{"id":11541,"date":"2018-04-30T08:00:33","date_gmt":"2018-04-30T12:00:33","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=11541"},"modified":"2026-04-18T06:41:11","modified_gmt":"2026-04-18T11:41:11","slug":"electrophilic-aromatic-substitutions-2-nitration-and-sulfonation","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2018\/04\/30\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\/","title":{"rendered":"Electrophilic Aromatic Substitutions (2) &#8211; Nitration and Sulfonation"},"content":{"rendered":"<ul>\n<li>Aromatic rings undergo nitration and sulfonation through the electrophilic aromatic substitution mechanism.<\/li>\n<li>Aromatic rings can undergo\u00a0<strong>nitration<\/strong> when treated with nitric acid HNO<sub>3<\/sub> in addition to the strong acid H<sub>2<\/sub>SO<sub>4<\/sub>.<\/li>\n<li>This leads to the formation of the nitronium ion NO<sub>2<\/sub>+ which is the active electrophile.<\/li>\n<li>Aromatic rings can undergo <strong>sulfonylation<\/strong> when treated with sulfur trioxide (SO<sub>3<\/sub>) in the presence of strong acid (H<sub>2<\/sub>SO<sub>4<\/sub>).<\/li>\n<li>The active electrophile is HSO<sub>3<\/sub>(+)<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-15914\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/0-summary-of-aromatic-nitration-of-benzene-and-sulfonation-using-hno3-h2so4-or-so3-h2so4-mechanism.gif\" alt=\"summary of aromatic nitration of benzene and sulfonation using hno3 h2so4 or so3 h2so4 mechanism\" width=\"630\" height=\"443\" \/><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Recap: The Three Stages Of Electrophilic Aromatic Substitution Reactions<\/a><\/li>\n<li><a href=\"#two\">Addition of NO<sub>2<\/sub> (Nitration)<\/a><\/li>\n<li><a href=\"#three\">Nitration of Benzene<\/a><\/li>\n<li><a href=\"#four\">The Nitronium Ion (NO<sub>2<\/sub>+) Is The Key Electrophile In Aromatic Nitration<\/a><\/li>\n<li><a href=\"#five\">Addition of SO<sub>3<\/sub>H (Sulfonation)<\/a><\/li>\n<li><a href=\"#six\">Summary: Aromatic Sulfonation and Nitration<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#appendix\">Bonus Topic: Reversibility of Sulfonation<\/a><\/li>\n<li><a href=\"#quiz\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Recap: The Three Stages Of Electrophilic Aromatic Substitution Reactions<\/h2>\n<p>In the previous post on <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/18\/electrophilic-aromatic-substitutions-1-halogenation\/\">halogenation via electrophilic aromatic substitution<\/a>, we saw that this electrophilic aromatic substitution reaction proceeded in three distinct stages:<\/p>\n<ol>\n<li><strong>Activation.\u00a0<\/strong>Since halogens (Cl<sub>2<\/sub>, Br<sub>2<\/sub>) don&#8217;t usually react with aromatic molecules at a reasonable rate,\u00a0 a Lewis acid catalyst (e.g. FeCl<sub>3<\/sub>) is added to &#8220;activate&#8221; the electrophile toward attack<\/li>\n<li><strong>Attack of Electrophile By The Aromatic Ring.\u00a0<\/strong>The activated electrophile is attacked by the aromatic ring, resulting in a carbocation intermediate (this is the rate determining step, and Step 1 in the generic mechanism of electrophilic aromatic substitution).<\/li>\n<li><strong>Deprotonation.\u00a0<\/strong>The carbocation intermediate is deprotonated by a weak base, restoring aromaticity (Step 2 in the generic mechanism of electrophilic aromatic substitution).<\/li>\n<\/ol>\n<p>In today&#8217;s post we&#8217;ll cover the mechanism of two other important electrophilic aromatic substitution reactions that proceed through Br\u00f8nsted acid catalysis &#8211; nitration and sulfonylation.<\/p>\n<h2><strong><a id=\"two\"><\/a>2. Addition of NO<sub>2<\/sub> (Nitration)<\/strong><\/h2>\n<p><b>Don&#8217;t Try This At Home<\/b><\/p>\n<p>&#8220;Nitration&#8221; is the name we give to the process of attaching the nitro group (NO<sub>2<\/sub>) to a molecule.<\/p>\n<p>If we skipped this reaction in previous chapters,\u00a0 it&#8217;s because nitrating functional groups other than aromatic rings leads to some pretty explosive products.