Organic Chemistry · Conjugated Compounds and Ultraviolet Spectroscopy
Interpreting Ultraviolet Spectra: The Effect of Conjugation
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conjugation Alternating single and double bonds sharing a delocalized π system Full entry → changes UV spectra in a systematic, predictable way. When double bonds alternate — a conjugated π system — the molecular orbitals spread over more atoms and the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) shrinks. Since a photon's energy is E = hc/λ, a smaller gap means absorption at longer wavelength: the maximum shifts toward the red (a bathochromic shift Shift of absorption to longer wavelength (red shift) Full entry →). Conjugation also makes absorption more intense (higher ε), because the transitions become more allowed.
The polyene series makes the trend unforgettable: ethene ~175 nm, 1,3-butadiene ~217 nm, 1,3,5-hexatriene ~258 nm, 1,3,5,7-octatetraene ~304 nm — roughly 30–45 nm of red shift per added double bond. Chemists codify the pattern in the empirical Woodward–Fieser rules Empirical rules predicting λ_max of dienes and enones Full entry →, which predict λ_max for dienes and enones within about ±5 nm. When conjugation grows long enough to reach the visible region (400–700 nm), molecules become colored — the chemistry behind β-carotene's orange and the photochemistry of vision.
Why this matters
- Structure elucidation. A single λ_max reading answers a structural question: "how many double bonds are conjugated?" An alkene absorbs near 175 nm, a diene near 220 nm, a triene near 260 nm — the wavelength climbs with every added double bond.
- Color and materials. Dyes, inks, food colorants, and sunscreens all work by tuning conjugation. Sunscreen molecules absorb UV-A/UV-B through extended conjugated systems, converting harmful photons into heat before they reach skin.
- Biology. The pigment that lets you read this page began as a conjugated polyene: 11-cis-retinal absorbs a photon and isomerizes to all-trans-retinal, triggering the signal that becomes vision. β-carotene, the orange pigment of carrots, is the same family of molecules.
- Analytical monitoring. Polymer degradation, fat oxidation, and photobleaching change conjugation — and all show up immediately in the UV spectrum.
The college version
Core Concepts
Why conjugation shifts absorption to longer wavelength
Two adjacent, alternating π bonds share electrons, so their π orbitals combine into delocalized molecular orbitals spanning all the atoms. As the number of interacting orbitals grows, the energy levels pack closer together: the highest occupied level rises and the lowest unoccupied level falls. The HOMO–LUMO gap Energy difference between the highest occupied and lowest unoccupied molecular orbitals Full entry → therefore decreases with conjugation length. The photon energy needed to excite an electron is exactly that gap:
ΔE = hcλ
A smaller gap means a smaller photon energy — and since energy is inversely proportional to wavelength, absorption moves to longer wavelength. The same delocalization intensifies the transition: more participating atoms grow the transition dipole, so ε rises from ~104 for a single alkene toward 105 for long polyenes.
The polyene series: a built-in ruler
Measure λ_max as the conjugated chain grows:
| Compound | Conjugated double bonds | λ_max (nm) | ε (L mol⁻¹ cm⁻¹) |
|---|---|---|---|
| ethene | 1 | ~175 | ~1 × 10⁴ |
| 1,3-butadiene | 2 | 217 | 2.1 × 10⁴ |
| 1,3,5-hexatriene | 3 | 258 | 3.5 × 10⁴ |
| 1,3,5,7-octatetraene | 4 | 304 | 5.4 × 10⁴ |
| β-carotene | 11 | ~450 | ~1.4 × 10⁵ |
Each additional double bond adds roughly 30–45 nm and roughly doubles ε. Extrapolate and you see why β-carotene — an 11-double-bond polyene — finally reaches the visible: 450 nm is blue-violet light, so it absorbs blue and reflects the rest, appearing orange.
The Woodward–Fieser rules
These empirical rules predict λ_max for conjugated dienes (and α,β-unsaturated carbonyls, "enones") from the parent structure plus substituent increments:
- Base values: 214 nm for an acyclic or heteroannular (double bonds in different rings) diene; 253 nm for a homoannular diene Diene with both double bonds in the same ring Full entry → (double bonds in the same ring, e.g., 1,3-cyclohexadiene).
- Increments: +30 nm for each additional double bond extending the conjugation; +5 nm for each alkyl substituent or ring residue; +5 nm for each exocyclic double bond (a double bond attached to the diene system from outside).
