Carbocation

by

[I Manoja Devi][Msc org Chem][101309503051]

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TABLE OF CONTENTS

CARBCATION:

Structure

Stability

Generation of Carbocation’s

Reaction’s of Carbocation’s

Application’s

Non-Classical Carbocation’s

Detection of Carbocation’s

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CARBOCATION

Carbocation’s are the intermediate’s formed during heterolytic cleavage of covalent bond in alkyl halides.They are short lived species formed during the course of reaction.

RX R+ + X-

Majority of organic reactions proceeds via the formation of short lived species which are highly reactive are known as “Reactive Intermediates”. They are unstable, cannot be isolated to study during the course of reaction because of their less stability and high reactivity.

Ex: Carbocation, Carbanion, Carbon free radical, Carbine, Nitrene, Benzyne - Unstable.

Ylides & Enamines - Stable

REACTANTS REACTIVE INTERMEDIATES PRODUCTS

Carbocation’s are intermediate’s in several kinds of reactions. The more stable one have been prepared in solution & in some cases even as solid salts. In solution, the carbocation may be free (in polar solvents) or it may exist as an ionpair, which means that it is closely associated with a negative ion called a counter ion or gegenion. Ionpairs more likely in nonpolar solvents.

STRUCTURE:

Carbocations are species bearing a formal "+" charge on carbon. They have sp2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents (see diagrams shown to the right). Carbocation’s are "hypovalent" species, in as much as they have only three shared pairs of electrons around carbon, instead of the usual four. Of course, this incomplete octet around carbon makes carbocation’s very unstable and very reactive..

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There are basically two types of carbocation’s: those that are not stabilized by resonance effects, and those that are stabilized by resonance (either through lone pair electrons on adjacent atoms or through conjugated pi bonding electrons).

Simple Alkyl Carbocation’s:There are four possible degrees of alkyl substitution on a carbocation: three attached alkyl groups, two attached groups, one attached group, or no attached alkyl groups. These would be referred to as tertiary (3°), secondary (2°), primary (1°), or methyl carbocation’s, respectively. These four possibilities have been shown below.

Structure

Name tert-butyl cation isopropyl cation ethyl cation methyl cation

(type) (a 3° carbocation) (a 2° carbocation) (a 1° carbocation)

Relativestability

most stablenext-to-most

stablenext-to-least

stableleast stable

An essential requirement for such stabilization is that the carbocation should be planar due to effective delocalization. According to Quantum Mechanical Calculation’s, planar (sp2) configuration is more stable than the pyramidal (sp3) by ~84 KJ (20 K.cal) mol-1.

As planarity is departed leads to instability of the carbocation & consequent difficulty in its formation increase very rapidly. This has been seen in extreme inertness of 1-bromo tryptycene to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation.

Br

Ph C Ph SN1 No Reaction

Ph

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The expected planar structure of even simple cation’s has been confined by analysis of N.M.R & I.R spectra of species such as Me3C+SbF6

- ; they thus parallel the trialkyl boron's, R3B with which they are isoelectronic.

STABILITY:

Order of stability of examples of tertiary (III), secondary (II), and primary (I) alkyl carbocations

The sequence is resulting from increasing substitution of the cationic carbon atom leading to increasing delocalization of the positive charge by Inductive, Hyperconjugation, Mesomeric & solvation effect’s. stability of Me3C+ is borne out by the fact that it may often be formed under vigorous conditions, by the isomerisation of other first formed carbocation’s & also by the observation that it remained unchanged after heating at 1700 in SbF5 / FSo3H for 4 weeks.

CH3F + SbF5 CH3+SbF6

-

1) INDUCTIVE EFFECT:

Displacement of sigma electrons towards the more electronegative atom is called “Inductive Effect”. All electron donating groups containing lonepair of electron’s are ‘+I’ groups & all electron withdrawing groups are ‘–I’ groups.

C+ R

R groups (+I groups) donate the electrons towards carbocation & increase in the stability of carbocation takes place. Therefore +I groups increase the stability of carbocation.

C+ W

W groups (-I groups) withdraw the electrons from carbocation so,stability of carbocation decrease. Therefore –I groups decrease the stability of carbocation.

Ex : Stability Order: C+ OCH3 > C+ CN

2) HYPERCONJUGATION EFFECT:

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This is also called as “No bond resonance effect”. Greater the –CH3 groups attached to carbocation, greater will be the stability.

