{"id":10170,"date":"2016-09-26T14:17:54","date_gmt":"2016-09-26T18:17:54","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10170"},"modified":"2022-10-31T05:23:45","modified_gmt":"2022-10-31T10:23:45","slug":"uv-vis-spectroscopy-absorbance-of-carbonyls","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2016\/09\/26\/uv-vis-spectroscopy-absorbance-of-carbonyls\/","title":{"rendered":"UV-Vis Spectroscopy: Absorbance of Carbonyls"},"content":{"rendered":"<p><strong>UV-Vis Spectroscopy Of Carbonyls (C=O Bonds)<\/strong><\/p>\n<p>UV-Visible spectroscopy is not just about C-C pi bonds. C-O pi bonds can absorb UV light as well!<\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">A Quick Review on UV-Vis<\/a><\/li>\n<li><a href=\"#two\">C=O Bonds Show An Absorbance Maximum Around 300 nm<\/a><\/li>\n<li><a href=\"#three\">This is actually\u00a0 a n-&gt; pi* transition, not pi to pi*\u00a0 (!)<\/a><\/li>\n<li><a href=\"#four\">Wait. Why not pi to pi* ?\u00a0<\/a><\/li>\n<li><a href=\"#five\">Carbonyls also participate in conjugation<\/a><\/li>\n<li><a href=\"#six\">Summary: UV-Vis Spectroscopy Of Carbonyls<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. A Quick Review Of What We&#8217;ve Learned So Far About UV-Vis<\/h2>\n<p>In our <a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/16\/introduction-to-uv-vis-spectroscopy\/\"><strong>last post<\/strong> <\/a>we showed that molecules with C-C pi (\u03c0) bonds absorb light in the UV-visible region, which promotes electrons from (bonding)\u00a0\u03c0\u00a0orbitals to (anti bonding) \u03c0* orbitals.<\/p>\n<p>We saw that<\/p>\n<ul>\n<li>the energy required for the transition depends mostly on the extent of <strong>conjugation<\/strong> (i.e. the number of consecutive pi bonds, roughly speaking).<\/li>\n<li>an alkene with\u00a0little or no<strong>\u00a0conjugation<\/strong> (e.g. ethene, CH<sub>2<\/sub>=CH<sub>2<\/sub>) possesses\u00a0a large\u00a0<strong>energy gap<\/strong> (\u0394E) between the bonding and anti bonding orbitals, which requires <strong>more energetic<\/strong>\u00a0(shorter wavelength) photons for excitation. For ethene, maximum absorbance occurs at about 170 nm, in the UV region.<\/li>\n<li>as conjugation increases, <strong>the energy gap\u00a0\u0394E decreases<\/strong>, pushing the wavelength of maximum absorbance (\u03bb<sub>max<\/sub>) toward the <strong>visible<\/strong> (less energetic photons, longer wavelength) . For example, \u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Beta-Carotene\" target=\"_blank\" rel=\"noopener noreferrer\">\u03b2-carotene<\/a> (the orange pigment in carrots) with 11 conjugated pi bonds, absorbs in the visible (\u03bb<sub>max\u00a0<\/sub>=\u00a0470 nm).<\/li>\n<\/ul>\n<p>Because 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?<\/p>\n<p>For example, do C=O pi bonds also absorb light in the UV\/visible region?<\/p>\n<ul>\n<li><span style=\"line-height: 1.5;\">The short answer is: \u00a0<strong>yes<\/strong>.<\/span><\/li>\n<\/ul>\n<ul>\n<li><span style=\"line-height: 1.5;\">The medium sized answer is: yes, but the main transition of interest is not a\u00a0pi-pi* transition &#8211; it&#8217;s slightly different. \u00a0<\/span><\/li>\n<\/ul>\n<p>The long answer is.. well, here\u2019s the long answer.<\/p>\n<h2><strong><a id=\"two\"><\/a>2. Absorbance of C=O bonds Show A Maximum Around 300 nm<\/strong><\/h2>\n<p>Let\u2019s start with one of the simplest compounds with a C=O bond: 2-propanone, otherwise known as acetone.<\/p>\n<p>Question: Does acetone absorb UV or visible light?<\/p>\n<p>Answer: You betcha. Here\u2019s the UV-Vis absorption spectrum for 2-propanone (acetone).<\/p>\n<p>[The key piece of information to glean from that spectrum is that\u00a0<strong>there is an absorbance\u00a0<\/strong><b>maximum at about 275 nm<\/b>, in the ultraviolet.]<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-15444\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet.png\" alt=\"uv absorption of acetone shows maximum at about 275 in ultraviolet\" width=\"600\" height=\"450\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet.png 800w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-300x225.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-768x576.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-320x240.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-640x480.