We already know that the use of polar, aprotic solvents increases the reactivity of nucleophiles in SN2 reactions, because these solvents do not ‘cage’ the nucleophile and keep it from attacking the electrophile. In laboratory synthesis reactions, halides often act as leaving groups. Iodide, which is the least basic of the four main halides, is also the best leaving group – it is the most stable as a negative ion. Fluoride is the least effective leaving group among the halides, because fluoride anion is the most basic. As long as the two of the groups attached to the carbon being attacked are small hydrogens, the repulsions which happen do not require much energy. If the groups attached to the carbon are larger, though, like methyl groups, the transition state energy increases, the activation energy increases, and the reaction becomes much slower.
Thus, the presence of electron withdrawing groups at o- and p-positions (but not at m-positions) w.r.t. halogen atom activates the aryl halides towards nucleophilic substitution reaction. Elimination reactions (especially Beta-elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. In pathway ‘a’, water acts as a nucleophile – this is, of course, the familiar SN1 reaction. This alternative pathway is called an elimination reaction, and in fact with the conditions above, both the substitution and the elimination pathways will occur in competition with each other.
Thus 81% of the theoretical yield is actually isolated, which is a very respectable yield, that would please most chemists. S N2 mechanisms always proceed via rearward attack of the nucleophile on the substrate. This process results in the inversion of the relative configuration, going from starting material to product. This inversion is often called the Walden inversion, and this mechanism is sometimes illustrated as shown in Figure 3. At the transition state, the electrophilic carbon and the three ‘R’ substituents all lie on the same plane. Ethers do not generally react with nucleophiles under neutral or basic conditions.
This occurs because the nucleophilic attack is always on the back lobe of the carbon atom acting as the nucleus. The following diagram illustrates these two types of nucleophilic attacks, where the frontside attack results in retention of configuration; that is, the product has the same configuration as the substrate. The backside attack results in inversion of configuration, where the product’s configuration is opposite that of the substrate. A stereospecific reaction is one in which different stereoisomers react to give different stereoisomers of the product.
The addition and its counterpart, the elimination, are reactions which change the number of substituents on the carbon atom, and form or cleave multiple bonds. Double and triple bonds can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms which are named after the respective reaction order.
Cyclic rearrangements include cycloadditions and, more generally, pericyclic reactions, wherein two or more double bond-containing molecules form a cyclic molecule. An important example of cycloaddition reaction is the Diels–Alder reaction (the so-called [4+2] cycloaddition) between a conjugated diene and a substituted alkene to form a substituted cyclohexene system. This is one of the most useful methods for the mild formation of C–C bonds. An important class of redox reactions are the electrochemical reactions, where electrons from the power supply are used as the reducing agent. These reactions are particularly important for the production of chemical elements, such as chlorine or aluminium.
However, CH₃ is extremely unstable and it cannot wait for the OH to form a bond. Hence it can only form a bond in Sₙ² reaction because Sₙ² is a transitional reaction and the detachment and attachment mcguff blog process happens at the same time. The rate of reaction depends on the concentration of both the substrate and the nucleophile. In haloarenes, the carbon atom attached to halogen is sp2 hybridised.
