In other words, the (n→π*) transition at 275 nm that we’ve spent so much time talking about is very weak relative to the (π→π*) transition. So as it turns out, that “peak” at 275 nm (n→π*) we were looking at turns out to be a molehill, next to the (π→π*) mountain at about 195 nm in the deeper UV. If you zoom out, you’ll see that there’s a much stronger transition around 190 nm. Glad you asked. If you take a quick look back at the UV-Vis absorption spectrum of acetone, above, you’ll note that the X-axis gets cut off around 240 nm or so. So what about the pi to pi* transition? Doesn’t that happen too? What About Pi to Pi* Transitions for C=O? It is this (n→π*) transition which is responsible for the peak at around 275 nm.Ĥ.Being higher in energy, transitions between electrons in the non-bonding orbital to the pi* orbital have a smaller ΔE and therefore absorb at longer wavelength.These orbitals are absent in typical alkenes such as ethylene Carbonyl groups contain non-bonding electrons that are in an orbital intermediate in energy between the bonding pi orbital and the anti bonding pi* orbital.Huh? Let’s look at a simple molecular orbital drawing of acetone. Carbonyl (C=O) Groups Tend To Show Weak Absorbances At (Roughly) 300 nm That Correspond To Transitions Between Non-Bonding Orbitals and Pi* Orbitals It’s a transition from a non-bonding orbital (n) to the pi* orbital (n→π*). Now: as we’ll see in a minute, there is a pi to pi* ( π→π*) transition for acetone in the UV, but that peak at 275 nm is NOT a pi to pi* transition. Wouldn’t you reasonably expect *more* energy to be required to promote an electron from pi(π) to pi* (π*)? If anything, C=O π bonds are stronger than C=C π bonds. If you have an astonishingly good memory you may recall from the last post (or from my introduction above) that the absorption max for ethene (CH 2=CH 2) is about 170 nm.Īn absorption around 275 nm means that longer wavelength and therefore less energetic photons are required for this transition. Here’s the UV-Vis absorption spectrum for 2-propanone (acetone). Question: Does acetone absorb UV or visible light?Īnswer: You betcha. Let’s start with one of the simplest compounds with a C=O bond: 2-propanone, otherwise known as acetone. Absorbance of C=O bonds Show A Maximum Around 300 nm The medium sized answer is: yes, but the main transition of interest is not a pi-pi* transition – it’s slightly different.For example, β-carotene (the orange pigment in carrots) with 11 conjugated pi bonds, absorbs in the visible (λ max = 470 nm).īecause the post was so damn long, we never got around to addressing a key question: does this apply to other types of pi bonds as well?įor example, do C=O pi bonds also absorb light in the UV/visible region? as conjugation increases, the energy gap ΔE decreases, pushing the wavelength of maximum absorbance (λ max) toward the visible (less energetic photons, longer wavelength).For ethene, maximum absorbance occurs at about 170 nm, in the UV region. ethene, CH 2=CH 2) possesses a large energy gap (ΔE) between the bonding and anti bonding orbitals, which requires more energetic (shorter wavelength) photons for excitation. an alkene with little or no conjugation (e.g.the number of consecutive pi bonds, roughly speaking). the energy required for the transition depends mostly on the extent of conjugation (i.e.In our last postwe showed that molecules with C-C pi (π) bonds absorb light in the UV-visible region, which promotes electrons from (bonding) π orbitals to (anti bonding) π* orbitals. A Quick Review Of What We’ve Learned So Far About UV-Vis Summary: UV-Vis Spectroscopy Of Carbonylsġ.Carbonyls also participate in conjugation.This is actually a n-> pi* transition, not pi to pi* (!).C=O Bonds Show An Absorbance Maximum Around 300 nm.UV-Vis Spectroscopy Of Carbonyls (C=O Bonds)