# benzene substitution reactions

In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring. There may be different ratios of ortho– to para– and there may be small amounts of meta-, but don’t get bogged down in the details. But this is completely wrong. React benzene with haloalkane. There are two positions ortho– to the initial substituent and two positions meta– to it. 16.4 ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE 755 Step 2 Reaction of the benzene p electrons with the electrophile to form a carbocation inter- mediate. The following four-part illustration shows this mechanism for the bromination reaction. Substituents with heteroatoms connected to the aromatic ring are significantly more ring-activating than alkyl groups, because resonance electron-donating effects are possible. Substitution could actually occur on five positions around the ring, but two pairs are related by symmetry. Classify these two groups as activating or deactivating groups: 5. The direction of the reaction. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Nucleophilic Substitution, Elimination & Addition Reactions of Benzene Derivatives 1. The mechanism for the nitrobenzene reaction occurs in six steps. Substitution Reactions of Benzene Derivatives, Substitution Reactions of Benzene and Other Aromatic Compounds, Substituents determine the reaction direction by resonance or inductive effect, Activating groups (ortho or para directors), Substituents determine the reactivity of rings, http://en.wikipedia.org/wiki/Activating_group, http://en.wikipedia.org/wiki/Deactivating_group, http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/PDF/12_12_14.pdf, Predict the direction of the electrophile. Because these mechanisms are different from what's gone before (and from each other), there isn't any point in dealing with them in a general way. (pp. Examples of deactivating groups in the relative order from the most deactivating to the least deactivating: -NO2, -CF3> -COR, -CN, -CO2R, -SO3H > Halogens. In addition, the para product would be expected to be preferred over the ortho product, due to steric considerations. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Resonance effect is the conjugation between the ring and the substituent, which means the delocalizing of the $$\pi$$ electrons between the ring and the substituent. The reactivity of aromatic pi bonds in SEAr reactions is very sensitive to the presence of electron-donating groups (EDG) and electron-withdrawing groups (EWG) on the aromatic ring. The ion formed in this step isn't the final product. This has the effect of breaking the delocalisation, although not completely. You have to remember that it is impossible to get a positive ion on its own in a chemical system - so Y- is simply the negative ion that was originally associated with X+. Remember, there are three different positions on the benzene ring where a new substituent can attach, relative to the original substituent. Nitrobenzene, C6H5NO2, undergoes the reaction millions of times more slowly. As described above NH2is an activating group which speeds up the reaction and NO2 is deactivating group that slows down the reaction. In most of benzene's reactions, the electrophile is a positive ion, and these reactions all follow a general pattern. Have questions or comments? . Phenol, C6H5OH, undergoes nitration a thousand times faster than benzene does. Thus; stabilizing the intermediate step, speeds up the reaction; and this is due to the decrease of the activating energy. In the mid-twentieth century, physical organic chemists including Christopher Ingold conducted a number of kinetic studies on electrophilic aromatic substitution reactions. Substitution versus Addition Reactions. Step 3 Loss of a proton from the carbocation to give a new aromatic compound. The first substitution is going to be ortho and/or para substitution since we have a halogen subtituent. It is just an intermediate. Electron-withdrawing substituents on an aromatic ring are ring-deactivating, making it harder for further substitution reactions to occur. American Chemical Society. Sauls, Thomas W., Walter H. Rueggeberg, and Samuel L. Norwood. That is, they control where the new substituent appears in the product. Alkylation of benzene is when an alkyl group substitutes a H atom in benzene ring. This is known as the ortho-para directing effect. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability. If we want to understand these data, we need to think about things like π-donation, π-acceptance, inductive effects and cation stability. This is what you need to understand for the purposes of the electrophilic substitution mechanisms: Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached. In some cases, there may be multiple effects, and the overall influence of the substituents is determined by the balance of the effects. Amines, for example, are very powerful ring-activating substituents, due to the ability of the lone pair on the nitrogen to stabilize the carbocation intermediate through resonance: Other ring-activating groups are shown below (in these figures, the R group can be a hydrogen). These are mostly carbonyl-containing groups, as well as alkyl halides. When substitution does occur on an aromatic ring with deactivating group already attached, it tends to occur specifically at the meta position – deactivating groups are generally meta-directing. 1. Example of electrophilic substitution = … The hydrogen at the top isn't new - it's the hydrogen that was already attached to that carbon. 2. Simply draw the "+" in the middle of the ring. Also the size of the halogen effects the reactivity of the benzene ring that the halogen is attached to. In cases where the subtituents is esters or amides, they are less activating because they form resonance structure that pull the electron density away from the ring. Substituents that make the benzene moor electron-poor can retard the reaction. Because benzene acts as a nucleophile in electrophilic aromatic substitution, substituents that make the benzene more electron-rich can accelerate the reaction. Where did this come from? 3.1 Halogenation of Benzene • Substitution of a halogen for H on a benzene ring via electrophilic aromatic substitution: • The electrophile in the reaction is generated using a Lewis acid catalyst. Reactions of Benzene & Its Derivatives Chapter 22 Organic Lecture Series 2 Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: + + Chlorobenzene Halogenation: H Cl2 Cl FeCl3 HCl + + Nitrobenzene Nitration: HNOHNO3 2 H2 SO4 H2 O

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