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

As we have already learned SN2, what would happen if the substrate is hindered and the nucleophile is a weak one? SN1 will happen. Unimolecular : SN1 is called unimolecular substitution because only one molecule (the substrate) is involved in determining the rate of reaction. This would not happen if the reaction is one step (nucleophile attacks and leaving group leaves the substrate). This reaction must involves multi steps: one is rate-liming (slow) and the other is fast (doesn't affect the rate). As it is not a concerted reaction, an intermediate is formed. Substitution : the nucelophile (from the solvent) substitutes the leaving group. SN1 is also called solvolysis because the solvent acts as the nucleophile (SN1 happens when we put a bulky substrate in a solvent). Let's look at the general mechanism: Unimolecular Substitution Step 1 is a rate-determining step which is a slow ionization that gives a carbocation intermediate. This step is slow and has a high...