (λmax)
Step 1 · Find predicted λmax (λmax)
Use λmax = λ₀ + 5 × nR + 5 × nₑₓₒ + 30 × nₑₓₜ
Start from the base value and add each group’s increment.
λmax = 214 + 5 × 3 + 5 × 1 + 30 × 0
λmax = 214 + 15 + 5
λmax = 234 nm → The rules are good to about 5 nm; each group on the diene nudges λmax a little longer.
Check
Put the numbers back in. Both sides should match.
234 = 214 + 5 × 3 + 5 × 1 + 30 × 0 ✓
Topic check
See what you know: 5 questions on this topic
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