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-360x270.png 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-720x540.png 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-uv-absorption-of-acetone-shows-maximum-at-about-275-in-ultraviolet-760x570.png 760w\" sizes=\"(max-width: 600px) 100vw, 600px\" \/><\/p>\n<p>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<sub>2<\/sub>=CH<sub>2<\/sub>) is about 170 nm.<\/p>\n<p>An absorption around 275 nm means that <strong>longer wavelength<\/strong> and therefore <strong>less energetic\u00a0<\/strong>photons are\u00a0required for this transition.<\/p>\n<p>Doesn&#8217;t that seem weird?<\/p>\n<p>If anything, C=O \u03c0 bonds are <em>stronger<\/em> than C=C \u03c0 bonds. [You can look it up]. Wouldn\u2019t you reasonably expect *more* energy to be required to promote an electron from pi(\u03c0) \u00a0to pi* (\u03c0*)?<\/p>\n<p>What gives?<\/p>\n<p>Now: as we&#8217;ll see in a minute, there <em><strong>is<\/strong><\/em> a pi to pi* ( \u03c0\u2192\u03c0*) transition for acetone in the UV, <strong>but that peak\u00a0at 275 nm is NOT a pi to pi* transition<\/strong>. It&#8217;s a transition from a non-bonding orbital (n) to the pi* orbital (n\u2192\u03c0*).<\/p>\n<h2><a id=\"three\"><\/a>3. Carbonyl (C=O) Groups Tend To Show Weak Absorbances At (Roughly) 300 nm That Correspond To Transitions Between Non-Bonding Orbitals and Pi* Orbitals<\/h2>\n<p><em>Huh<\/em>? Let\u2019s\u00a0look at a simple molecular orbital drawing of\u00a0acetone.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15445\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-simple-molecular-orbital-diagram-acetone-showing-pi-and-pi-star-and-non-bonding-orbital-nonbonding-to-pi-star-is-most-important-for-uv-vis-pi-to-pi-star-is-about-190.gif\" alt=\"simple molecular orbital diagram acetone showing pi and pi star and non bonding orbital nonbonding to pi star is most important for uv vis pi to pi star is about 190\" width=\"630\" height=\"606\" \/><\/p>\n<p><span style=\"color: #993366;\"><em>[This is a somewhat\u00a0simplified picture. For a more detailed MO diagram for that also includes a more thorough discussion about the nature of the non-bonding orbitals,\u00a0I highly recommend <a style=\"color: #993366;\" href=\"https:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/virttxtjml\/photchem.htm#phot5b\">Reusch&#8217;s online textbook entry here<\/a>. ]<\/em><\/span><\/p>\n<p>A few important things to note:<\/p>\n<ul>\n<li>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.\u00a0<strong>These orbitals are <em>absent<\/em> in typical alkenes such as ethylene [CH<sub>2<\/sub>=CH<sub>2<\/sub>]\u00a0<\/strong>[note 1]<\/li>\n<li>Being higher in energy, transitions between electrons in the\u00a0non-bonding orbital to the pi* orbital have a smaller\u00a0\u0394E and therefore absorb at longer wavelength.<\/li>\n<li>\u00a0<strong>It is this (n\u2192\u03c0*) transition which is responsible for the peak at around 275 nm.\u00a0<\/strong><\/li>\n<\/ul>\n<hr \/>\n<h2><a id=\"four\"><\/a>4. What About Pi to Pi* Transitions for C=O?<\/h2>\n<p>So what about the pi to pi* transition? Doesn&#8217;t that happen too?<\/p>\n<p>Glad you asked.\u00a0If you take a quick look back at the UV-Vis absorption spectrum of acetone, above, you&#8217;ll note that the X-axis gets cut off around 240 nm or so. There&#8217;s a reason for that (mwah-ah-ah).<\/p>\n<p>If you zoom out, you&#8217;ll see that there&#8217;s a much stronger transition around 190 nm. <span style=\"color: #993366;\"><em>[I went looking for a decent full-size UV spectrum of acetone, and the diagram below is the best I could find. I didn&#8217;t make this image and it is not my intellectual property. I found it <a style=\"color: #993366;\" href=\"http:\/\/www.chemconnections.org\/organic\/chem226\/226topics-12.html\"><strong>here<\/strong><\/a>. ]<\/em><\/span><\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15446\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption.jpg\" alt=\"uv vis absorption of acetone showing lambdamax of 195 corresponding to pi to pi star absorption\" width=\"450\" height=\"226\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption.jpg 680w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption-300x151.jpg 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption-320x161.jpg 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption-640x322.jpg 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-uv-vis-absorption-of-acetone-showing-lambdamax-of-195-corresponding-to-pi-to-pi-star-absorption-360x181.jpg 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>So as it turns out, that &#8220;peak&#8221; at 275 nm\u00a0(n\u2192\u03c0*)\u00a0 we were looking at turns out to be a <strong>molehill<\/strong>, next to the (\u03c0\u2192\u03c0*) \u00a0<strong>mountain<\/strong> \u00a0at about 195 nm in the deeper UV.