Why divinyl ketone derivatives absorb UV radiation as if their enone π-systems were separate?

Question

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1. Restating the question in plain language

A student has learned the Wood‑Fieser (WF) rules for estimating the longest‑wavelength UV‑vis absorption (λ_max) of conjugated carbonyl compounds (enones).
When a divinyl‑ketone (a carbonyl flanked by two C=C double bonds) is encountered, the WF tables tell us to ignore one of the two vinyl groups and to calculate λ_max only from the conjugated diene that is directly attached to the carbonyl.

The student therefore wonders:

Why does the “far” vinyl group not contribute to the UV‑vis absorption?
Why do two different molecules—one having the extra vinyl group (compound A) and one lacking it (compound B)—show essentially the same λ_max?

In other words: What is the electronic‑structure reason that the π‑system of the carbonyl plus the adjacent diene behaves as if the second vinyl group were absent?

2. Step‑by‑step explanation

2.1 Review of the relevant electronic transitions

Transition	Type of excitation	Typical wavelength range (nm)
π → π*	electron promoted from a bonding π orbital to an antibonding π* orbital	200–300 (isolated C=C), 300–350 (conjugated C=C)
n → π*	non‑bonding lone‑pair (n) on O → antibonding π* of C=O	300–350 (isolated carbonyl), red‑shifted when conjugated

For an enone (C=O conjugated to a C=C), the most intense band is the π → π* transition of the combined conjugated system (C=C‑C=O). Conjugation lowers the energy (red‑shifts) because the π‑system is extended: the HOMO becomes more delocalised (higher energy) and the LUMO more stabilised (lower energy).

2.2 What the Wood‑Fieser rules actually count

The WF rules are empirical: they start from a base value for a simple chromophore (e.g., an isolated enone) and then add increments for each of the following:

additional double bonds conjugated to the chromophore (≈+30 nm each)
alkyl substituents attached to the conjugated chain (≈+5 nm each)
exocyclic double bonds (≈+5 nm each)

The crucial word is “conjugated to the chromophore.” Only those π‑bonds that can delocalise electron density through overlapping p‑orbitals with the carbonyl contribute an increment.

2.3 Geometry of a divinyl‑ketone

Consider the generic skeleton:

   C=C   C=C
    \   /
     C=O

The carbonyl carbon is sp²‑hybridised; its p‑orbital (π) is perpendicular to the plane of the carbonyl C=O bond. The two adjacent C=C bonds each lie in a plane that can be coplanar with the carbonyl π‑system, or they can be twisted.

In practice, the most stable conformation of a divinyl‑ketone places only one of the two C=C bonds in the same plane as the carbonyl π‑orbital (the conjugated side). The other C=C is forced, by steric and orbital‑symmetry reasons, to adopt a torsion angle of ~90° relative to the carbonyl π system.

Why does this happen?

Orbital symmetry – The carbonyl π‑orbital can overlap with only one adjacent p‑orbital at a time. If both C=C bonds tried to align, the central carbon would have to be sp³‑like (four p‑orbitals), which is impossible.
Steric strain – Simultaneous coplanarity would bring the two alkenyl substituents too close, raising the energy.
Hybridisation of the carbonyl carbon – It is already sp²; adding a second p‑orbital to the same carbon would break the trigonal planar geometry.

Consequently, only one vinyl group is truly conjugated to the carbonyl. The other vinyl group is essentially an isolated alkene that is orthogonal to the carbonyl π system, so its p‑orbitals do not overlap with the carbonyl π‑orbitals.

2.4 Molecular orbital picture

2.4.1 The conjugated side (enone + diene)

The relevant π‑system:

C=C–C=O–C=C   (4 conjugated double bonds)

The linear combination of p‑orbitals creates four π molecular orbitals (two bonding, two antibonding). The HOMO is a delocalised π orbital spread over the whole chain; the LUMO is the corresponding π* orbital.

The π → π* excitation involves moving an electron from this delocalised HOMO to the delocalised LUMO, giving a single, strong absorption whose wavelength is predicted by the WF increments for a conjugated diene attached to an enone.

