Chemistery

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

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...

As I love jumping from here and there, let's talk about Fischer Esterification! While you may think that chemical reactions go in one way and never turn back. The truth is that a chemical reaction is always in an equilibrium! Here is the Fischer Esterification, which is always a reaction between a carboxylic acid and an alcohol. Note that the source of the oxygen of the ester group and the reaction is always in an equilibrium. Fisher Esterification  Now, let's revise the concept of equilibrium and the math behind it like how to calculate the Keq. We consider concentration when we look at an equilibrium. The initial concentration is Mi for both the acetic acid and the ethanol. When the equilibrium is established, a concentration of x of ester and water is formed. In a closed environment, the product can only be formed from the reactants. So the reactants lost a concentration of x to form the product. The final concentration of the reactants is Mi-x . ...