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: ...

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 ...

                                          Wittig Reaction Wittig reaction turns a carbonyl ( compound that has C=O) to an alkene by reacting a carbonyl (aldehyde or ketone) with a phosphorus ylide . It is a very useful reaction that turns a C=O to a desired C=C. Making ylide Let's talk about the phosphorus ylide . It has no overall charge, but it has a negatively charged carbanion that is bonded to a positively charged phosphorus. It is prepared by a two-step reaction sequence - an SN2 in which a triphenylphosphine attacking an unhindered alkyl halide (making a positively charged phosphorus), followed by a proton abstraction by a strong base (usually butyllithium ). We know that Phosphorus and Sulfur can form more than 4 bonds using the d orbitals . One may think that the ylide should have a double bond instead of having the carbon and phosphorus ch...

Hoffman elimination and Cope elimination are amine chemistry. Both reactions are concerted and both favor the Hofmann product (less-substituted alkene). Hofmann Elimination  Leaving group  The general form of an amine is R-NH2. The amide ion is a strong base hence a poor leaving group. So if we want an amine to undergo an elimination, we have to make a better leaving group first. We do this by exhaustive methylation (usually with methyl iodide) to convert the leaving group into a quaternary ammonium salt which can leave as a neutral amine. E2 Mechanism Hofmann elimination follow a E2 , concerted reaction mechanism which needs a strong base . The geometry is specific here (like a typical E2): anti-coplanar between the proton being abstracted and the leaving group. The quaternary ammonium salt is reacted with silver oxide to become a hydroxide salt to generate the strong base needed. Heat is applied and the Hofmann product is the major product.  Hofmann Eli...

Dehydration of alcohols This is a reversible acid-catalyzed reaction. In fact, it is a common way to turn an alkene  into an alcohol. To increase the yield of the product, the alkene  produced is usually distilled off (an alkene has a lower boiling point than its alcohol due to the lack of hydrogen bonding) to shift the equilibrium to the product side. Concentrated sulfuric acid is used as a catalyst to protonate the -OH group to a better leaving group, H2O. Then, a E1 mechanism is followed: 1. ionization (water leaves) to a crabocation 2. a weak base (water or HSO4-) abstracts the proton to form an alkene.  Dehydration of alcohol Cracking (alkane) An industrial (large scale and least expensive) way to make alkene is by the catalytic cracking of alkane (e.g. petroleum). A long chain of alkane is heated with catalyst (e.g. platinum) to form small alkenes (around 6 carbon atoms). This process of dehydrogenation is endothermic, but it has a positive entropy change...