2.4.2 The “non‑conjugated” vinyl group

Because its p‑orbital is orthogonal, its π and π* orbitals remain localized on that alkene alone. The lowest excitation of that isolated vinyl group lies near ~200 nm (π → π* of a simple alkene). That band is:

Much higher in energy (shorter λ) than the enone‑diene band, and
Usually weak (low molar absorptivity) compared with the intense π → π* of the extended conjugated system.

Thus, in a normal UV‑vis scan (200–400 nm), the dominant band is the one from the conjugated enone‑diene; the isolated vinyl’s band is either hidden in the far‑UV or appears as a small shoulder that is ignored in the WF bookkeeping.

2.5 Experimental evidence

Spectra of simple divinyl‑ketones (e.g., 2,5‑hexadien-1‑one) show a single strong band at ≈ 300 nm that matches the prediction for a conjugated triene (C=C‑C=O‑C=C).
A comparable spectrum of a molecule lacking the remote vinyl group (e.g., 3‑methyl‑2‑buten‑1‑one) shows essentially the same λ_max (within a few nm).

The additional vinyl contributes only a weak, high‑energy band that does not affect the measured λ_max in the region used for WF calculations.

2.6 Putting it together – why the WF rule “ignores” one vinyl

Only one vinyl can be coplanar with the carbonyl π‑system → only that side is truly conjugated.
The other vinyl is orthogonal → its π‑orbitals do not mix with the carbonyl π‑orbitals, so it does not lower the HOMO–LUMO gap of the chromophore.
Consequently, the absorption maximum is governed solely by the conjugated portion (the enone plus the adjacent diene).
The WF tables are built from empirical data that already reflect this behaviour; they therefore add increments only for the conjugated diene and ignore the remote vinyl.

Hence compounds A (with the extra vinyl) and B (without it) display essentially the same λ_max, because the extra vinyl does not participate in the chromophore responsible for the observed transition.

3. Final answer

The UV‑vis λ_max of a divinyl‑ketone is determined only by the conjugated enone–diene fragment. The second vinyl group is forced out of conjugation (its p‑orbitals are orthogonal to the carbonyl π‑system) and therefore does not lower the HOMO–LUMO energy gap. As a result, its presence does not shift the λ_max, and Woodward‑Fieser calculations correctly ignore it. Compounds A and B therefore have essentially identical λ_max values.

4. Common mistakes

Mistake	Why it’s wrong	How to avoid it
Assuming every double bond adds a WF increment	The rule adds increments only for bonds that are conjugated (i.e., coplanar and able to delocalise). The remote vinyl in a divinyl‑ketone is orthogonal, so it does not count.	Draw the molecule, indicate the plane of the carbonyl π‑orbital, and check which C=C bonds can be coplanar with it.
Counting the extra vinyl as an “exocyclic” double bond	“Exocyclic” increments apply when the double bond is directly attached to the conjugated ring system. The remote vinyl is not exocyclic to the conjugated chain; it is isolated.	Verify the connectivity: the extra vinyl is attached to the carbonyl carbon, not to the conjugated π‑system.
Attributing a high‑energy UV band to the observed λ_max	The isolated vinyl absorbs in the far‑UV (< 250 nm). The measured λ_max for enones is usually in the 300–350 nm region, dominated by the conjugated system.	Compare the spectrum: the strong band near 300 nm is the one to use for WF calculations; ignore weak far‑UV shoulders.
Neglecting conformational effects	Assuming the molecule can adopt a fully planar conformation leads to the idea that both vinyl groups are conjugated. In reality, steric and orbital constraints prevent this.	Consider the preferred conformation (torsion ~90° for one side) when deciding which double bonds are conjugated.
Using the “sum of all double bonds” rule for λ_max	This rule applies only to simple polyenes, not to systems containing a carbonyl where conjugation is limited by the carbonyl geometry.	Apply the specific WF tables for enones (or α,β‑unsaturated carbonyls), not the polyene tables.

By keeping these points in mind, you can correctly decide which parts of a molecule contribute to its UV‑vis absorption and apply the Woodward‑Fieser rules without error.

Original question: Why divinyl ketone derivatives absorb UV radiation as if their enone π-systems were separate? on Chemistry Stack Exchange, licensed CC BY-SA.