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Reaction Intermediates

Reaction intermediates are generated by the breaking of covalent bond of the substrate. They are short-lived species and are highly reactive.

There are six types of reaction intermediates: 

(1) Carbocation 

(2) Carbanion 

(3) Free radical

(4) Carbene 

(5) Benzyne

(6) Nitrene

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Carbocations

An organic species which has a carbon atom bearing six electrons in its outermost orbit and has a positive charge is called a carbocation.

  • It has three bond pairs with empty p-orbital. Its hybridization is sp2.
  • Shape of carbocation is trigonal planar.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

[Note: Triphynylmethyl carbocation has propeller shape.]

  • There are six electrons in the outermost orbit of carbocations carbon hence its octet is incomplete. All six electrons are paired.
  • It is charged electrophile. 
  • It is diamagnetic in character.
  • It is formed by heterolytic bond fission. 
  • It reacts with nucleophiles.

Question for Introduction to Reaction Intermediates & Carbocations
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Which type of reaction intermediate has a trigonal planar shape and a positive charge?
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Classification of Carbocation

(i) Classical Carbocation: A classical carbocation is an ion containing a positively charged carbon atom which has six electrons that take part in three chemical bonds. We can name this carbon atom as a three-coordinate positive carbon.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

To ensure maximum stability, the carbon atom should have eight valence electrons. But in the carbocation, there are only six electrons in the carbon atom having a positive charge. Therefore, it tends to share two more electrons from an electronegative species. This makes the carbon atom stable and neutralizes the positive charge. This is the reason for the high reactivity of classical carbocations. However, the energy of a classical carbocation is low compared to the energy of the corresponding nonclassical carbocation. But this difference in their energies is very small.

(ii) Non-Classical Carbocation: A nonclassical carbocation is an ion containing a positively charged carbon in a three-center two-electron center. This means, there are three atoms sharing two electrons in these carbocations. This type of electron sharing is named as delocalization of the electrons. 

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

The most common example of a nonclassical carbocation is 2-norbornyl cation. It exists in a less symmetrical three-center two-electron structure. There is very little difference in the energy between classical and nonclassical carbocations. Therefore, it is very difficult to distinguish them experimentally.

Question for Introduction to Reaction Intermediates & Carbocations
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Which type of carbocation is characterized by a positively charged carbon atom with six electrons and tends to share two more electrons from an electronegative species to neutralize the positive charge?
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Generation of Carbocation

Through direct ionisation of Introduction to Reaction Intermediates & Carbocations | Organic Chemistry bond where X leaves the molecule with bonding pair.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(i) From Alkyl halides: By SN1 reaction conditions, by Lewis acids

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(ii) From alkene/alkynes:

By adding H+ (acids)like, H2SO4, H3PO4, TsOH, TfOH, BsOH etc. on alkene or alkyne.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Note: HNO3 is not used because it is oxidizing agent & oxidized alkene into aldehyde & ketones

(iii) From alcohol

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(v) From Acyl halides:

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Note: In some intramolecualr f.c. reactions (in Haworth synthesis etc.) both acylation & alkylation may give carbonation.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(vi) From Primary amine: Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

[Note: Aromatic diazonium salts are more stable than aliphatic]

Question for Introduction to Reaction Intermediates & Carbocations
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Which of the following methods can be used to generate carbocations?
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Fate of Carbocation

Carbocations are most often short-lived transient species and undergo three basic types of reactions.

(i) Combination with a nucleophile

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(ii) Elimination of proton

Carbocation may loose a proton from the adjacent atom.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(iii) Addition to an unsaturated linkage

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability of Carbocation

Stability of Carbocation can be gained by

  • Ring expansion
  • Ring contraction
  • Rearrangement
  • Aromaticity
  • Resonance or mesomeric effect
  • Inductive effect
  • Hyperconjugation

Reactions of Carbocation

After formation of carbonation, we have to follow the following flow chart for the reaction.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(i) Stability of carbocation by ring expansion: It takes place when carbocation will formed adjacent to small ring

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Another example-

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(ii) Ring Contraction

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Another Example:

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(iii) Rearrangement of carbocation in electrophilic addition reaction.

(a) By 1, 2-hydride shift

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Mechanism of the 1, 2 hydride shift

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(b) By 1, 2-methyl shift

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(c) By 1, 2-phenyl shift

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(iv)  Stability of Carbocation by Aromaticity

(a) Cations in which positive charge is present on carbon of aromatic system is known as aromatic carbocation.