9 Hyperconjugative Structure’s.

6 Hyperconjugative Structure’s.

3 Hyperconjugative Structure’s.

3) RESONANCE / MESOMERIC EFFECT:

Higher the resonance in the structure , higher will be the stability.

The Resonance Stabilization of the Allylic Carbocation

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The Resonance Stabilization of the Benzyl Carbocation:

As in the Benzyl C+ , if H’s are replaced by the Phenyl rings the stability still increases.

Ph H H

Ph C+ > Ph C+ > Ph C+

Ph Ph H

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Over All Order Of Stability:

Tropylium C+ > TriPhenyl Methyl C+ > C6H5–CH2+ > CH2=CH–CH2

+ >

The Cyclo Propyl rings present on the carbocation also stabilized.

In this carbocation,the P-Orbitals of CH2+ are parallel to P-Orbitals of Cyclo Propyl carbon

atom’s. Delocalization of positive charge is increased to maximum extent. Therefore stability is increased.

Order:

H

C+ > H C+ > H C+ > H C+

H H

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4) SOLVATION EFFECT:

Solvation Effect increases the stability of carbocation. The carbocation should be inert to the solvent. It should not react with solvent to give some other product.

Stabilization can also occur again by delocalization through the operation of a Neibouring Group Effect resulting in the formation of a “Bridged Carbocation”. Thus the action of SbF5

in liquid So2 on P-MeOC6H4CH2CH2Cl(1) results in formation of (2) rather than the expected cation (3). Phenyl ring acting as Neighbouring Group.

OMe : OMe OMe SbF5/SO2

SbF5Cl-

-700

CH2 CH2+ H2C CH2 Cl H2C CH2

(3) (1) (2)

Such species with a bridging phenyl group are known as “Phenonium Ions”. The NG Effect is even more pronounced with ‘-OH’ rather than an ‘-OMe’ substituted in P-Position . solvlysis is found to occur ~106 times more rapidly under comparable conditions & matter’s can be so arranged as to make possible the isolation of a bridged intermediate , albeit not now a carbocation.

GENERATION OF CARBOCATION:

1) From Alkene’s:

In presence of acidic media, alkenes are protonated and give carbocation’s.

CH2 CH2 + H+ +CH2 CH3

Alkyl Carbocation

+ H+ +

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2) From Alcohol’s:

Alcohols on treatment with concentrated acids get protonated and the protonated alcohols lose a molecule of water to form carbocations.

R OH + H+ R OH2+ R+

-H2O O O OCH3 C OH + H+ CH3 C +OH CH3 C+

H -H2O Acyl Carbocation

3) From Diazonium Ion’s:

The alkyl Diazonium Ion’s are unstable & decompose at room temperature to give carbocation’s.

R N+ N R+ + N2

4) From Alkyl Halides:

The Alkyl Halides on ionization give carbocation’s.

AlCl3

R X R+ + X-

Or Ag+

(X=I,Br,Cl)

The process is accelerated by the presence of powerful Ion-Solvation medium or metal ion’s such as Ag+ Ions. In place of alkyl halides, alkyl tosylates & alkyl mesylates can also be used.

5) From Acyl Halides:

The acyl halides on treatment with anhydrous Alcl3 gives a complex, which decomposes to give alkyl carbocation’s.

O Anhydrous OR C Cl AlCl

3 RCO+---Cl-AlCl3 R C+ + AlCl4+

Acyl Carbocation

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REACTIONS OF CARBOCATION’S:

The carbocation’s being very reactive & unstable, undergo reaction’s (as soon as they are formed) to give stable product’s. in general carbocation’s undrgo following types’s of

a) Elimination of a proton.b) Reaction with Nucleophile.c) Addition to unsaturated compound’s.d) Molecular rearrangement’s

Elimination of a proton:

A carbocation may lose a proton to form an alkene. Thus, 1-propyl carbocation (which can be generated from 1-amino propane by diazotization, followed by decomposition of the formed diazonium salt ) may eliminate a hydrogen ion to form an alkene (propane). Alternatively, 1-propyl carbocation may rearrange to more stable secondary carbocation, which may lose a proton to give propene.

