Inductive Effect

Definition: “The polarization of a sigma (single covalent) bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect.” OR

“An inductive effect is the pull of electron density through sigma (σ) bonds caused by electronegativity differences of atoms.”

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Salient features of inductive effect:

• It is a permanent effect that arises due to electronegativity difference between two atoms forming a sigma bond.
• It influences the chemical and physical properties of compounds.
• Inductive effect is represented by an arrow head in the middle of the covalent bond pointing in the direction of the displacement of electrons.

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Explanation:

• When a σ-bond (single covalent bond) is formed between atoms of different electronegativity, the electron density is shifted more towards the more electronegative atom of the bond.

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• This causes a permanent state of bond polarization, where the more electronegative atom acquires a slight (partial) negative charge (δ^–) and the other less electronegative atom has a slight (partial) positive charge (δ⁺).

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• This transmission of induced charges along a chain of sigma bonded carbon atoms is known as inductive effect.

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• The inductive effect decreases as we move along a chain of carbon atoms, away from the electronegative atom.

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• The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol δ (delta) and the shift of electron density is shown by an arrow that points from δ⁺ end to δ^– end of the polar bond.

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• Example: Let us consider cholorethane (CH₃CH₂Cl) in which the C – Cl bond is a polar covalent bond. It is polarised in such a way that the carbon-1 gains some positive charge (δ⁺) and the chlorine atom acquires some negative charge (δ^–).

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• In turn carbon-1, which has developed partial positive charge (δ⁺) draws some electron density towards it from the adjacent C-C bond.

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• Consequently, some positive charge (δδ⁺) develops on carbon-2 also, where δδ⁺ symbolises relatively smaller positive charge as compared to that on carbon – 1.

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• In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarization of σ-bond caused by the polarization of adjacent σ-bond is referred to as the inductive effect.

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• This effect is passed on to the subsequent bonds also but the magnitude of inductive effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds.

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Types of Inductive Effect:

• The inductive effect is related to the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom.
• Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen.

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[1]. Electron withdrawing effect or Negative Inductive Effect (-I effect):

• When an electronegative atom or group (more electro negative than the H-atom) is attached to the terminal of the carbon chain in a compound, the electrons are displaced in the direction of the attached atom or group and is said to have electron attracting or negative inductive effect (-I).

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• Electron-Withdrawing Groups:
• Halogens (-X), hydroxy (-OH), carbonyl groups (C=O) and many other groups such as nitro (-NO₂), cyano (-CN), carboxy (-COOH), ester (-COOR), aryloxy (-OAr, e.g. – OC₆H₅), etc.

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• Following are various groups arranged in the decreasing order of their -I effect.

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[2]. Electron releasing/donating effect or Positive Inductive effect (+I effect): When an electro positive atom or group that is electron donating in nature (and repels electrons more strongly than hydrogen) is attached to the terminal of the carbon chain in a compound, the electrons are displaced away from the attached atom or group and is said to have electron releasing or positive inductive effect (+ I).

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• Electron Donating Groups: The alkyl groups like methyl (–CH₃) and ethyl (–CH₂–CH₃) etc. can help stabilize positive charges in reactions such as protonation of bases.
• Following are the various groups in the decreasing order of +I effect.

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Factors Affecting Inductive Effect:

Inductive effect of an atom or functional group is a function of that group’s (1). bonding order and charge, (2). position within a structure and (3). Electronegativity.

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[1]. Bonding Order and Charge: It is important to consider both the electronegativity and bonding order when analyzing the inductive potential of an atom.

• For example, oxygen in a hydroxyl group (OH) is electron withdrawing by induction (-I) because the oxygen atom is relatively electronegative and is uncharged in that bonding arrangement.

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• However, oxygen in an "alkoxide” (conjugate base of an alcohol, written as RO⁻) structure is electron donating (+I) by induction because in this bonding order, the oxygen atom has an "excess" of electron density.

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• Thus an OH group would help to stabilize a negative charge within a structure, while it's ionized form, the alkoxide, would stabilize a positive charge.

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[2]. Bonding Position:

• The strength of the inductive effect produced by a particular atom or functional group is dependent on it's position within a structure.