<\/p>\n<p>In other words, the\u00a0(n\u2192\u03c0*) transition at 275 nm that we&#8217;ve spent so much time talking about is <strong>very weak<\/strong>\u00a0relative to the (\u03c0\u2192\u03c0*) transition.<strong>\u00a0<\/strong><span style=\"color: #993366;\"><em>\u00a0[sometimes a term called \u201cepsilon, \u03b5\u201d is used to denote this difference in magnitude of absorption].\u00a0<\/em><\/span><\/p>\n<p>Why might that be? It has to do with differences in\u00a0<strong>orbital overlap<\/strong>. In order for an electron to transition\u00a0from one orbital to another, two conditions must be met.<\/p>\n<ul>\n<li>First, as previously discussed, the orbital has to interact with a photon of appropriate energy \u0394E.<\/li>\n<li>Second,\u00a0\u00a0there has to be significant <strong>overlap<\/strong> between the orbitals in space. We generally don&#8217;t discuss this for\u00a0(\u03c0\u2192\u03c0*) and\u00a0(\u03c3\u2192\u03c3*) transitions because each pair of bonding and anti bonding orbitals occupies the same region of space. If you look back to the diagram for the location of the n orbitals in acetone compared to the location of the pi* orbitals, you might notice that they are <strong>essentially at right angles to each other.<\/strong> Poor orbital overlap means that even if the electron has sufficient energy \u00a0\u0394E to make the transition, the transition is considerably less likely to\u00a0occur since the excited electron will be less likely to be occupying an area of space corresponding to the higher-energy orbital. [Extra detail: you might recall that orbitals are 3-dimensional volumes where the probability of encountering an electron is 95%. Therefore, there <em>is<\/em> some electron density outside of the volumes\u00a0we typically consider &#8220;orbitals&#8221;]<\/li>\n<\/ul>\n<h2><a id=\"five\"><\/a>5. Carbonyls Also Participate in Conjugation<\/h2>\n<p>Carbonyls can also participate in conjugation with C-C pi bonds. This leads to an increase in the overall\u00a0\u03bb<sub>max<\/sub> of the molecule.\u00a0<sub>\u00a0\u00a0<\/sub>For instance, the absorbance of the alkene 2-methyl pent-2-ene is below 200 nm, as is the\u00a0\u03c0\u2192\u03c0* absorbance of 4-methyl pentane-2-one (below).<\/p>\n<p>In mesityl oxide, where the alkene and C=O group are in conjugation with each other, the absorption maximum moves to longer wavelength at 228 nm. This is similar to the difference between the absorbance of ethene (174 nm) and butadiene (217 nm).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15447\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-carbonyls-pi-bonds-participate-in-conjugation-and-lead-to-absorbance-at-higher-wavelengths-such-as-mesityl-oxide-lambda-max-228-similar-to-butadiene-217.gif\" alt=\"carbonyls pi bonds participate in conjugation and lead to absorbance at higher wavelengths such as mesityl oxide lambda max 228 similar to butadiene 217\" width=\"600\" height=\"449\" \/><\/p>\n<p><strong>Note<\/strong>The absorbance maximum can be sensitive to the identity of the solvent as well as the identity of the substituents on the alkene. [<a href=\"#notetwo\">Note 2<\/a>].<\/p>\n<h2><a id=\"six\"><\/a>6. Summary: UV-Vis Spectroscopy Of Carbonyls<\/h2>\n<p>Absorbance\u00a0in the rough neighbourhood of 270-300 nm is common for molecules containing a C=O group (such as ketones and aldehydes) and this corresponds to a (n\u2192\u03c0*)<strong>\u00a0<\/strong>transition.<\/p>\n<p>These absorbances tend to be weak, relative to\u00a0(\u03c0\u2192\u03c0*) transitions. Still, observing this absorbance can be an important clue in the structure determination of unknown compounds.<\/p>\n<p><strong>In the next post<\/strong> we\u2019ll go into practical details of using UV-Vis in structure determination.<\/p>\n<p><span style=\"color: #993366;\"><em>[Again, for a more in depth look on the subject of C=O absorbance, go to <a style=\"color: #993366;\" href=\"https:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/virttxtjml\/photchem.htm\">Reusch<\/a>.