\u00a0\u00a0 For example, nitration of the tri-ol glycerol produces the infamous <a href=\"https:\/\/en.wikipedia.org\/wiki\/Nitroglycerin\">nitroglycerin<\/a>, with three\u00a0<em>excellent\u00a0<\/em>leaving groups (<span style=\"color: #993366;\">ONO<sub>2<\/sub> ;<em> technically, these are &#8220;nitrate&#8221; groups, not nitro groups<\/em><\/span>). The slightest disturbance to this liquid leads to a chain reaction resulting in the rapid generation of hot gas. [<a href=\"#noteone\"><em>Note<\/em><em> 1<\/em><\/a><em>]<\/em><\/p>\n<p><a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/mim\/environmental\/html\/nitroglyc_text.htm\">4 moles of liquid nitrogylcerin creates 35 moles of gas<\/a>: 6 moles of N<sub>2<\/sub>, 12 moles of CO, 10 moles of H<sub>2<\/sub>O, and 7 moles of O<sub>2<\/sub>.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15915\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-nitration-of-alcohols-like-glycerol-produces-highly-unstable-nitrates-that-can-detonate-hno3-h2so4.gif\" alt=\"nitration of alcohols like glycerol produces highly unstable nitrates that can detonate hno3 h2so4\" width=\"630\" height=\"229\" \/><\/p>\n<p><strong><em>[Note to readers: <span style=\"text-decoration: underline;\">don&#8217;t try to make nitroglycerin<\/span>].\u00a0<\/em><\/strong><\/p>\n<p>My dad and his neighborhood buddies all made black powder and other explosives when they were kids, but making &#8220;nitro&#8221; was considered the Holy Grail.\u00a0 One day, my dad got all the material ready for making\u00a0 nitroglycerin, but on the day when it came to actually making the stuff, he chickened out.\u00a0 Wise move, Dad. It&#8217;s pretty hard to to do surgery when you are missing a couple of fingers.<\/p>\n<p>Similarly, nitration of cellulose (a polymer of the sugar glucose) produces &#8220;<a href=\"https:\/\/en.wikipedia.org\/wiki\/Nitrocellulose\">nitrocellulose<\/a>&#8220;, which besides being used in smokeless powder, is a polymer used as film stock in the early days of the film industry. &#8220;Celluloid&#8221;, as it was called, is not a contact explosive, but it does have the unfortunate property of being prone to spontaneous combustion. <span style=\"color: #993366;\"><em>(Hollywood later moved to cellulose acetate, a.k.a. &#8220;safety film&#8221;. )<\/em><\/span><\/p>\n<h2><strong><a id=\"three\"><\/a>3. Nitration of Benzene<\/strong><\/h2>\n<p>In contrast to nitration of alcohols, the nitration of benzene produces relatively stable nitro compounds that are much more difficult to detonate. For example, the high-explosive TNT (2,4,6-trinitrotoluene) is formed by triple nitration of toluene.\u00a0Another explosive, <a href=\"https:\/\/en.wikipedia.org\/wiki\/RDX\">RDX<\/a> comes from nitration of trihydro-1,3,5-triazine. [<a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<p>The key reagent for nitration is nitric acid, HNO<sub>3<\/sub>. By itself, nitric acid is a relatively slow-acting electrophile, especially in the presence of a poor nucleophile such as benzene. <span style=\"color: #993366;\"><em>[Note &#8211; in the case of phenol and other aromatic rings with strongly activating groups, HNO<sub>3<\/sub> by itself is sufficient for nitration].\u00a0<\/em><\/span><\/p>\n<p>Upon addition of acid, however &#8211; the usual choice being sulfuric acid, H<sub>2<\/sub>SO<sub>4<\/sub>\u00a0&#8211;\u00a0a rapid reaction with benzene ensues.<\/p>\n<p>The key to the process is protonation of OH on nitric acid, which converts it to H<sub>2<\/sub>O.\u00a0 Being a much better leaving group than HO(-), H<sub>2<\/sub>O is rapidly lost from\u00a0nitric acid to give the highly reactive &#8220;nitronium ion&#8221;, NO<sub>2<\/sub><sup>+<\/sup>.