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• For example, the  inductive effect takes place through covalent bonds, and its influence decreases markedly with distance – thus a chlorine atom two carbons away from a carboxylic acid group has a decreased effect compared to a chlorine just one carbon away.

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• Definition of Electronegativity: It is a measure of the tendency of an atom to attract a bonding pair of electrons.

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[3]. Electronegativity: Inductive effect also depends on electronegativity.

• The more electronegative the atoms and the closer it is to the site of the negative charge, the greater the (-I) effect.

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• More electronegative atoms stabilize regions of high electron density by an electron withdrawing inductive effect.

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Example 1:

• Chlorine atoms are electronegative than a hydrogen and thus have a -I effect.

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• Alkyl groups – methyl, ethyl, and the like – are weak electron donating groups, and thus stabilize nearby carbocations because the electrons around the neighboring carbons are drawn towards the nearby positive charge, thus slightly reducing the electron poverty of the positively-charged carbon.

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• The greater the number of alkyl groups attached to the central carbon atom, the more stable is the carbocation. Thus tert-butyl carbocation is the most stable because electron-donating groups exert positive inductive effects to reduce the positive charge on the carbon atom.

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Why alkyl groups are showing positive inductive effect?

• Though the C-H bond is practically considered as non-polar, but actually there is partial positive charge on hydrogen atom and partial negative charge on carbon atom.

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• Therefore, each hydrogen atom acts as electron donating group.

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• This in turn makes an alkyl group, an electron donating group.

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Applications of Inductive Effect

[1]. Inductive Effect and Stability of a Molecule: The inductive effect can be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to it.

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Explanation:

• For example, if an atom has a positive charge and is attached to a −I group its charge becomes 'amplified' and the molecule becomes more unstable.

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• Similarly, if an atom has a negative charge and is attached to a +I group its charge becomes 'amplified' and the molecule becomes more unstable.

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• In contrast, if an atom has a negative charge and is attached to a −I group its charge becomes 'de-amplified' and the molecule becomes more stable.

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• Similarly, if an atom has a positive charge and it is attached to a +I group its charge becomes 'de-amplified' and the molecule becomes more stable.

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• The above explanation is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability.

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Examples:

Stability of carbonium ions.

• Presence of groups showing +I effect increases the stability of carbocation while presence of groups showing – I effect decreases their stability.
• Stability of carbocation (also called carbonium ions) increases with increase in number of alkyl groups due to their +I effect. The alkyl groups release electrons to carbon, bearing positive charge and thus stabilizes the ion.
• The order of stability of carbonium ions is given below:

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• Stability of free radicals: In the same way, the stability of free radicals increases with increase in the number of alkyl groups. Thus the stability of different free radicals is:

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Stability of carbanions: 

• However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density. 

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• Thus the order of stability of carbanions is:

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[2]. Inductive Effect and Strength of a Carboxylic Acid:

• The inductive effect also plays a vital role in deciding the acidity and basicity of a molecule.
• Any group or atom, which is highly electronegative help in removing the hydrogen atom as proton and the group or atom which is less electronegative than hydrogen atom makes the removal of proton difficult.

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• Hence (–I) effect group increases acidic strength and (+I) effect groups decreases the acidic strength of carboxylic acid.

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• As the number of -I groups attached to a molecule increases, its acidity increases; as the number of +I groups on a molecule increases, its basicity increases.

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[5]. Inductive Effect and Reactivity of Alkyl halides: 

• Alkyl halides are more reactive than the corresponding alkanes due to presence of C––X bond which is polar due to I effect, furthermore reactivity increases with increase of branching.

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• Due to –I effect of halogen atom, the carbon attached to the halogen acquires a partial positive charge and hence is readily attacked by a nucleophilie (negatively charged ion) CH₃→Cl or CH₃^δ+ Cl^δ-.

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• Alkyl halides undergo nucleophilic substitution reactions. As the number of +I showing R groups on the carbon with the leaving group increases, the rate of an S_N1 reaction increases.

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[6]. Effect on Bond lengths: 

• Since the inductive effect leads to ionic character in the bond, the increase in –I effect usually decreases the bond length.

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[7]. Dipole moment: 

• Since, inductive effect leads to a dipolar character in the molecule, it develops some dipole moment in the molecule, which increases with the increase in the inductive effect.