NaNo2/HCl CH3CH2CH2NH2 CH3CH2CH2 N+ N 1-Amino propane 0-50C Propyl diazonium cation

-N2

H+ CH3 CH CH2 - CH3 CH CH2

+

Propene H

Primary carbocation

Rearrangement +

CH3 CH CH3

Secondary carbocation(More Stable)

-H+

CH3 CH CH2

Propene

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Reaction with Nucleophiles:

A carbocation may combine with a nucleophile to form a new bond. For ex: proply carbocation on reaction with Br- forms n-proply bromide. Similarly, secondary propyl carbocation reacts with Cl- ion to form isopropyl chloride.

CH3 CH2 CH2+ + Br- CH3 CH2 CH2Br

Propyl carbocation Propyl bromide

+

CH3 CH CH + Cl- CH3 CH CH3

Secondary carbotion Cl

Isopropyl chloride

The reaction of a carbocation with a neutral nucleophile such as water gives a protonated alcohol. Tertiary butyl carbocation , for ex; reacts with water(neutral nucleophile) to give protonated tertiary butyl alcohol, which eliminates a proton to give tertiary butyl alcohol.

CH3 CH3 CH3

+

CH3 C+ + O H CH3 C O H CH3 C OH CH3 H H3C H CH3

Tertiary butyl water t-butyl alcoholCarbocation (Neutral

nucleophile)

H + -H

+

CH3 CH2+ + NH3 CH3 CH2 N H CH3 CH2 NH2

Ethyl H Ethyl amineCarbocation

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Addition to unsaturated compounds :

A carbocation may react with an alkene to produce another carbocation.

+

CH2 CH2 + CH3 CH3 CH2 CH2+

Propyl carbocation

CH3 CH3 CH3 CH3

+

CH3 C CH2 + C CH3 CH3 C CH2 C CH3 + CH3 CH3

2-Methyl propene

Molecular rearrangements:

A carbocation undergoes molecular rearrangement to produce a more stable carbocation.

i. A primary carbocation on rearrangement gives a more stable carbocation (20 C+ or 30 C+) Rearrangement +CH3CH2CH2

+ CH3 CH CH3

10 carbocation 1,2 Hydride shift 20 carbocation

ii. A secondary carbocation on rearrangement gives a tertiary carbocation.

CH3 H3C CH3

RearrangementCH3 C CH CH3 CH3 C C CH3

+ 1,2 alkyl shift + CH3 H

3,3 Dimethyl 2-butyl 2,3 Dimethyl-2-butyl Carbocation(20 C+) carbocation(30 C+)

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In some cases molecular rearrangements involving carbocation’s may lead to ring expansion. For ex:

HO CH2NH2 HO CH2N2+ HO CH2

+

NaNO2/HCl 0-50 -N2

1-Amino methyl

Cyclohexanol

+

O -H+ OH

Cyclo Heptanone

STEREOCHEMISTRY OF REARRANGEMENT:

In rearrangements involving carbocation’s , there are three centre’s of steriochemical interest, the configuration of the carbon atom from which migration takes place (migration origin), the configuration of the carbon atom to which migration takes place (migration terminus) & the configuration of the migrating group (if it is a chiral).

It was observed that in intramolecular rearrangement, the migration of group proceeds through the formation of a bridged intermediate. Thus, the migrating group gets associated with the migration terminus before it has completely broken its connection with the migration origin. Due to this there is no chance for change in its configuration i.e, there is retention of configuration in a chiral migration group ® . Further evidences show that the configuration at both the chiral centers (migration terminus & migration origin) is inverted which is a typical of a molecular rearrangement’s.

H H R

H3C H CH3

Y +R CH3 H3C H R H R H H

CH3 H3C H H3C Z Z- H3C Z

APPLICATIONS:

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Generation of carbocation’s in a large number of synthetic sequences is very well established. Thus , Friedal-Craft Reaction is useful for introducing - COCH3 group in an aromatic moiety (acylation) & also for introducing alkyl group (alkylation). Some common examples of acylation are:

COCH3

AlCl3

+ CH3COCl

HO OH HO OH

AlCl3

+ CH3COCl COCH3

The yield of the acyl products is good (70-80%). Some examples of alkylation are given below.

AlCl3 R + X-R

AlCl3 CH(CH3)2 + (CH3)2CHCl

There are some other useful reactions involving carbocations For ex: Baeyer Villeger Oxidation, Beckmann Rearrangement, Pincol-Pinacolone Rearrangement & Wagner Meerwein Rearrangement.

WAGNER-MEERWEIN REARRANGEMENT:

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The rearrangement which takes place by involving the change in the carbon skeleton of the reactant via the formation of carbocation intermediate is known as Wagner Meerwein Rearrangement.