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15916\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-nitration-of-benzene-using-hno3-h2so4-giving-nitronium-ion-mechanism-step-1.gif\" alt=\"nitration of benzene using hno3 h2so4 giving nitronium ion mechanism step 1\" width=\"630\" height=\"262\" \/><\/p>\n<p><em><span style=\"color: #993366;\">[note: the mechanism drawn above is a little oversimplified. Two equivalents of H<sub>2<\/sub>SO<sub>4<\/sub> are usually best &#8211; protonate HNO<sub>3<\/sub> twice (not once)\u00a0 followed by loss of water, and then deprotonation of HNO<sub>2<\/sub>+<\/span>.<a id=\"backup\"><\/a> <\/em><\/p>\n<p>To see an image of the mechanism\u00a0 <a href=\"\" class=\"custom-tooltip\" data-image=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-full-mechanism-for-preparation-of-nitronium-ion-from-nitric-acid-with-h2so4.gif\" data-link=\"\" data-title=\"\" data-text=\"\"> hover here for a pop-up image <\/a> or open image link <a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-full-mechanism-for-preparation-of-nitronium-ion-from-nitric-acid-with-h2so4.gif\">here<\/a><\/p>\n<h2><a id=\"four\"><\/a>4. The Nitronium Ion (NO<sub>2<\/sub>+) Is The Key Electrophile In Aromatic Nitration<\/h2>\n<p>The nitronium ion is what really gets the job done here.<\/p>\n<p>You might recall that this is just like what we saw with the halogens, where use of a Lewis acid in that case produced an activated electrophile that reacted much more quickly with benzene.<\/p>\n<p>So what happens when the nitronium ion meets benzene?<\/p>\n<p>In the first (and rate-determining) step of electrophilic aromatic substitution, the nitronium ion (NO<sub>2<\/sub><sup>+<\/sup>) is attacked by a pair of pi electrons on the aromatic ring to give a carbocation intermediate, forming C\u2013N and breaking C\u2013C (pi) and N\u2013O (pi).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15917\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-electrophilic-nitration-of-benzene-with-nitronium-ion-no2-mechanism-step-1.gif\" alt=\"electrophilic nitration of benzene with nitronium ion no2+ mechanism step 1\" width=\"600\" height=\"188\" \/><\/p>\n<p>The second step of electrophilic aromatic substitution, which is relatively fast, is an acid-base reaction. A weak base (such as water, or the HSO<sub>4<sup>\u2013<\/sup><\/sub>\u00a0ion left after protonation of HNO<sub>3<\/sub>) removes a proton from carbon bearing the nitro group, breaking C\u2013H and re-forming C\u2013C pi. <strong>Aromaticity is restored.<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15918\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-step-2-of-nitration-of-benzene-deprotonation-restoring-aromaticity.gif\" alt=\"step 2 of nitration of benzene deprotonation restoring aromaticity\" width=\"600\" height=\"221\" \/><\/p>\n<p>The last two steps we covered previously in the <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/11\/09\/electrophilic-aromatic-substitution-the-mechanism\/\">generic electrophilic aromatic substitution mechanism<\/a>, and they are actually very similar between all electrophilic aromatic substitution reactions.<\/p>\n<h2><strong><a id=\"five\"><\/a>5. Addition of SO<sub>3<\/sub>H (Sulfonation)\u00a0<\/strong><\/h2>\n<p>The sulfonyl group, SO<sub>3<\/sub>H can also be added to an aromatic ring via electrophilic aromatic substitution.<\/p>\n<p>In this case the electrophilic reagent is sulfur trioxide, SO<sub>3\u00a0<\/sub>\u00a0(a gas) which can be introduced by bubbling through the solvent.\u00a0 On its own, SO<sub>3<\/sub> is not particularly reactive with aromatic rings, but as we&#8217;ve seen, addition of an acid can increase electrophilicity (and reaction rate) considerably. [<span style=\"color: #993366;\"><em>See post: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2010\/04\/21\/the-power-of-acid-catalysis\/\">The power of acid catalysis<\/a>]<\/em><\/span><\/p>\n<p>Like nitric acid, sulfur trioxide is &#8220;activated&#8221; by the addition of a proton from sulfuric acid.<span style=\"color: #993366;\"><em> [Note: this combination of SO<sub>3<\/sub> and H<sub>2<\/sub>SO<sub>4<\/sub> is called &#8220;<a style=\"color: #993366;\" href=\"https:\/\/en.wikipedia.org\/wiki\/Oleum\">fuming sulfuric acid<\/a>&#8220;, or &#8220;oleum&#8221; if you want to go for some really old school nomenclature.