Inductive and Mesomeric Effects: Introduction and Types of Electronic Effects

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Learning Outcomes

After studying this lesson, you shall be able to:

Understand inductive effects

Introduction

  • You are already familiar with various electronic effects.
  • But to understand the properties of organic molecules, like acidity, basicity, stability of various species, and to understand the organic reactions and their mechanism, it is very important that we visit the electronic effects operating especially with a detailed study of conjugated chemical bonding.
  • During most of the organic reactions, reagents either undergo heterolysis to give rise to electrophiles and/or nucleophiles. or they undergo homolyses to give rise to free radicals.
  • Electrophiles being electron deficient search for electron rich site in the organic substrate to attack and form the bond.
  • But, if there are more than one electron rich sites in the organic substrate, the electrophile obviously would prefer to attack on the most electron rich site.
  • On the other hand, nucleophiles being electron rich, search for electron deficient sites in the organic substrate.
Electronic Effects
  • How these electron deficient and electron rich sites are created (or occur) in the substrate and how can we predict them by looking at the structure of the substrate so that we may predict the attack and hence the course of the reaction?
  • This can be done by understanding and analysing the electronic effects operating or present in the substrate.
  • Electronic effects are the effects originating or present in the organic molecules due to which the reactivity at one part of a molecule is affected by electron attraction or repulsion originating in another part of a molecule.
  • Let us know about different types of electronic effects.

Types of Electronic Effects

There are four types of electronic effects mainly, viz.

  • inductive effect,
  • Mesomeric (or resonance) effect,
  • electrometric effect and
  • hyper conjugative effect.

Of these, electrometric effect is temporary while rest are permanent and are exhibited as the dipole moment of the molecule.

  • If we consider hydrocarbons, they are non-polar. But the compound having functional groups i.e.. having hetero atom (i.e.. , atoms other than carbon) or groups are polar.
  • While studying all these effects, we shall consider how the presence of a hetero atom/group introduces polarity in the organic molecules.
  • It is in the context of this group which is known to exert different types of effects on the remaining part of the molecule.

Inductive Effect

  • Ideally, a covalent bond between two atoms must share the pair of electrons equally.
  • If the electronegativities of the two atoms are different, this sharing of electrons is not equal, and the more electronegative atom gets a fractional negative charge due to the greater attraction of the shared pair of electrons towards itself and the other atom gets a fractional positive charge.
  • Fractional or partial negative charge is represented by δ- (pronounced as delta minus) and
  • Fractional or partial positive charge is represented by δ + (pronounced as delta plus) ,
  • δ signifying the very less magnitude of charge
  • For example, ( halogen) bond is polar with C having a δ + charge and the halogen having a charge.
  • The magnitude or value of these fractional charges increases with an increase in the electronegativity of the halogen i.e.. for halogens, it is in the order .
  • Let us consider a carbon chain, with the end carbon joined to a more electronegative atom say Cl, then Cl will attract electrons (σ electrons between and ) more towards itself.
  • Hence, the carbon atom will become slightly electron deficient and get a fractional positive charge due to a difference of electronegativity.
  • This polarization is not restricted till here, but it induces polarity further and hence the name as inductive effect.

MCQs

1. Which of the following attack on electron rich sites of the substrate

a) Electrophile

b) Nucleophiles

c) Both

d) None of the above

Answer: A

2. Which of the following exhibited temporary dipole moment of the molecule.

a) inductive effect,

b) Mesomeric (or resonance) effect,

c) electrometric effect

d) hyper conjugative effect

Answer: C

#Understand inductive effects

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