The formation of carbocations is sometimes accompanied by a structural rearrangement. Such rearrangements take place by a shift of a neighbouring alkyl group or hydrogen, and are favoured when the rearranged carbocation is more stable than the initial cation. The addition of HCl to 3,3-dimethyl-1-butene, for example, leads to an unexpected product, 2-chloro-2,3-dimethylbutane, in somewhat greater yield than 3-chloro-2,2-dimethylbutane, the expected Markovnikov product. This surprising result may be explained by a carbocation rearrangement of the initially formed 2º-carbocation to a 3º-carbocation by a 1,2-shift of a methyl group.

PINACOL-PINACOLONE REARRANGEMENT:

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The pinacol rearrangement or pinacol-pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. This rearrangement takes place under acidic conditions. The name of the reaction comes from the rearrangement of pinacol to pinacolone.

The reaction of 2,3-di-(3-pyridyl)-2,3-butanediol (1) in H2SO4 was studied.

BECKMANN REARRANGEMENT:

The rearrangement of Oximes in presence of acid catalyst PCl5/PCl3 to form n-substituted amine is Beckmann Rearrangament

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BAEYER VILLEGER OXIDATION:

The Baeyer-Villiger oxidation is an organic reaction in which a ketone is oxidized to an ester by treatment with peroxy acids or hydrogen peroxide.[1][2] Key features of the Baeyer-Villiger oxidation are its stereospecificity and predictable regiochemistry.[3]

Mechanism:

NON CLASSICAL CARBOCATION:

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Carbocation’s which are more than tri-coordinate are called Non-classical carbocation’s. These carbcation’s are cyclic, bridged ions and possess a 3 center bond in which 3 atoms share two electrons (3 center, 2e- bonding) ex.,the norbornyl carbocation.It is well known that acetolysis of both exo-2-norbornyl brosylate and endo-2-norbornyl brosylate produce exclusively exo-2-norbornyl acetate. The exo isomer is more reactive than the endo isomer. Further optically active exo-brosylate gives completely racemic exo-acetate.

Both acetolysis proceed via formation of a carbocation. Ionization of exo-brosylate is assisted by the c1-c6 bonding electrons.(neighbouring single bond participation) & led to the formation of non-classical carbocation as an intermediate.

This intermediate is achiral & has a plane of symmetry passing through c-4,c-5,c-6 and the mid point of c-1,c-2 bond.c6 is penta coordinate and serves as a bridging atom in the cation.

The non-classical ion is stabilized relative to a secondary ion by c-c sigma bond delocalization..

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Some other examples of non-classical carbocations are given below:

+ +

DETECTION OF CARBOCATION:

Majority of carbocation are unstable, cannot be isolated, and studied. Therefore they can be detected by NMR &IR spectroscopy.

NMR spectroscopy:

Tertiary butyl fluoride on reaction with SbF5 produces tertiary carbocation. In NMR

spectroscopy, tertiary butyl fluoride gives a singlet at 3-4ppm. After reaction with SbF5 tertiary carbocation also gives a singlet but there is an increase in the chemical shift. The increase in the chemical shift of carbocation than reactants indicates the formation of intermediate.

CH3 CH3

SbF5

CH3 C F H3C C+ + SbF6 –

Liq So3

CH3 CH3

Singlet singlet 3-4ppm 4-5ppm

H3C CH3 H3C CH3

SbF5

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CH3 C C CH3 CH3 C C CH3

Liq So3

Br Br Br+ Singlet singlet 3-4ppm (increase in chemical shift than reactant)

IR SPECTROSCOPY:

In the below reaction, when IR spectroscopy is observed, peaks corresponding to the Sp3C-H stretching and C-F stretching is observed in reactant. But when SbF5 is added, peak corresponding to C-F stretching is disappeared indicating that some intermediate is formed may be carbocation.

CH3 CH3

SbF5

CH3 C F H3C C+ + SbF6 –

Liq So3

CH3 CH3

The gas-phase IR spectra of the diamantyl and triamantyl carbocations are presented.

BIBLIOGRAPHY

First reference. Advanced Organic Chemistry (4th Edition) – Jerry March.Mechanism in Organic Chemistry (6th Edition) – Petersykes.Organic Reaction Mechanisms (3rd Edition) – V.K.Ahulwalia & Rakesh Kumar Parashar.

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Additional references. Internet