\u00a0 ]<\/em><\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15919\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-sulfur-trioxide-activation-through-protonation-with-h2so4-conjugate-acid-better-electrophile.gif\" alt=\"sulfur trioxide activation through protonation with h2so4 conjugate acid better electrophile\" width=\"600\" height=\"195\" \/><\/p>\n<p>In the rate determining step, the highly electrophilic SO<sub>3<\/sub>H(+)\u00a0 is then attacked by the aromatic ring to give the carbocation intermediate, forming C-S and breaking C-C (pi).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15920\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-electrophilic-aromatic-sulfonation-of-benzene-mechanism-attack-of-benzene-on-so3h.gif\" alt=\"electrophilic aromatic sulfonation of benzene mechanism attack of benzene on so3h\" width=\"600\" height=\"245\" \/><\/p>\n<p>As with all electrophilic aromatic substitutions, the C-H bond is then deprotonated with a weak base to regenerate the C\u2013C pi bond and restore aromaticity, providing the sulfonic acid product.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15921\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-sulfonation-of-benzene-mechanism-step-2-deprotonation-restoring-aromaticity.gif\" alt=\"sulfonation of benzene mechanism step 2 deprotonation restoring aromaticity\" width=\"630\" height=\"203\" \/><\/p>\n<p>And&#8230; that&#8217;s really it for the key mechanisms of nitration and sulfonation.<\/p>\n<h2><a id=\"six\"><\/a>6. Summary: Aromatic Sulfonation and Nitration<\/h2>\n<p>If you start comparing reactions, you should see that the key mechanism of electrophilic aromatic substitution doesn&#8217;t really change <strong>except for the identity of the electrophile<\/strong>.<\/p>\n<p>What really give each reaction its unique flavor is how the electrophile is activated, either by Lewis acid catalysis (chlorination, bromination) or Br\u00f8nsted acid catalysis (nitration, sulfonation).<\/p>\n<p>In the next post, we&#8217;ll cover the fifth and sixth of the six key electrophilic aromatic substitution reactions: Friedel-Crafts alkylation and Friedel-Crafts acylation, and how they are used to form new carbon-carbon bonds on aromatic rings.<\/p>\n<p><strong>Next Post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/05\/17\/friedel-crafts-alkylation-acylation\/\">Friedel-Crafts Alkylation and Acylation<\/a><\/strong><\/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\/2018\/05\/17\/friedel-crafts-alkylation-acylation\/\" class=\"\"><span>EAS Reactions (3) \u2013 Friedel-Crafts Acylation and Friedel-Crafts Alkylation<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/11\/26\/sulfonyl-blocking-groups-aromatic-synthesis\/\" class=\"\"><span>Aromatic Synthesis (3) \u2013 Sulfonyl Blocking Groups<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/18\/electrophilic-aromatic-substitutions-1-halogenation\/\" class=\"\"><span>Electrophilic Aromatic Substitutions (1) \u2013 Halogenation of Benzene<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/nitration-of-aromatic-groups\/\" class=\"\"><span>Nitration of aromatic groups (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/30\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\/\" class=\"\"><span>Sulfonation of Arenes to give sulfonic acids (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/05\/30\/intramolecular-friedel-crafts-reactions\/\" class=\"\"><span>Intramolecular Friedel-Crafts Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/09\/26\/activating-and-deactivating-groups-in-electrophilic-aromatic-substitution\/\" class=\"\"><span>Activating and Deactivating Groups In Electrophilic Aromatic Substitution<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/electrophilic-aromatic-substitution-practice-problems\/\" class=\"\"><span>Electrophilic Aromatic Substitution Practice Problems (MOC Membership)<\/span><\/a><\/li><\/ul><\/div>\n<p><a id=\"noteone\"><\/a><strong>Note 1.