\u00a0 We&#8217;re really skimming the surface here, but it is enough for our purposes.]<\/em><\/span><\/p>\n<hr \/>\n<h2><a id=\"notes\"><\/a>Notes<\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/27\/uv-vis-spectroscopy-some-practice-questions\/\" class=\"\"><span>UV-Vis Spectroscopy: Practice Questions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/08\/23\/structure-determination-case-study-deer-tarsal-gland-pheremone\/\" class=\"\"><span>Structure Determination Case Study: Deer Tarsal Gland Pheromone<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/16\/introduction-to-uv-vis-spectroscopy\/\" class=\"\"><span>Introduction To UV-Vis Spectroscopy<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/11\/23\/quick_analysis_of_ir_spectra\/\" class=\"\"><span>Infrared Spectroscopy: A Quick Primer On Interpreting Spectra<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/11\/29\/ir-spectroscopy-some-simple-practice-problems\/\" class=\"\"><span>IR Spectroscopy: 4 Practice Problems<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2022\/02\/08\/1h-nmr-how-many-signals\/\" class=\"\"><span>1H NMR: How Many Signals?<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1.\u00a0<\/strong> It should be noted that non-bonding orbitals\u00a0<em>are<\/em> present in species such as the allyl cation, allyl anion, and other ions of odd-numbered pi systems.<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2.<\/strong> For carbonyls, generally <a href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01546a021\">more polar solvents lead to higher\u00a0\u00a0\u03bb<sub>max\u00a0<\/sub>values<\/a>, as does the presence of substituents (such as methyl groups) on the alkene.<\/p>\n<p><strong>Bonus Topic<\/strong>: Azo Dyes<\/p>\n<p>Since we&#8217;re on the subject, let&#8217;s briefly explore another system where both\u00a0n\u2192\u03c0* and\u00a0\u03c0\u2192\u03c0* transitions are observed: <a href=\"https:\/\/en.wikipedia.org\/wiki\/Azo_compound\"><strong>azo dyes<\/strong><\/a>.<\/p>\n<p>Azo dyes are the kind of thing that you&#8217;ve likely seen a million times without specifically knowing\u00a0<em>what<\/em> they are.<\/p>\n<p>For example, the yellow color of highway markings? That&#8217;s <a href=\"https:\/\/en.wikipedia.org\/wiki\/Pigment_Yellow_10\">Pigment Yellow 10<\/a>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15448\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-yellow-road-highway-markings-due-to-azo-dyes-pigment-yellow-10-has-nitrogen-nitrogen-double-bond.jpg\" alt=\"yellow road highway markings due to azo dyes pigment yellow 10 has nitrogen nitrogen double bond\" width=\"450\" height=\"300\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-yellow-road-highway-markings-due-to-azo-dyes-pigment-yellow-10-has-nitrogen-nitrogen-double-bond.jpg 440w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-yellow-road-highway-markings-due-to-azo-dyes-pigment-yellow-10-has-nitrogen-nitrogen-double-bond-300x200.jpg 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-yellow-road-highway-markings-due-to-azo-dyes-pigment-yellow-10-has-nitrogen-nitrogen-double-bond-320x213.jpg 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-yellow-road-highway-markings-due-to-azo-dyes-pigment-yellow-10-has-nitrogen-nitrogen-double-bond-360x240.jpg 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>Azo dyes are commonly used in colouring textiles, plastics, and many other substances not intended for human consumption (they&#8217;re generally banned as food additives).<\/p>\n<p>The key structural feature of an azo compound is a N=N linkage. One of the simplest azo compounds is <a href=\"https:\/\/en.wikipedia.org\/wiki\/Azobenzene\">azobenzene<\/a>, where each nitrogen is connected to an aromatic ring. Slight modifications to the benzene ring can dramatically modify the color of the molecule.\u00a0\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Aniline_Yellow\">Aniline Yellow<\/a>, discovered in 1861, was the first azo compound to find commercial use as a dye, and countless derivatives of azobenzene have been synthesized since then. [The synthesis is via\u00a0<strong><a href=\"https:\/\/en.wikipedia.org\/wiki\/Azo_coupling\">diazo coupling<\/a><\/strong> &#8211; we won&#8217;t get into that here].