About Nitration of Benzene. If you are going to substitute an -NO2 group into the ring, then the electrophile must be NO2+. Because the nitronium ion is a good electrophile, it is attacked by benzene to produce Nitrobenzene. This is accompanied by cleavage of the sulfuric acid H-O bond, with the electrons going onto the oxygen: Now that the nitric acid oxygen is protonated (and positively charged), it is a good leaving group; oxygen, which is electronegative, doesn’t like to be positively charged, and wants to take the electrons from the N-O bond. Sulfonation of benzene is a reversible reaction. Benzene reacts with halogens in the presence of Lewis acid like FeCl3, FeBr3 to form aryl halides. This is formed by reaction between the nitric acid and the sulphuric acid. Nitronium ion (NO 2 +) is formed as first step in the mechanism, this formation is facilitated by the presence of concentrated sulfuric acid which behaves as a catalyst. Instead, it is done by converting benzene into an arenediazonium salt which is then replaced by fluorine by reacting it with fluoroboric acid (HBF4). Generation of electrophile. “On the Mechanism of Sulfonation of the Aromatic Nucleus and Sulfone Formation.” The Journal of Organic Chemistry 66 (1955): 455-465. ( Log Out /  Plus, handling F2 is not really what you want to do unless you absolutely have to, and are trained to do so. THE NITRATION OF BENZENE This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and a mixture of concentrated nitric acid and concentrated sulphuric acid. Sulfuric acid (H2SO4) helps overcome this by converting nitric acid into nitronium ions (NO2+), which are much better electrophiles. New York: VCH Publishers, Inc., 1989. Draw an energy diagram for the nitration of benzene. In the same way, FeBr3 is used as the Lewis acid activator for generating the source of Br+. The mechanism for nitration of benzene: Step 1: Nitric acid accepts a proton from sulphuric acid and then dissociates to form nitronium ion. Mechanism. You can also subscribe without commenting. Malhotra, Ripudaman, Subhash C. Narang, and George A. Olah. Bisulfite ion ( HSO4-) takes a proton from intermediate carbocation (σ complex) to give nitrobenzene. The benzene ring then nucleophilically attacks a nitronium ion, and re-aromatization of the subsequent complex leads to the nitrobenzene product. This reaction proceeds via the formation of an electrophilic nitronium ion (NO2+) from nitric acid (HNO3), resulting from proton transfer from sulfuric acid (H2SO4). Having nitrogen present in a ring is very useful because it can be used as a directing group as well as a masked amino group. The mechanism for the benzene nitration reaction is very similar to the mechanisms of the other electrophilic aromatic substitution reactions. It is secondary, there two conjugated double bonds, which in turn are conjugated with the empty p orbital of the positively charged carbon. Required fields are marked *. Benzene can be converted into benzenesulfonic by reacting it with fuming sulfuric acid which is prepared by adding sulfur trioxide (SO3). If you need a sulfonation of the aromatic ring, then use a concentrated solution of H2SO4. The electrophile is the “nitronium ion” or the “nitryl cation”, NO2+. Change ), You are commenting using your Twitter account. Sauls, Thomas W., Walter H. Rueggeberg, and Samuel L. Norwood. Because of the new sigma bond formed, this intermediate is called a sigma complex. The electrophile in the nitration of benzene is the +NO2 (the nitronium ion), which is formed by protonation of HNO3 by H2SO4 (yep, sulfuric acid is powerful). This significantly speeds up the reaction making it feasible. The electrophilic substitution reaction of benzene is done with the help of three steps. Sometimes, Fe may be shown instead of FeBr3, but don’t worry, it is the same thing as Fe as it reacts with Br2 to form the catalyst FeBr3 in situ (in the reaction mixture). It’s all here – Just keep browsing. Concentrated nitric acid and concentrated sulphuric acid comprise the nitrating mixture. As such, there is a strong drive to re-aromatize the benzene ring. It will always be attached to something else. Reagent : for benzene, HNO 3 in H 2 SO 4 / heat Electrophilic species : the nitronium ion ( i.e. What is/are the required reagent(s)for the following reaction: 2. Once the electrophile is formed, you can see that the mechanisms for the nucleophilic attack (and subsequent re-aromatization) are nearly identical: The nitration of benzene is an electrophilic aromatic substitution reaction, in which a nitro group (-NO2) is introduced onto a benzene ring. On the other hand, the arenium ion is not the worst carbocation you will ever see. Arthur, the H2SO4 protonates the HNO3 enabling the production of a viable electrophile the nitryl cation (NO2+) which can then attack the benzene ring. Regardless of what electrophile is used, the electrophilic aromatic substitution mechanism can be divided into two main steps.In step 1 the π electrons of benzene attack the electrophile which takes two electrons of the six-electron aromatic system. The electrophile attacks benzene ring to form an intermediate carbocation. The hydroxonium ion, H3O+, isn't involved. 3. When hydrogen atoms on the benzene ring is replaced by a nitrogroup (NO2).