\u00a0<\/strong>I am told that if nitroglycerin is neutralized with sodium bicarbonate, it is &#8220;reasonably&#8221; safe to work with,\u00a0<span style=\"color: #993366;\"><em>so long as you do not breathe it or get it on your skin<\/em>. <\/span><em><span style=\"color: #993366;\">(reasonably safe still means, &#8220;don&#8217;t do it&#8221;)<\/span>.\u00a0<\/em> Upon prolonged standing, nitroglycerin develops\u00a0 a brown-red color, which is a sign that it is going &#8220;off&#8221;.<\/p>\n<p>I don&#8217;t know, but I wonder if the detonation process of nitroglycerin starts with homolytic cleavage of the weak O-NO<sub>2<\/sub> bond, to give the very nasty (and strongly colored)\u00a0 \u2022NO<sub>2<\/sub> radical. This can abstract a hydrogen atom from nitroglycerin, and then &#8211; <strong><em>shabang!<\/em>\u00a0<\/strong>&#8211; homolytic cleavage everywhere.<\/p>\n<p>Nitrogen tetroxide (which is in equilibrium with 2 moles of\u00a0 \u2022NO<sub>2<\/sub>) is a useful propellant in rocket engines and the like.<\/p>\n<p><a id=\"notetwo\"><\/a><strong>Note 2<\/strong>. Preparation of TNT, and structure of RDX.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15925\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-preparation-of-tnt-via-triple-nitration-of-toluene-hno3-and-h2so4.gif\" alt=\"preparation of tnt via triple nitration of toluene hno3 and h2so4\" width=\"630\" height=\"285\" \/><\/p>\n<h2><strong><a id=\"appendix\"><\/a>Sulfonation Is Reversible\u00a0<\/strong><\/h2>\n<p><em>See: <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/11\/26\/sulfonyl-blocking-groups-aromatic-synthesis\/\">Aromatic Synthesis (3) &#8211; Sulfonyl Blocking Groups for a full post on this topic.<\/a><\/em><\/p>\n<p>Aromatic sulfonyl groups have a very interesting property.\u00a0 If treated with a strong enough acid in the absence of SO<sub>3<\/sub>, the sulfonic acid group can be removed.<\/p>\n<p>Let&#8217;s have a look at how this happens.<\/p>\n<p>We&#8217;ve been so busy talking about different electrophiles (chlorine, bromine, nitronium, sulfonium&#8230;)\u00a0 that we neglected to mention that H+ can act as an electrophile too.<\/p>\n<p>We&#8217;ve ignored this because the reaction is so easily reversible &#8211; protonation of an aromatic ring is expected to be quickly followed by deprotonation back to the (stable, aromatic) starting material.<strong> It&#8217;s a cul-de-sac<\/strong>.<\/p>\n<p>However, in the particular case of sulfonyl groups (and one other group, which I&#8217;ll mention in the next post) protonation of the ring can have important consequences,\u00a0<strong>IF\u00a0<\/strong>done in the absence of SO<sub>3<\/sub>.<\/p>\n<p>First, let&#8217;s see what &#8220;protonation of the ring&#8221; looks like.<\/p>\n<p>If the ring is protonated at the exact same carbon as that bearing the sulfonyl group [<em><span style=\"color: #993366;\">you likely don&#8217;t need to know this, but this is called the &#8220;<a style=\"color: #993366;\" href=\"http:\/\/www.chem.ucla.edu\/~harding\/IGOC\/I\/ipso_hydrogen.html\">ipso<\/a>&#8221; carbon<\/span>],\u00a0<\/em>we generate a carbocation intermediate.<\/p>\n<p>Does the product look familiar? It should &#8211; it&#8217;s the exact same carbocation intermediate that is generated when benzene attacks protonated SO<sub>3<\/sub>, in &#8220;step 2&#8221; of sulfonation.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15922\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-in-presence-of-strong-acid-sulfonation-can-be-reversed-step-1-gives-carbocation.gif\" alt=\"in presence of strong acid sulfonation can be reversed step 1 gives carbocation\" width=\"600\" height=\"290\" \/><\/p>\n<p>Most of the time, this intermediate will just be deprotonated to regenerate the aromatic sulfonic acid.