Enones have separate bases (acyclic or six-membered-ring, 215 nm; five-membered ring, 202 nm; α,β-unsaturated aldehyde, 210 nm) with their own increments (α-alkyl +10, β-alkyl +12, γ-and-beyond +18, exocyclic +5, extended conjugation +30). The rules are empirical, accurate to about ±5 nm, and work best for substituted dienes — bare polyenes deviate more.
From spectra to color: complementary colors
A colored compound absorbs in the visible range (400–700 nm); the eye sees the light that is not absorbed. Absorbed and perceived colors are therefore complementary: absorbing blue-violet (~450 nm) looks orange; absorbing green (~520 nm) looks red; absorbing red (~650 nm) looks green. Two consequences: a compound's color reports its longest conjugated segment, and "white" objects absorb nothing visible while "black" ones absorb everything.
Conjugation in vision
The light-sensitive molecule in the retina, rhodopsin, contains 11-cis-retinal, a conjugated polyenal. When 11-cis-retinal absorbs a visible photon, it isomerizes to the all-trans geometry; that shape change triggers a protein cascade that sends a nerve signal to the brain. Vision is, at root, a photoisomerization of a conjugated polyene — detailed in the next topic.
How It Works / Step-by-Step Process
To predict and interpret λ_max:
- Count the conjugated double bonds and note whether they share a ring (homoannular) or not (heteroannular/acyclic).
- Select the base value — 214 nm (acyclic/heteroannular diene Diene with double bonds in different rings Full entry →) or 253 nm (homoannular diene); use the enone bases for α,β-unsaturated carbonyls.
- Add increments: +30 nm per extended double bond, +5 nm per alkyl substituent or ring residue, +5 nm per exocyclic double bond.
- Compare with the measured λ_max (agreement within ±5 nm supports the structure; a large discrepancy means the chromophore was misidentified).
- Relate to energy with ΔE = hc/λ (per photon) or NA hc/λ (per mole) when you need the gap in kJ/mol.
- If λ_max ≥ 400 nm, expect visible color — and predict it with the complementary-color logic.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Bathochromic shift | Hypsochromic shift | Bathochromic = longer wavelength (red); hypsochromic = shorter wavelength (blue) |
| Isolated double bonds | Conjugated double bonds | Isolated C=C groups absorb near 175 nm regardless of count; conjugation red-shifts the band |
| Absorbed color | Perceived color | A molecule looks like the complement of what it absorbs (450 nm absorption → orange appearance) |
| Woodward–Fieser rules for all chromophores | Valid for dienes and enones | The rules are empirical and specific to these systems; other chromophores need other correlations |
| One diene base value | Two bases | Acyclic/heteroannular: 214 nm; homoannular: 253 nm — mixing them up ruins the prediction |
| Exact rule predictions | Approximate predictions (±5 nm) | The rules are empirical; use them for trends and structure checks, not exact numbers |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of electrons as kids on a playground slide: the taller the slide, the more energy they need to climb to the top. Conjugation builds a longer, flatter slide — the more alternating double bonds, the smaller the climb, so gentler, longer-wavelength light can make the jump. A short slide needs harsh UV light; a very long one can be climbed by ordinary sunlight, which is why long polyenes look colorful — carrots are orange because β-carotene's eleven double bonds make a slide exactly the right height to swallow blue light.
Worked example
Example 1: Predicting λ_max for 2,3-dimethyl-1,3-butadiene
Apply the Woodward–Fieser rules to 2,3-dimethyl-1,3-butadiene, a diene with two methyl substituents on the inner carbons of the conjugated system.
- Base value (acyclic diene): 214 nm
- Two alkyl substituents: 2 × 5 = 10 nm
Summing the increments:
λmax(predicted) = 214 nm + 10 nm = 224 nm
The measured value is about 226 nm — within the ±5 nm accuracy of the rules. Note the reasoning chain: 1,3-butadiene itself is predicted at 214 nm (measured 217 nm); the two methyl groups, which donate electron density into the π system, account for the additional red shift of roughly 10 nm.
Example 2: How much energy does conjugation save?
