Addition Reaction

Addition reaction is a chemical process in which two or more molecules combine to form a single molecule. They are typical of unsaturated organic compounds such as alkenes, alkynes, aldehydes, and ketones.

In this article, we will look into what addition reaction is, types of addition reaction, its examples, applications, and the difference between addition and substitution reaction.

What is Addition Reaction?

Addition reaction is a chemical process in organic chemistry where two or more molecules combine to form a larger molecule, known as the adduct. This reaction is limited to compounds with multiple bonds, such as alkenes and alkynes, and compounds with carbon-hetero double bonds, like carbonyl or imine groups. There are several types of addition reactions, including electrophilic addition, nucleophilic addition, free-radical addition, and cycloadditions.

Addition reactions are essential in forming many organic compounds and are encountered in processes such as polymerization. It involves breaking double or triple bonds to form new single bonds. Addition reactions are essential in synthesizing various organic compounds and are used in analytic chemistry to identify the existence and number of double bonds in a molecule.

Addition Reaction Definition

Addition reaction is a chemical process in which an atom or group of atoms is added to a molecule to form a larger one (the adduct).

Addition reactions are limited to chemical compounds with multiple bonds, such as molecules with carbon–carbon double bonds, triple bonds, and compounds with rings.

Types of Addition Reactions

Addition reactions are essential in forming many organic compounds and are used to form large molecules by combining two or more molecules. Addition reaction are basically classified into two types which include:

• Polar Addition Reaction
• Non-Polar Addition Reaction

The detailed explanation of each of these reactions is given below:

Polar Addition Reactions

A polar addition reaction is a chemical reaction where an electrophile or nucleophile adds to multiple bonds to form an adduct. Polar addition reaction are further divided into two types which are:

• Electrophilic Addition Reactions: Involves the addition of an electrophile to a double bond.
• Nucleophilic Addition Reactions: Involves the addition of a nucleophile to a double bond.

Electrophilic Addition Reaction

In an electrophilic addition reaction, a reactant with a double bond is attacked by an electrophile, which adds the electrophile to the double bond. The mechanism of electrophilic addition can be illustrated using the example of adding hydrogen halide to an alkene, such as adding HCl to propene.

Step 1: The pi bond of the alkene acts as a nucleophile and attacks the hydrogen of the hydrogen halide, forming a carbocation intermediate and a halide ion.

CH₃CH=CH₂ + HCl → CH₃CH₂⁺+ Cl ^-

Step 2: The carbocation intermediate then reacts with the halide ion, forming the addition product.

CH₃CH⁺+ Cl ^- → CH₃CH₂Cl

Nucleophilic Addition Reaction

In a Nucleophilic Addition Reaction, a chemical compound with a double bond reacts with a nucleophile, adding the nucleophile to the double bond. The mechanism of nucleophilic addition can be illustrated using the example of adding a nucleophile to a carbonyl compound, such as adding a Grignard reagent to a ketone.

Step 1: The nucleophile, such as a Grignard reagent, attacks the carbonyl carbon, leading to the formation of an alkoxide intermediate.

R₂C=O + R'MgX → R₂C(OMgX)R'

Step 2: The alkoxide intermediate then picks up a proton from the solvent, forming the addition product.

R₂C(OMgX)R' + H₂O → R₂C(OR')R' + MgXOH

Non-Polar Addition Reactions

Nonpolar addition reactions involves the addition of nonpolar molecules or nonpolar functional groups to unsaturated compounds in the absence of a polar solvent or catalyst. Non-polar addition reaction are further divided into two types which are:

• Free Radical Addition Reactions: Involves the addition of a free radical to a double bond.
• Cycloaddition Reactions: Involves the addition of two or more unsaturated molecules to form a cyclic product.

Free Radical Addition Reactions

Free radical addition reactions involve the addition of free radicals to unsaturated substrates, such as alkenes which result in formation of a new covalent bond.

One example of a radical addition reaction is the addition of hydrogen bromide (HBr) to an alkene in the presence of a peroxide initiator. This reaction results in the anti-Markovnikov addition of H and Br to the alkene, where the bromine ends up on the less substituted carbon of the alkene.

Cycloaddition Reactions

Cycloaddition reactions involve the formation of cyclic compounds from two or more reactants. These reactions are characterized by the formation of multiple new bonds and the creation of a cyclic structure in the product.

One example of a cycloaddition reaction is the Diels-Alder reaction, which consists of the reaction of a conjugated diene with a dienophile to form a cyclic compound. An example of this reaction is the reaction of 1,3-butadiene with ethene to form cyclohexene.

Example of Addition Reaction

Examples of addition reactions include:

Hydrogenation of Alkenes

Hydrogenation is addition of hydrogen to an alkene in the presence of a catalyst to form an alkane. The double bond in the alkene is broken, and the hydrogen atoms are added across the two carbons of the double bond.

Example: Ethene reacts with hydrogen in the presence of a nickel catalyst to produce ethane.

C₂H₄ + H₂ → C₂H₆

Halogenation of Alkanes

Halogenation involves the addition of a halogen, such as bromine, to an alkane under high temperature which results in formation of alkyl halide.

Example: The addition of chlorine to methane, which is called chlorination of methane.

CH₄ + Cl₂→ CH₃Cl + HCl

Hydrohalogenation of Alkynes

Hydrohalogenation of alkynes involves the addition of a hydrogen halide (such as HCl or HBr) across the carbon-carbon triple bond of an alkyne to form a halogenated alkene.

Example: 2-butyne (methylacetylene) reacts with hydrogen chloride gas (HCl) to form 3-chloro-2-butyne.

CH₃​C ≡ CH + HCl → CH_3​CHCl

Polymerization

Polymerization is the process of joining monomers to form a polymer. In the case of addition polymerization, monomers containing a double bond react to form a polymer without losing any other atoms.

Example: Polymerization of ethene form polyethylene by adding ethene molecules to form a long-chain polymer.

CH₂ =CH₂​ → (-CH₂-CH₂-)_n

Application of Addition Reaction

Applications of addition reactions in chemistry include building molecular complexity, identifying double bonds, and organic synthesis in various industries.

• Addition reactions have applications in various industries, including food, pharmaceuticals, adhesives, and coatings, through organic chemical synthesis to build specific target molecules.
• In the food industry, addition reactions are involved in hydrogenating fats, which convert unsaturated fats into saturated fats. However, this process can lead to the conversion of cis double bonds to the trans configuration, which has drawbacks.

Difference Between Addition and Substitution Reaction

The comparison between Addition and substitution reaction is given below:

Read More,

• Types of Chemical Reaction
• Combination Reaction
• Electrophile and Nucleophile