All About Addition Reactions

Definition

The polar opposite of an addition reaction is an elimination reaction. For example, dehydration converts alkene to alcohol by reverse tracing the hydration process.

The two fundamental types of polar addition processes are electrophilic and nucleophilic addition. Non-polar addition processes involve free-radical addition and cycloadditions. Adding polymerization is a type of polymerization that includes addition operations. The product’s structure may dictate how rapidly it reacts to expel a leaving group, resulting in an addition–elimination reaction chain.

Addition reactions occur in unsaturated organic compounds with a carbon-to-carbon double bond, alkynes with a carbon-to-carbon triple bond, and aldehydes and ketones with a carbon-to-oxygen double bond. A process in which double or triple bonds are broken entirely or partially in order to admit extra atoms or groups of atoms in a molecule is known as an addition reaction. Addition reactions to alkenes and alkynes are commonly referred to as saturation reactions because they lead the carbon atoms to become saturated with the largest number of linked groups.

A frequent addition reaction is a hydrochlorination of propene (an alkene), which has the equation CH₃CH = CH₂ + HCl → CH₃C+HCH₃ + Cl^– → CH₃CHClCH₃.

The positively charged hydrogen chloride ion, H+, joins one of the pair of carbon atoms joined by double bonds—in this case, the less alkylated carbon atom—followed by the negatively charged chloride ion, Cl, joining the other carbon atom.

In reactions to aldehydes and ketones, the order of events is reversed, with the negatively charged component of the reagent being added to the carbon atom first, then the positively charged component to the oxygen atom. As a result, a methyl ketone (CH3(C = O)R, where R is an alkyl group) reacts with hydrogen cyanide in the following way: 

Electron-seeking (electrophilic) reagents are expected to initiate propene hydrochlorination or the addition of alkenes in general, whereas electron-rich (nucleophilic) reagents initiate the addition of alkynes, aldehydes, and ketones. Other forms of addition reactions include catalyzed addition processes, such as the self-addition of alkenes (catalyzed by acids) or the hydrogenation of alkenes, aldehydes, and ketones (catalyzed by metals); cyclic compound formation and chain mechanism of addition reactions.

Addition Reaction Examples:

Halogenation

C₂H₄ + Br₂ → C₂Br₂H₄

Ethene + Bromine → Dibromoethane

• Hydration:

C₂H₄ + H₂O → C₂H₅OH

Ethene + Water → Ethanol

Hydrogenation

C₂H₄ + H₂ → C₂H₆

Ethene + Hydrogen → Ethane

Polymerization

(C₂H₃Cl)n → -CH₂-CHCl-CH₂-CHCl-CH₂-CHCl-

Vinyl Chloride → Polyvinyl Chloride

Addition Reaction of Alkyne

C₂H₂ + HI → C₂H₃I

Ethyne + Hydrogen Iodide → Ethylene Iodide

Types of Addition Reaction

• Nucleophilic addition reaction
• Electrophilic addition reaction 
• Free radical addition reaction

In Addition Reaction With Nucleophiles

The species that transfer a pair of electrons to create a covalent bond are known as nucleophiles, and they are also known as electron-rich species. In electrophilic unsaturated compounds, nucleophilic addition reactions are common. 

Nucleophilic addition reactions are the most common form of aldehydes and ketones. The presence of larger groups surrounding the sp² hybridized carbon reduces electrophilicity, creates impediments for the nucleophile, and so reduces the possibility of nucleophilic addition in organic compounds.

Electrophilic Addition Reaction

Electrophiles accept a pair of electrons in order to establish a new covalent bond. They are commonly known as electron-deficient species.

The electrophile is attacked by the pi electrons of the unsaturated species in an electrophilic addition process, and the nucleophile targets the electron-deficient species in the following step.

The major substances that undergo electrophilic addition reactions are alkenes and alkynes.

The Kharasch effect, also known as the Anti-Markovnikov addition reaction, is a free radical addition reaction. Only when the reagent is HBr and the solvent is peroxide can this unique addition reaction occur. Free radical addition is not seen in any other species. It controls how many hydrogen-free radicals are added to the intermediate form.

When unsymmetrical alkynes or alkenes react with HX (particularly HBr) in the presence of peroxide, a free radical is generated on the carbon with fewer hydrogen attached to it, according to Anti-Markovnikov’s rule.

Conclusion

An addition reaction is any of a series of chemical reactions in which an atom or group of atoms is added to a molecule. Addition reactions occur in unsaturated organic compounds with a carbon-to-carbon double bond, alkynes with a carbon-to-carbon triple bond, and aldehydes and ketones with a carbon-to-oxygen double bond. A process in which double or triple bonds are broken entirely or partially in order to admit extra atoms or groups of atoms in a molecule is known as an addition reaction.