Nomenclature and General Principles: Types of Reactions in Organic Compounds (For CBSE, ICSE, IAS, NET, NRA 2022)

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Types of Reactions in Organic Compounds

A chemical reaction is accompanied by breaking of some bonds and by making of some others.

The different types of reactions in organic compounds are:

  • Substitution
  • Elimination
  • Addition
  • Molecular Arrangement

Breaking of Covalent Bond – Types of Bond Fission

  • Chemical reactions involve breaking of one or more of the existing chemical bonds in the reactant molecules and formation of new bonds leading to products.
  • The breaking of a covalent bond is known as bond fission.

There are two types of bond fission:

Homolytic Fission

The fission of a covalent bond with equal sharing of bonding electrons is known as Homolytic fission.

Homolytic Fission
  • The neutral species so formed are known as free radicals.
  • Free radicals are neutral but reactive species having an unpaired electron and these can also initiate a chemical reaction.

Heterolytic Fission

  • The fission of a covalent bond involving unequal sharing of bond electrons is known as Heterolytic fission.
  • This type of bond fission results in the formation of ions.

The heterolytic fission of a hypothetical molecule is given:

  • The ion which has a positive charge on the carbon atom, is known as the carbonium ion or carbocation.
  • An ion with a negative charge on the carbon atom is known as carbanion.

Electrophiles

  • An electrophile is an electron deficient species and it may be positive charge or neutral molecule.
  • Examples are , , , , , , etc.

Nucleophiles

  • A nucleophile is negatively charged or electron rich neutral species.
  • Examples of nucleophiles are , , , etc.

Electron Displacement in a Covalent Bond

The effects involving displacement of electrons in the substrate molecules (molecule under attack of a reagent) are known electron displacement or electronic effect.

Some of these effect are given below:

Inductive Effect

  • The transmission of induced charges along a chain of bonded carbon atoms is known as inductive effect.
  • In a covalent bond between the two dissimilar atoms, the shared electron pair is attracted more towards the atom having greater electronegativity.
  • Consider an example of haloalkanes. The halogen atom (X) being more electronegative than carbon atom, pulls the bonded electrons of the bond. Thus , the bond is polarized as shown below:

The carbon atom gets a partial + ve charge and halogen atom a partial negative charge . This positively charged attracts bonded electrons of bond, thus making atom a little less positive than .

Inductive Effect

Many of the properties of organic compound such as acidic strength of carboxylic acids are explained on the basis of inductive effect.

Groups with – I Effect (Electron Withdrawing Groups)

  • Any atom or group of atoms that withdraws electrons more strongly than the H-atom is said to have effect.
  • Various group arranged in decreasing order of their effect are as follow:

Groups with + I Effect (Electron Releasing Groups)

  • Any atom or group of atoms that repels electrons more strongly than hydrogen, is said to have effect.
  • Various group arranged in decreasing order of their effect are as follow:

Electrometric Effect

  • This type of temporary electron displacement takes place in compounds containing multiple covalent bonds.
  • It involves the complete transfer of electrons resulting into the development of + ve and – ve charges within the molecule.
  • This is takes place in the direction of more electronegative atom.
  • The electrometric effect is represented by the symbol E.
  • It is known as + E effect when displacement of electron pair is away from the atom or group, or – E effect when the displacement is towards the atom or group.
  • For example, in carbonyle group
Electrometric Effect

Resonance

  • In this phenomenon in which a number of organic molecules can be represented by two or more structures which are called resonating or canonical structure.
  • None of those structures explains all the properties of the compound.
  • For example, the benzene molecule may be represented by the following two resonating structures, I and II.
The Structures of Resonance

Hyperconjugation

  • Hyperconjugation is also known as no-bond resonance.
  • It involves the conjugation of (sigma) bond with (pi) bond.
  • For example, hyperconjugation in propene can be represented as follows:
The Structures of Hyperconjugation

The structures II to IV have no bond between one of the H-atom and the C-atom.

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