<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15449\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-general-structure-of-azo-compound-and-aniline-yellow.gif\" alt=\"general structure of azo compound and aniline yellow\" width=\"600\" height=\"161\" \/><\/p>\n<p>Here&#8217;s the UV-Vis spectrum of Aniline Yellow, <a href=\"http:\/\/www.chemtube3d.com\/DyeAnilineyellow.htm\">as calculated by\u00a0ChemTube 3D<\/a>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15450\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d.png\" alt=\"uv vis spectrum of aniline yellow calculated by chemtube 3d\" width=\"630\" height=\"193\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d.png 1239w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-300x92.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-768x235.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-1024x313.png 1024w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-320x98.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-640x196.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-360x110.png 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-720x220.png 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-1080x330.png 1080w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-800x245.png 800w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-uv-vis-spectrum-of-aniline-yellow-calculated-by-chemtube-3d-760x232.png 760w\" sizes=\"(max-width: 630px) 100vw, 630px\" \/><\/p>\n<p>Note how it is <em>qualitatively<\/em> similar to that of acetone: a strong absorbance on the left (towards the UV) and a weak absorbance on the right (towards the visible).<\/p>\n<p>In contrast to acetone, however, where the weak absorbance is at 260 nm,\u00a0the weak absorbance in Aniline Yellow is in the visible region of the spectrum at about 460 nm.\u00a0It is this absorbance at 460 nm that is responsible for the color of Aniline Yellow.<\/p>\n<p>By analogy to acetone, the weak transition is an\u00a0(n\u2192\u03c0*) transition and the strong transition around 360 nm is a\u00a0\u03c0\u2192\u03c0* transition.<\/p>\n<h3><strong>Photoisomerization<\/strong><\/h3>\n<p>What&#8217;s even more interesting about azobenzenes and their derivatives (e.g. Aniline Yellow) is the phenomenon of <em>photoisomerization,\u00a0<\/em>where absorption of specific frequencies of light can lead to isomerization of<em>\u00a0<\/em><em>trans<\/em> isomers to\u00a0<em>cis<\/em> isomers and vice-versa.<\/p>\n<p>Absorbance of UV light by\u00a0<em>trans<\/em>-azobenzene (a \u03c0\u2192\u03c0* transition) leads to isomerization to\u00a0<em>cis<\/em>-azobenzene. Contrariwise, absorbance of visible light (blue light) by\u00a0<em>cis<\/em>-azobenzene (the \u00a0n\u2192\u03c0* transition) results in conversion back to the\u00a0<em>trans<\/em>-isomer [so does leaving\u00a0<em>cis<\/em>-azobenzene in the dark, a process known as thermal relaxation].\u00a0The mechanism for this process is still not completely settled.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15451\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F4-photoisomerization-of-trans-azobenzene-to-cis-azobenzene-by-uv-light-about-300-to-400-nm-and-blue-light-leads-to-cis-to-trans-isomerization.gif\" alt=\"photoisomerization of trans azobenzene to cis azobenzene by uv light about 300 to 400 nm and blue light leads to cis to trans isomerization\" width=\"630\" height=\"265\" \/><\/p>\n<p>Pretty neat that you can target a specific isomer merely by changing the frequency of light.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>UV-Vis Spectroscopy Of Carbonyls (C=O Bonds) UV-Visible spectroscopy is not just about C-C pi bonds. C-O pi bonds can absorb UV light as well! Table <\/p>\n","protected":false},"author":1,"featured_media":15444,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[386],"tags":[1128,940,1129,941,1130,1127,1119,1120],"post_folder":[],"class_list":["post-10170","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-spectroscopy-2","tag-absorbance","tag-homo","tag-isomerization","tag-lumo","tag-nonbonding-orbitals","tag-pi-to-pi","tag-uv","tag-uv-vis"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>UV-Vis Spectroscopy: Absorbance of Carbonyls - Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"In UV-Vis spectroscopy, isolated carbonyls absorb around 300 nm. This is due to a n to pi* transition, not a pi to pi* transition! 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