Compare the HOMO–LUMO gaps of 1,3-butadiene (λ_max = 217 nm) and β-carotene (λ_max ≈ 450 nm), using the energy–wavelength relation per mole of photons:
ΔE = NA hcλ
First compute the combined constant, NA hc = (6.022 × 1023 mol-1)(6.626 × 10-34 J · s)(2.998 × 108 m/s) = 0.1196 J · m/mol.
For butadiene, λ= 217 × 10-9 m:
ΔE = 0.1196 J · m/mol217 × 10-9 m = 5.51 × 105 J/mol ≈ 551 kJ/mol
For β-carotene, λ= 450 × 10-9 m:
ΔE = 0.1196 J · m/mol450 × 10-9 m = 2.66 × 105 J/mol ≈ 266 kJ/mol
Going from 2 to 11 conjugated double bonds cuts the electronic transition energy roughly in half (551 → 266 kJ/mol). The unit check: J·m/mol divided by m leaves J/mol, converted to kJ/mol by dividing by 1000. This halving of the energy gap is precisely why the longest polyenes absorb visible light: their transition energies (below ~300 kJ/mol) fall inside the energy range of visible photons, while short alkenes need energetic UV photons of ~550 kJ/mol.
Key takeaways
- Conjugation lowers the HOMO–LUMO gap, shifting λ_max to longer wavelength (bathochromic/red shift) and raising ε.
- Polyene ruler: ethene 175 → butadiene 217 → hexatriene 258 → octatetraene 304 nm; roughly +30–45 nm per double bond.
- Woodward–Fieser rules (dienes): base 214 nm (acyclic/heteroannular) or 253 nm (homoannular); +30 nm per extended double bond, +5 nm per alkyl substituent/ring residue, +5 nm per exocyclic double bond. Accuracy ~±5 nm.
- Enones have separate bases (acyclic/6-ring 215 nm; 5-ring 202 nm; aldehyde 210 nm) and increments.
- Absorbed and perceived colors are complementary: β-carotene absorbs ~450 nm (blue) and looks orange.
- 11-cis-Retinal photoisomerizes to all-trans in vision — a conjugated polyene at the heart of sight.
- ΔE = hc/λ connects every spectral shift to an energy gap in kJ/mol.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why does extending conjugation shift λ_max to longer wavelength?
Show answer
Conjugation delocalizes the π electrons over more atoms, which raises the HOMO and lowers the LUMO, shrinking the HOMO–LUMO gap; since ΔE = hc/λ, a smaller gap means longer-wavelength absorption.
Arrange in order of increasing λ_max: ethene, 1,3,5,7-octatetraene, 1,3-butadiene, 1,3,5-hexatriene.
Show answer
Ethene (~175 nm) < 1,3-butadiene (217 nm) < 1,3,5-hexatriene (258 nm) < 1,3,5,7-octatetraene (304 nm).
Using the Woodward–Fieser rules, predict λ_max for 2,3-dimethyl-1,3-butadiene (base 214 nm, +5 nm per alkyl substituent).
Show answer
Base 214 nm + 2 × 5 nm = 224 nm predicted (measured ~226 nm, within ±5 nm).
Why does β-carotene appear orange?
Show answer
It absorbs blue-violet light near 450 nm; the eye sees the complementary (unabsorbed) light, which is orange.
A compound absorbs at 300 nm. What is its HOMO–LUMO gap in kJ/mol? (Use NA hc = 0.1196 J · m/mol.)
Show answer
ΔE = 0.1196/(300 × 10-9) = 3.99 × 105 J/mol ≈ 399 kJ/mol.
A homoannular diene absorbs at 258 nm with no substituent increments applied. What does this tell you about the base value and the structure?
Show answer
A homoannular diene's base value is 253 nm; absorption near 258 nm (253 + small increments) is consistent with a homoannular diene such as 1,3-cyclohexadiene — the higher base already accounts for the ring fusion.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- conjugation
- Alternating single and double bonds sharing a delocalized π system
- HOMO–LUMO gap
- Energy difference between the highest occupied and lowest unoccupied molecular orbitals
- bathochromic shift
- Shift of absorption to longer wavelength (red shift)
- hypsochromic shift
- Shift of absorption to shorter wavelength (blue shift)
- homoannular diene
- Diene with both double bonds in the same ring
- heteroannular diene
- Diene with double bonds in different rings
- Woodward–Fieser rules
- Empirical rules predicting λ_max of dienes and enones
- complementary color
- The color perceived when its complement is absorbed
Sources & references
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