Predict UV/visible absorption from a chromophore

A useful UV/visible prediction begins with molecular structure. Look for π systems, conjugation, aromatic rings, carbonyl groups and heteroatoms with nonbonding electrons.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Predict UV/visible absorption from a chromophore

Find the chromophore before predicting a band

A useful UV/visible prediction begins with molecular structure. Look for π systems, conjugation, aromatic rings, carbonyl groups and heteroatoms with nonbonding electrons.

Then identify a plausible occupied-to-unoccupied transition and compare its energy qualitatively. Structure alone rarely justifies one exact wavelength or colour.

What a defensible prediction contains

A chromophore is the part of a molecule whose electronic structure gives rise to an absorption. An auxochrome is a substituent, often with a lone pair, that can modify a chromophore's wavelength or intensity through interaction with it.

A prediction should name the structural feature, name the likely transition, and state a wavelength trend or spectrum region only as precisely as the evidence permits.

Absorption in the ultraviolet does not automatically make a substance coloured. Visible colour requires significant absorption within roughly 400–700 nm, leaving the complementary light to be seen.

A structure-to-spectrum reasoning chain

An alkane mainly offers σ→σ* transitions, so its strongest simple absorption is at short wavelength. An alkene adds a π→π* transition with a smaller gap and therefore longer wavelength.

A carbonyl compound offers both π→π* and usually weaker n→π* absorption. Aromatic and conjugated systems have several delocalised π levels, often reducing the lowest electronic gap.

Solvent, substituents and molecular geometry can shift λmax and change intensity. Without data or a stated empirical rule, predict relative behaviour rather than a precise number.

Worked example: ethane, ethene and ethanal

Ethane has only σ-bond transitions in the simple model. Ethene contains C=C and therefore a π→π* chromophore at longer wavelength than ethane's σ→σ* absorption.

Ethanal contains a C=O group, so both π→π* and weak n→π* absorption are plausible. The lone-pair-to-π* transition is typically the lowest-energy of these carbonyl transitions.

Try this

Rank ethane, ethene and ethanal by the types of lowest-energy transition available, without assigning exact λmax values.

Check your answer

Ethane: σ→σ*; ethene: π→π*; ethanal: often n→π* as the lowest-energy transition, with π→π* also present.

Practise a cautious prediction

For an unfamiliar molecule, circle the chromophore and write its likely occupied and vacant orbital types. Then use the relative gap to predict shorter or longer wavelength.

If the predicted band remains in the ultraviolet, say that visible colour is not established.

Try this

Cyclohexene and cyclohexane are compared. Which is expected to absorb at longer wavelength, and why?

Check your answer

Cyclohexene, because its C=C group permits a lower-energy π→π* transition than cyclohexane's σ→σ* transition.

Apply the method to a carbonyl

Propanone is CH₃COCH₃.

Use the same method: circle the chromophore, list its occupied and vacant orbital types, then say what the structure alone cannot tell you.

Try this

Predict the important UV/visible transitions of propanone and state one limit of your prediction.

Check your answer

The C=O chromophore has oxygen lone pairs (n), a π bonding orbital and a vacant π* orbital, so expect n→π* and π→π* transitions. The structure does not fix an exact λmax or show that propanone is coloured; that needs spectral or solvent information.

Common mistake: chromophore means coloured

Many chromophores absorb only ultraviolet radiation. A compound appears coloured only if its absorption reaches the visible region with sufficient intensity.

Another mistake is to name a functional group without explaining the electron transition. Link every prediction to occupied and unoccupied orbitals.

Try this

Correct: ‘Ethene is visibly coloured because C=C is a chromophore.’

Check your answer

The C=C group gives UV π→π* absorption, but ordinary ethene does not absorb sufficiently in the visible region to appear coloured.

Check your understanding

Use the chain structure → chromophore → occupied and vacant orbitals → transition → relative energy → wavelength region.

Next, explain in molecular-orbital terms why extending conjugation usually shifts absorption to longer wavelength.

Try this

What makes a UV/visible prediction chemically justified rather than a functional-group guess?

Check your answer

It identifies the chromophore and an allowed or plausible transition, then links the energy gap to a cautious wavelength prediction.