(b) Aromatic carbocations are so stable that even their solid states are known. For example, tropolium carbocations as tropolium bromide is a yellow solid. In fact tropolium carbocation is about 1011 times more stable than triphynylmethyl carbocation.

(c) Cations obeying Huckel (4n + 2) rule are stable because they are aromatic and there is complete delocalization of positive charge.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(v) Stability of carbocation by resonance:

(a) Allyl carbocation:Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

The stability of primary, secondary and tertiary allyl carbocations can be compared by

(a) Inductive effect (b) Hyperconjugation (c) Resonance

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(b) Phynylmethyl carbocation

The stability of phenylmethyl carbocations can be explained by resonance.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Phenylmethyl carbocations are more stable than allyl carbocations due to the number of resonating structures.

(c) Cyclopropylmethyl carbocation

(i) These carbocations are very stable carbocations. They are more stable than benzyl carbocations.

(ii) Stability of cyclopropyl methyl carbocations increases with every cyclopropyl group. Thus additions cyclopropyl group has a cumulative additive effect on the stability. Thus.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(iii) The special stability is a result of conjucation between the bent orbitals of the cycloproyl ring and the vecant-porbital of the cationic carbon. This type of bonding is called as banna bonding.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(e) Vinyl carbocation

When the positive charge is present on vinylic carbon then carbocation is known as vinyl carbocation;

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

This carbocation is the least stable because a positive charge is present on the electronegative carbon (sp-hybridized).

(vi) State of hybridization and stability

The positive charge is more stabilized on the less electronegative carbon atom. Hence, increasing s-character increases electronegativity, and its capability to stabilize positive charge decreases.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(decreasing stability with increasings character in its state of hybridization)

Question for Introduction to Reaction Intermediates & Carbocations
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Which of the following factors contributes to the stability of carbocations?
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Alkyl carbocations

The stability of alkyl carbocations can be explained by:

(a) Inductive effect (b) Hyperconjugation

According to these inductive effects the stability order is as follows:

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Bridge heads carbocations

Bridge heads cannot attain planar configuration. Therefore, a carbocation is never formed at the bridgehead.

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Examples for illustration

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

(When + change is on the C of benzene ring then resonance effect don’t work)

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability order: A < B < C < D

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability order: A > C > D > B

(R effect is equal to at O & P but I effect is distance dependent)

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order :D > B > A > C

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order :D > B > A > C

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order: C > A > B > E > F > D

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order: D > E > A > C > B

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order: D > C > A > F > E > B

Introduction to Reaction Intermediates & Carbocations | Organic Chemistry

Stability Order: F > E > C > A > B > D Wrong

Stability Order: F > E > C > D > B > A Right

The document Introduction to Reaction Intermediates & Carbocations | Organic Chemistry is a part of the Chemistry Course Organic Chemistry.
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FAQs on Introduction to Reaction Intermediates & Carbocations - Organic Chemistry

1. What are reaction intermediates in chemistry?
Ans.Reaction intermediates are transient species that occur during the conversion of reactants to products in a chemical reaction. They are formed in one step and consumed in the next, making them difficult to isolate. Common types of reaction intermediates include carbocations, carbanions, and free radicals.
2. How are carbocations classified based on their structure?
Ans.Carbocations can be classified based on their carbon atom's degree of substitution: primary (attached to one other carbon), secondary (attached to two other carbons), and tertiary (attached to three other carbons). Tertiary carbocations are generally more stable than secondary, which are more stable than primary due to the inductive and hyperconjugation effects.
3. What are the common methods for generating carbocations?
Ans.Carbocations can be generated through various methods, including the ionization of alkyl halides, dehydration of alcohols, and by the addition of strong acids to alkenes. In these reactions, the removal of a leaving group or a proton leads to the formation of a positively charged carbon atom.
4. What are the possible fates of a carbocation once it is formed?
Ans.Once formed, a carbocation can follow several pathways: it can undergo nucleophilic attack by a nucleophile to form a new bond, rearrange to a more stable carbocation via hydride or alkyl shifts, or lose a proton to form an alkene. The specific fate often depends on the stability of the carbocation and the surrounding reaction conditions.
5. Why is the stability of a carbocation important in chemical reactions?
Ans.The stability of a carbocation is crucial because it influences the reaction pathway and rate. More stable carbocations (like tertiary) are more likely to form and persist during a reaction, leading to faster reaction rates and different products. Understanding carbocation stability helps predict reaction mechanisms and outcomes in organic chemistry.
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