Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Organic Chemistry

IIT JAM : Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

[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.

Classification of Carbocation

(i) Classical Carbocation:

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(ii) Non-Classical Carbocation: These are usually less stable than classical carbocation.

They have special but weak stabilization by σ bond π bond which is not allylic & benzylic. (showing remote stabilization), discovered by Olah (1996), 

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Generation of Carbocation

Through direct ionisation of Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev bond where X leaves the molecule with bonding pair.

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(ii) From alkene/alkynes:

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

(iii) From alcohol

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(v) From Acyl halides:

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(vi) From Primary amine: Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(ii) Elimination of proton

Carbocation may loose a proton from the adjacent atom.

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(iii) Addition to an unsaturated linkage

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Another example-

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(ii) Ring Contraction

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Another Example:

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(iii) Rearrangement of carbocation in electrophilic addition reaction.

(a) By 1, 2-hydride shift

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Mechanism of the 1, 2 hydride shift

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(b) By 1, 2-methyl shift

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(c) By 1, 2-phenyl shift

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(v) Stability of carbocation by resonance:

(a) Allyl carbocation:Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev


(b) Phynylmethyl carbocation

The stability of phenylmethyl carbocations can be explained by resonance.

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

(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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev


(e) Vinyl carbocation

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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


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 intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Bridge heads carbocations

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev


Examples for illustration

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Stability order: A < B < C < D

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Stability order: A > C > D > B

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Stability Order :D > B > A > C

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

Stability Order :D > B > A > C

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

Introduction to Reaction intermediate - Reaction intermediate Chemistry Chemistry Notes | EduRev

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

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

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