<\/p>\n<p>However, in the case of SO<sub>3<\/sub>H, there is another pathway to restore aromaticity that is not energetically available in the case of most other substituents. Instead of losing H, the intermediate can lose SO<sub>3<\/sub>H(+), which leads to the formation of benzene.<\/p>\n<p>This is the\u00a0<em>reverse<\/em> of electrophilic attack!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15923\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-reversibility-of-sulfonation-part-2-loss-of-so3-instead-of-h-restores-aromaticity.gif\" alt=\"reversibility of sulfonation part 2 loss of so3 instead of h+ restores aromaticity\" width=\"600\" height=\"378\" \/><\/p>\n<p>Now here&#8217;s where the &#8220;absence of SO<sub>3<\/sub>&#8221; part comes in. In the presence of a high concentration of SO<sub>3<\/sub>, benzene would just attack protonated SO<sub>3<\/sub> again and re-form the sulfonic acid.<\/p>\n<p>But if we just add acid in the\u00a0<em>absence<\/em> of SO<sub>3<\/sub>, then this is less likely to occur. Furthermore, if we vent the reaction (say, by bubbling an inert gas like argon through the reaction mixture, which would eventually carry away any gaseous SO<sub>3<\/sub> along with it), then gaseous SO<sub>3<\/sub> will be slowly removed from the system. If you recall the principle of Le Ch\u00e2telier, this means that our sulfonic acid is guaranteed to slowly revert back to benzene.<\/p>\n<p>&#8220;OK, fine.\u00a0 It&#8217;s reversible. So what?&#8221; you may rightly ask.<\/p>\n<p>Well, this means that SO<sub>3<\/sub>H can be useful as a &#8220;blocking group&#8221; under some conditions. Again, we&#8217;ll have more to say on this when we get to synthetic strategy, but I&#8217;ll leave you with a challenge: how would you generate\u00a0<em>ortho-<\/em>chloromethylbenzene from methylbenzene without any of the\u00a0<em>para-<\/em> product?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15924\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-practice-problem-sulfonation-blocking-group-strategy-to-give-ortho-chlorotoluene-with-no-para.gif\" alt=\"practice problem sulfonation blocking group strategy to give ortho chlorotoluene with no para\" width=\"600\" height=\"250\" \/><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2018\/04\/FN-blocking-group-e1524773444676.png\"><br \/>\n<\/a>Answer in the comments.<\/p>\n<hr \/>\n<h2><strong><a id=\"quiz\"><\/a>Quiz Yourself!<\/strong><\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1101-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1109-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0518-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0519-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3104-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3105-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0520-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<p>For more detailed references on these reactions, consult the sections in the <a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\">reaction guide<\/a>. The references here are highlights.<\/p>\n<p>Nitration:<\/p>\n<ol>\n<li><strong>Kinetics and Mechanism of Aromatic Nitration<\/strong><br \/>\nJ. GILLESPIE, E. D. HUGHES, C. K. INGOLD, D. J. MILLEN &amp; R. I. REED<br \/>\n<em>Nature<\/em> volume 163, pages 599\u2013600 (<strong>1949<\/strong>)<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/www.nature.com\/articles\/163599b0?proof=true\">10.1038\/163599b0<\/a><br \/>\nProf. Christopher Ingold carried out a lot of early mechanistic studies on electrophilic aromatic nitration, and laid the groundwork for our current understanding of the reaction. He first proposed that the nitric acid-sulfuric acid mixture (known as \u201cmixed acid\u201d) used in nitration generated the nitronium ion, [NO<sub>2<\/sub>]<sup>+<\/sup>, and that this was the reactive species in electrophilic nitration.The Nobel Laureate late Prof. George A. Olah also published a lot of work studying the mechanism of nitration, resulting in at least 50+ papers and a book on the topic. Along the way, he isolated the nitronium ion as a stable salt (which is now commercially available) and demonstrated that it underwent the same reactions as mixed acid system.