Inductive Effect: Execution in Organic Chemistry

This article details what is Inductive Effect and its Execution in Organic Chemistry.

what is Inductive Effect?

The covalent bond between the two atoms with dissimilar electronegativity, electron pair forming the bond is never shared absolutely equally. But is attracted a little more towards the more electronegative atom. Such a covalent bond is known as a polar bond.

For example, in compound C—X where X is more electronegative than C, and C—Z C is more electronegative than Z

It is a permanent effect that is transmitted along the carbon-carbon chain. As the length of the carbon chain increases inductive effect decreases long the carbon chain. But the effect is insignificant beyond the third carbon atom

Types of Inductive Effect:

The electron attraction and repulsion are compared with hydrogen as the reference in the molecule R3C—H as standard.

  1. Any atom or group that attracts electrons more strongly than hydrogen is said to have a – I effect
  2. while the atom or group that attracts electrons less strongly than hydrogen is said to have + I effect

The important atoms or groups which cause negative or positive inductive effect are arranged below in the order of decreasing effect.

Alkynyl, vinyl, and phenyl groups, because of sp and sp2 hybrid carbon atom exert a weak-I effect. Carbon atoms that are sp and sp2 hybridized are electronegative in character due to the dominance of ‘s’ character over ‘p’ character. [50% ‘s’ character in ‘sp’ and 33% ‘s’ character in sp2 hybrid carbon]

All alkyl and cycloalkyl groups because of sp3 hybrid carbon atom exert +I effect. An sp3 hybrid carbon atom is an electron releasing (donating) due to the dominance of ‘p’ character over ‘s’ character. [75% p character]

Inductive Effect: Execution in Organic Chemistry

1. Stability of intermediates:

Carbocation: An alkyl group stabilizes an electron-deficient carbocation in two ways:

  1. through an inductive effect, and
  2. through the partial overlap of filled orbitals with empty ones.

The inductive effect is a donation of electron density through the sigma bonds of the molecule. The positively charged carbon atom withdraws some electron density from the polarizable alkyl groups bonded to it.

Stability ofcarbocations 3° > 2° > 1 ° > methyl

SN1 reaction substituent effect: The rate-limiting step of the SN1 reaction is ionization to form a carbocation, a strongly endothermic process. Reactivity toward SN1 substitution mechanisms follows the stability of carbocations:

Alkyl groups hinder the SN2 by blocking the attack of the strong nucleophile, but alkyl groups enhance the SN 1 by stabilizing the carbocation intermediate.

Carbanions: that occur as intermediates in organic reactions are almost always bonded to stabilizing groups. They can be stabilized either by inductive effects or by resonance.

For example, halogen atoms are electron-withdrawing, so they stabilize carbanions through the inductive withdrawal of electron density

2. Addition of water to aldehydes and ketones

Electronegative atoms such as halogens attached to the carbon atoms next to the carbonyl group can increase the extent of hydration by the inductive effect.

Based on the number of halogen substituents and their electron-withdrawing power, they increase the polarization of the carbonyl
group. Already has a positively polarized carbonyl carbon, and halogen substituents make it even more prone to attack by water.

Rate of carbonyl group hydration:

3. Substituents affect the pKa

Inductive effects of nearby electronegative atoms can also have marked effects on the pKa of acids. Adding fluorines to acetic acid reduces the pKa. Trifluoroacetic acid (TFA) is a very strong acid.

Inductive effects occur by the polarization of σ bonds when the atom at one end is more electronegative than at the other. Fluorine is much more electronegative than carbon so each σ bond is very polarized, making the carbon atom more electropositive and stabilizing the carboxylate anion

4. How electron donation and withdrawal change chemical shifts

Electron-withdrawing groups -I effect Deshields the protons large chemical shift value is seen. While electron-donating groups +I effect fewer deshields the protons low chemical shift value.

5. Acid derivatives are best distinguished by infrared

Conjugation donates electrons into the π* orbital of the π bond and so lengthens and weakens C=O. The C=O the bond becomes more like a single bond and its stretching frequency moves towards the single bond region, that is, it goes down.

The inductive effect removes electrons from the π orbital and so shortens and strengthens the π bond. It becomes more like a full double bond and
moves up in frequency.

6. Selectivity between ortho and para positions

With large substituents, such as the amide, steric hindrance will be significant and it gives more para product. Also, the inductive electron-withdrawing effect decreases the amount of ortho substitution.

Oxygen and nitrogen, although they are electronegative, activate the ring towards attack by donating π electron density from their lone pairs. At the same time, the C–O or C–N σ bond is polarized back towards the O or N atom by withdrawing electron density from the σ framework.

This is inductive electron withdrawal—it affects the atoms nearest the O or N atom the most, and has the effect of decreasing the likelihood that attack will happen in the ortho positions

7. directing and activating effects

8. Inductive Effect: Addition–elimination reaction

An anion-stabilizing (electron-withdrawing) group ortho or para to a potential leaving group can be used to facilitate nucleophilic aromatic substitution.

Fluoride accelerates the first step through its inductive effect. It is the most electronegative element of all and it stabilizes the anionic intermediate, assisting the acceptance of electrons by the benzene ring.

How good a leaving group it might be does not matter: the rate of the second step—the step where fluoride leaves— has no effect on the overall rate of the reaction.

The activating anion-stabilizing substituent

Nitro is the best activating group, but the others will all perform well, especially when combined with a fluoride rather than a bromide as the leaving group.

9. The benzyne mechanism

show how the meta product might be formed, but why should it be formed? The attack could also occur at the ortho position, so why is there no ortho product?

There are two reasons: electronic and steric.

Electronically, the anion next to the electronegative oxygen atom is
preferred because oxygen is inductively electron-withdrawing. The same factor facilitates deprotonation next to Cl in the formation of the benzyne.

Sterically, it is better for the amide anion to attack away from the OMe group rather than come in alongside it. Nucleophilic attack on benzyne has to occur in the plane of the benzene ring because that is where the
orbitals are.

10. Amine synthesis using functional group interconversions

If the alkylating agent contains an inductive electron-withdrawing group, the product may be less reactive than the starting material—benzylamine was only alkylated once by the alkyl bromide

Alkylations with epoxides usually stop after the first step because the inductively electron-withdrawing hydroxyl group in the product makes it less nucleophilic than the starting material

11. Amine relative rate of reaction

The extra heteroatom, through an inductive effect, withdraws electron
density from the nitrogen atom, making it less nucleophilic and less basic. In this sense, morpholine can be a very useful base, less basic than triethylamine but somewhat more so than pyridine (pKa 5.2).

Inductive Effect: Execution in Organic Chemistry is an attempt to cover comprehensively all important applications with suitable examples. Hope this article helps the student to understand the concepts of chemistry built based on inductive effect.

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