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Note: Dipole moment is the measure of the separation of electric charge.

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Increasing dipole moment

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Electromeric Effect

• Definition: “The complete and spontaneous transfer of shared pair of π-electrons of a multiple bond to one of the bonding atom at the time of approaching of a reagent is called electromeric effect”. OR
• “The electromeric effect (E effect) refers to the polarity produced in a multiple bonded compound as it is approached by a reagent.” OR
• “The electromeric effect is an intramolecular movement of electrons from a pi bond to another atom in the molecule due to attack by a reagent.”

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• It is represented by the symbol E and can be indicated by curved arrows symbolizing the displacement of electron pairs, as in:

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Characteristic Features:

[1]. Electromeric effect (E effect) is a purely temporary and reversible effect & is brought into play only at the requirement of attacking agent. This effect will remain as long as the attacking reagent is present. As soon as the reagent is removed, the polarized molecule will come back to the original state.

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[2]. It is shown by those compounds containing multiple bond.

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[3]. Due to complete transfer or migration of two π-electrons on one of the bonded atom, E effect results in development of unit positive or unit negative charges on bonded atoms. This results in increased attraction between substrate and reagent which in turn, favours the phenomenon of bond formation. In other words, it “increases chemical reactivity of substrate.”

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[4]. The reagent being added to multiple bond (which is also called, addendum) must be polar or an ionic reagent carrying either (a) positive charge (known as an electrophile) or (b) negative charge (known as nucleophile).

[5]. Being temporary, electromeric effect does not affect any of the physical properties of substrate molecule.

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Types of Electromeric Effect:

• Electromeric effect can be classified into two types i.e. + E and - E effects based on the direction of transfer of the electron pair.

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[1]. +E effect: In the positive electromeric effect, the pi electrons of the multiple bond are transferred to that atom to which the attacking reagent gets attached. E.g. addition of H⁺ to an alkene.

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Explanation:

• If the attacking reagent is an electrophile (positively charged species), it will attract the electron pair of π-bond due to its positive charge or due to its electron deficiency.
• Hence the transfer of two electrons of π-bond of substrate molecule will take place towards that positively charged atom from an atom of a multiple bond, which is nearer to attacking reagent.

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• This will cause one unit negative charge on atom which gains electron and also simultaneously one unit positive charge on other atom of the π-bond.
• After the transfer of electrons takes place, the reagent gets attached to the atom where the electrons have been transferred to.

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• This is called, "positive electromeric effect" and is represented by symbol, “+E effect”. It always produces carbocation intermediate.

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• Example: The addition of acids to alkenes is an example of the +E effect. If a positive charge like H⁺ is brought near ethene (CH₂=CH₂), the electron pair moves towards the attacking reagent and so the reagent gets attached to the atom where the electrons have been transferred to.

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[2]. -E effect: When the pi electrons of the multiple bond are transferred to the atom to which the attacking reagent does not gets attached is called the negative electromeric effect. E.g. addition of cyanide ion (CN-) to an alkene or a carbonyl group.

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Explanation:

• If the attacking reagent is a nucleophile (some negatively charged species like cyanide ion i.e. CN^– or OH^–), it will repel the electron pair of π-bond due to its negative charge or due to its high electron density.

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• Therefore, transfer of two electrons of π-bond of substrate molecule will take place from the attacking reagent to an atom of a multiple bond, which is away from the attacking reagent.

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• This will cause one unit positive charge on atom which looses electron and also simultaneously one unit negative charge on another atom.
• Therefore, in the presence of attacking reagent, one bond is lost and this negatively charged attacking reagent links to the carbon having positive charge.

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• It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.

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• This is called, "negative electromeric effect" and is represented by symbol, “-E effect”. It always produces carbanion intermediate.

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Illustration of -E effect

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• The nucleophilic addition reaction between ethanal (which is also known as acetaldehyde) and hydrogen cyanide to yield acetaldehyde cyanohydrin (1-hydroxy-1-methylpropanenitrile) is an example of -E effect.

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• Here, oxygen of carbon-oxygen double bond becomes negatively charged due to -E effect caused by cyanide nucleophile.
• The other aspects of this reaction can be shown as shown above in case of +E effect.

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Difference between Inductive and Electromeric Effects

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