There are mainly four major type of chemical reactions in organic chemistry. These includes addition, elimination, substitution and pericyclic reaction. The above aspect is discussed by giving examples.

1. Addition Reaction

An addition reaction take place when two or more reactants combine to form a chemical product. Addition reactions take place in a unsaturated compound, which is called olefin. Olefin decolourise bromine water or potassium permanganate due to addition reaction. Hydrohalogenation is one of the addition, in which a hydrogen atom and a halogen atom are added to the olefin . For example, olefin reacts with hydrogen halide (HX) to give an alkyl halide as shown in Figure 1.1.

Addition Reaction

As per Markovnikov's rule, the addition of hydrogen ion to one of the carbon atom in the alkene creates a positive charge on the other carbon atom, which generates a carbocation intermediate. Thus, stability of carbocation is increased when more alkyl substitution on that carbon atom. Thus the negative part of the reagent (e.g. X of HX) attached to the carbon atom, which has lesser number of hydrogen atom or more number of alkylation as shown in Figure 1.2.

The hydrohalogenation of substituted olefin

Another addition reaction is hydration reaction of olefin compounds. In this reaction, the addition of water  to an olefin results the formation of an alcohol. In this reaction, an acid is act as catalyst as shown in Figure 1.3

Figure 1.3 Hydration of ethene to form ethanol

2. Elimination Reaction

In elimination reaction, the reactant is broken up into two products in presence of an base. Generally,  elimination reactions take place in alkane and alkyl halides. Dehydrohalogenation is an example of elimination in a haloalkane in presence of alcoholic basic solution. After refluxing the reaction mixture for couple of hours, the base helps in eliminating a hydrogen atom from the substrate and subsequently, removal of halide to form a double bond as shown in Figure 2.1

Figure 2.1 Dehydrohalogenation of bromoethane in presence of base, KOH to form ethene and water

As per Saytzeff's rule, elimination in alkyl halide generates predominately olefin with greater number of alkyl group as shown in Figure 2.2

Figure 2.2. Base induced elimination in iodo butane generate olefin with more alkyl group with elimination of H2O and KI

During the dehydration of an alcohol the hydroxyl group and a hydrogen atom are eliminated from the reactant. An alkene is formed as a product in the reaction, along with a water molecule as shown in Figure 2.3.

Figure 2.3. Dehydration of ethanol in presence of excess of dehydrating agent (e.g. sulfuric acid) and high temperature to form an alkene

3. Substitution Reaction

In substitution reaction, an exchange of element take place between a substrate and a reagent and the initial reactant is transformed to a new product.When hydroxyl group of an alcohol is replaced by a halogen atom in presence of hydrogen halide, an alkyl halide is formed. This reaction works fine at room temperature in presence of a tertiary alcohols, however, the reaction is slow and occur at high temperature in case of  a primary or secondary alcohol. Substitution of tert-alcohol is shown in Figure 3.1

Figure 3.1 Substitution reaction in tert-butyl alcohol to form tert-alkyl halide (X = Cl, Br).

Hydration of an alkyl halide is another type of substitution reaction, in which an alcohol can be formed through substitution as shown in Figure 3.2. Using a base (e.g. KOH, NaOH, etc) dissolved in water and refluxing the solution generates corresponding substituted product. In order get good yield, the base solution should be diluted in water and the reaction temperature should be low.

Figure 3.2 Substitution reaction in ethyl chloride to form ethyl alcohol