Chemistery

Nitration of benzene forms nitrobenzene . For it to take place, we need nitronium ion (strong electrophile ) to attack the benzene ring. We can do so by reaction nitric acid with an oxidizing agent. However, this will cause an explosion as the reaction is very exothermic. Therefore, we need a safer way, which is reacting nitric acid with sulfuric acid (catalyst, proton source). This way, we can form the electrophile , the nitronium ion. Sulfuric acid protonats the hydroxyl group to make it a better leaving group (water) to form nitronium ion. The reaction mechanism is as follows:  After nitronium ion is formed, it is reacted with benzene. The electrophilic nitronium ion attacks the benzene ring. Deprotonation follows to from the conjugated system back.  The nitro group of the product ( nitrobenzene ) can easily be reduced to amino group  by treating with active metals (Zn, Fe, Sn) in dilute acid (HCl). 

Carbocation is useful in EAS ( alkylation ) because it can be a good source of an electrophile . The carbocation electrophile attacks the benzene ring with the general EAS mechanism. Here, the electrophilicity of the carbocation is determined by its stability. There are two common methods to produce carbocation : from alkene and from alcohol. 1. Alkene with HF Recall back to nucleophilicity , fluoride ion is a weak nucleophile (as it is stable). Therefore, when the pi-bond of the alkene is protonated by HF, the given off fluoride ion does not attack the carbocation immediately. If a benzene ring presents, EAS occurs with the electrophilic carbocation alkylating the aromatic ring. Alkene with HF 2. Alcohol with Lewis Acid (BF3) Alcohol forms carbocation when it is treated with a Lewis Acid (commonly BF3). Note that BF3 is consumed in the reaction, so it is not a catalyst in this reaction. Carbocation from alcohol

The first step of EAS is highly endothermic as the aromaticity is lost. Therefore, we need a strong electrophile  to initiate the reaction. If we want to do bromination , we cannot use Br2 directly because it is not a strong electrophile  (Br2 has no open octect  and it is nonpolar, no formal charges). We enhance its electrophilicity  by using a Br2.FeBr3 (or Br2.AlBr3 ) intermediate (FeBr3 or AlBr3 is a Lewis acid , electron acceptor, so it withdraws the electrons from Br, making it much more polar). The Fe-Br bond is more polar so that the Br is a stronger  electrophile .  The rest follows the general mechanism pattern. The electrophile  attacks the benzene ring, forming a sigma complex. Then a proton is lost, giving off HBr and achieving aromaticity again. Chlorination is basically the same with the chlorine group instead of the bromine group.  EAS: Bromination The sigma complex is stabilized by resonance: Sigma Complex

Aromatic compounds are widely used in organic chemistry. To master the chemistry of aromatic compounds, we have to know EAS and NAS, which stands for Nucleophilic Aromatic Substitution and Electrophilic Aromatic Substitution respectively. Aromatic compounds are hydrocarbons contain a benzene (which has an aromatic ring). Let's have an overview of the steps of reaction happening in an EAS. Electrophilic attack the ring (resonance stabilized)  Base abstract proton  Here is the general two-step mechanism: EAS The intermediate is a resonance-stabilized carbocation  called  sigma complex (arenium ion) and it is not aromatic anymore. The first step is highly endothermic because of the loss of aromaticity (aromatic compound is more stable).   Here are the resonance forms: sigma complex After going through the general mechanism, we will then discuss the specific EAS reactions in the following order: Halogenation  (Bromination & Chlorination)  Sulfon