<\/li>\n<li><strong>Aromatic Substitution. VII. Friedel-Crafts Type Nitration of Aromatics<br \/>\n<\/strong>Stephen J. Kuhn and George A. Olah<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1961, <\/strong><em>83<\/em> (22), 4564-4571<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01483a016\">1021\/ja01483a016<\/a><\/li>\n<li><strong>Aromatic substitution. XXVIII. Mechanism of electrophilic aromatic substitutions<br \/>\n<\/strong>George A. Olah<strong><br \/>\n<\/strong><em>Acc. Chem. Res.,<\/em><strong> 1971, <\/strong><em>4<\/em> (7), 240-248<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ar50043a002\">10.1021\/ar50043a002<\/a><\/li>\n<li><strong>Unified Mechanistic Concept of Electrophilic Aromatic Nitration:\u2009 Convergence of Computational Results and Experimental Data<\/strong><br \/>\nPierre M. Esteves, Jos\u00e9 Walkimar de M. Carneiro, Sheila P. Cardoso, Andr\u00e9 H. Barbosa, Kenneth K. Laali, Golam Rasul, G. K. Surya Prakash, and George A. Olah<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>2003,<\/strong> <em>125<\/em> (16), 4836-4849<br \/>\n<strong>DOI:<\/strong> <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja021307w\">10.1021\/ja021307w<\/a><br \/>\nThis is the grand-daddy paper on nitration, summarizing a lifetime\u2019s worth of work on the subject. If you don\u2019t have time to read any of the other papers here, read this.Sulfonation:<\/li>\n<li><strong>Aromatic sulfonation with sulfur trioxide: mechanism and kinetic model<br \/>\n<\/strong>Samuel L. C. Moors, Xavier Deraet, Guy Van Assche, Paul Geerlings and Frank De Profta<br \/>\n<em> Sci.,<\/em> <strong>2017<\/strong>, <em>8<\/em>, 680<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/2017\/SC\/c6sc03500k#!divAbstract\">10.1039\/c6sc03500k<\/a><br \/>\nA very recent paper on a classic reaction. This paper is a computational analysis of electrophilic aromatic sulfonation, attempting to clarify the kinetics and mechanism of the reaction.<\/li>\n<li><strong>Electrophilic Aromatic Sulfonation with SO<sub>3<\/sub>: Concerted or Classic S<sub>E<\/sub>Ar Mechanism?<br \/>\n<\/strong>Gergana Koleva, Boris Galabov, Jing Kong, Henry F. Schaefer, III, and Paul von R. Schleyer<br \/>\n<em>Journal of the American Chemical Society<\/em><strong> 2011, <\/strong><em>133<\/em> (47), 19094-19101<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja201866h\">1021\/ja201866h<\/a><br \/>\nThe late Prof. P. v. R. Schleyer was a giant in <em>computational<\/em> Physical Organic chemistry. This is a computational study of aromatic sulfonation that attempts to clarify the mechanism. Computational modeling shows that a trimolecular reaction with 2 SO<sub>3<\/sub> molecules is actually energetically favored.Prof. Hans Cerfontain (U. Amsterdam) did a lot of work on electrophilic aromatic sulfonation, publishing over 100 papers on this topic. The following two papers are just a selection:<\/li>\n<li><strong>Aromatic sulphonation. Part 92. Sulphonation of the three methylphenols and the six dimethylphenols in concentrated aqueous sulphuric acid; and the lsomerization of some of the resulting sulphonic acids and of m-xylene-2-and o-xylene-3-sulphonic acid<br \/>\n<\/strong>Hans J. A. Lambrechts, Zwaan R. H. Schaasberg-Nienhuis and Hans Cerfontain<strong><br \/>\n<\/strong><em> Chem. Soc.,<\/em> <em>Perkin Trans. 2<\/em>, <strong>1985<\/strong>, 669-675<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1985\/P2\/P29850000669#!divAbstract\">10.1039\/P29850000669<\/a><\/li>\n<li><strong>Kinetics of the desulfonation of benzenesulfonic acid and the toluenesulfonic acids in aqueous sulfuric acid<br \/>\n<\/strong> C. M. Wanders, H. Cerfontain<strong><br \/>\n<\/strong><em>Rec. Trev. Chim. Pays-Bas<\/em><strong> 1967, <\/strong><em>86<\/em> (11), 1199-1216<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/abs\/10.1002\/recl.19670861106\">10.1002\/recl.19670861106<\/a><br \/>\nThe desulfonation of arylsulfonic acids is a synthetically useful reaction, and this paper examines the kinetics of the reaction.The following two procedures from <em>Organic Syntheses<\/em> highlight the utility of desulfonation, allowing access to <em>ortho<\/em>-substituted aromatics. Whether such a route would actually be conducted or not depends on the target compound. With the improved chromatographic separation techniques available today, one might just separate the isomers from a crude mixture.<\/li>\n<li><strong>o-BROMOPHENOL<br \/>\n<\/strong>Ralph C. Huston and Murel M. Ballard<strong><br \/>\n<\/strong><em>Org. Synth.<\/em><strong> 1934, <\/strong><em>14<\/em>, 14<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/www.orgsyn.org\/demo.aspx?prep=CV2P0097\">10.15227\/orgsyn.014.0014<\/a><\/li>\n<li><strong>2,6-DINITROANILINE<br \/>\n<\/strong>Harry P. Schultz<strong><br \/>\n<\/strong><em>Org. Synth.<\/em><strong> 1951<\/strong>, <em>31<\/em>, 45<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/www.orgsyn.org\/demo.aspx?prep=CV4P0364\">10.15227\/orgsyn.031.0045<\/a><\/li>\n<\/ol>\n<p>&#8216;,&#8217;Electrophilic Aromatic Substitutions (2) &#8211; Nitration and Sulfonation<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Aromatic rings undergo nitration and sulfonation through the electrophilic aromatic substitution mechanism. Aromatic rings can undergo\u00a0nitration when treated with nitric acid HNO3 in addition to <\/p>\n","protected":false},"author":1,"featured_media":15914,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1297],"tags":[162,1330,319,1231,1232,1329,1328],"post_folder":[],"class_list":["post-11541","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aromatic-reactions","tag-acid-catalysis","tag-blocking-group","tag-electrophilic-aromatic-substitution","tag-nitration","tag-sulfonylation","tag-sulfur-trioxide","tag-sulfuric-acid"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Nitration and Sulfonation Reactions In Electrophilic Aromatic Substitution<\/title>\n<meta name=\"description\" content=\"All about the nitration and sulfonation electrophilic aromatic substitution reactions of benzene, their mechanisms, examples, and more.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/30\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Nitration and Sulfonation Reactions In Electrophilic Aromatic Substitution\" \/>\n<meta property=\"og:description\" content=\"All about the nitration and sulfonation electrophilic aromatic substitution reactions of benzene, their mechanisms, examples, and more.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/30\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\/\" \/>\n<meta property=\"og:site_name\" content=\"Master Organic Chemistry\" \/>\n<meta property=\"article:publisher\" content=\"https:\/\/www.facebook.com\/Master-Organic-Chemistry-242610599108055\/\" \/>\n<meta property=\"article:published_time\" content=\"2018-04-30T12:00:33+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-04-18T11:41:11+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/0-summary-of-aromatic-nitration-of-benzene-and-sulfonation-using-hno3-h2so4-or-so3-h2so4-mechanism.gif\" \/>\n\t<meta property=\"og:image:width\" content=\"924\" \/>\n\t<meta property=\"og:image:height\" content=\"650\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/gif\" \/>\n<meta name=\"author\" content=\"James Ashenhurst\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"James Ashenhurst\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"16 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/2018\\\/04\\\/30\\\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/2018\\\/04\\\/30\\\/electrophilic-aromatic-substitutions-2-nitration-and-sulfonation\\\/\"},\"author\":{\"name\":\"James Ashenhurst\",\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/#\\\/schema\\\/person\\\/78d83ec7d02b4b7365bade2cedaef80c\"},\"headline\":\"Electrophilic Aromatic Substitutions (2) &#8211; 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