Carbocation Stability: Definition, Resonance and Stability

Carbocation is an organic species with a positive charge and a carbon atom with only six electrons in its outermost shell. With a flat structure, the carbocation has all the three covalent bonds in plane with the bond angle of 120 between them.

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Carbocation Stability Definition

Carbocations have a planar structure and their trivalent carbon can be referred to as the sp² hybridizes. The three constituents are placed in the corners of an equilateral triangle. Since there are only six valence electrons on carbon and all of them can be said to be in sigma bonds, the p orbital exceeding the plane either up or down, remains empty. 2 methyl propene can be reactive in the presence of H? in order to give rise to carbocation consisting of three alkyl substituents or tertiary ion of 3º and it can react in order to produce the carbonation consisting of one alkyl substituent having a base ion of 1º. Tertiary cation is more favoured than the primary one since the tertiary alkyl chloride can be said to be the only product that is visible. 

Carbocation stabilities can be determined by measuring the amount of energy to create the carbocation by dissociating the corresponding alkyl halide, whereas the tertiary alkyl halide separates to produce carbocation much more easily than primary or secondary that leads to tri-substituted carbocations which can be found to be more easily stable than the di-substituted or mono-substituted ones. The polar solvents can easily stabilise the ions which result in a low dissociation enthalpy, however there is no change in the carbonation stability.

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Reactivity and Stability of Carbocations

The relationship between reactivity and stability of carbocation is an important aspect to understand especially in cases of nucleophilic substituents. Carbocations can be formed through the organic reactions in order to produce organic compounds. Through this reaction tri-coordinated carbonation intermediate is produced which is entirely different from the nucleophilic or electrophilic substitution. 

• Carbocation is very unstable and brought as intermediate since stability is reciprocally proportional to the charge.
• When in the gaseous phase the ionization of 2-chloro-3-methyl propane is known as endothermic having a 153 Kcal per mol the rate of reaction will be slow at the room temp. 
• The activation energy of carbocation increases with solvents, hence the results of the gas phase are often disregarded. 
• The thermodynamic stability of carbocation is known by the measurement of hydride affinity.
• Hydride affinity is defined as enthalpy of reaction for the dissociation of RH- to yield R+ H-.
• The thermodynamic basis for comparing the relative stability of non isomeric carbocations is hydride affinity.
• If the hybride affinity price is a smaller amount, stability of carbocations can increase.
• Resonance/mesomeric impact > Hyperconjugative impact > Inductive impact

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Allylic Carbocation Stability

Similar to allylic radicals, allylic carbocation contains a double bond beside the carbon having a carbon-deficient. The allyl cation can be referred to as the simplest form of allylic carbocation. Since the allyl cation consists of only a single substituent on the positive charged carbon it can be said to be a primarily allylic carbocation. Allylic carbocation is more stable than the substituted alkyl carbocation due to the delocalization relation of the resonance interaction between the carbon bearing the positive charge and the pie bond present next to it.

A molecule having its charge distributed can be considered as more stable than a molecule having a localized charge. It has also been determined through experiments that a double bond of a vinyl group present adjacently imparts stability which is approximately equal to two alkyl groups. Therefore it can be said that the cation 2º isopropyl cations are more stable in comparison. 1º, 2º, 3º alkyl cation can be decided through the placement of the positive charge present in the more important contributing structure. 

CH₂ = CH-CH₂⁺

CH₃-CH⁺-CH₃

Reaction Intermediate 

Reaction intermediate can be found in Elimination Reaction, Electrophilic Addition Reaction and Substitution Nucleophilic Unimolecular reaction. The tertiary carbocations are more stable than secondary because we have three electron donating groups and in secondary it'll be two and for primary it is one. Methyl Carbocation is a very weak electron donating group. It can donate electron density to the carbocation by the means of the inductive effects i.e., through the sigma bond that is the way it can stabilize and also by hyperconjugation and resonance effects.

Carbocation Stability Order

Carbocation Stability

While comparing the carbocations it must be kept in mind that the standards for every cation is the substrate from which it is produced. Through experiments we know that as the carbocation decreases in order, its stability decreases. It has been observed that the primary and methyl carbocations are so unstable that they can hardly be seen in any solutions. We can say that the more the stability of the carbocation intermediate, the faster is its formation. When the charge of a system is delocalized it is considered to be more stable irrespective of its positive or negative charge. 

The carbocation stability order can also be explained by considering that when the alkyl groups bond to a carbon which is positively charged and exerts electron density in the direction of the carbon and helps in delocalizing the cation having the positive charge. Therefore the electrons that release the ability of alkyl groups that bonds to a cationic carbon can have either of the two results: Inductive effect or hyperconjugation. 

Methyl cationEthyl cationIsopropyl cationTert-butyl cation

Carbocations have more difference in stability than that of free radicals. The tert butyl radical is 12 Kcal more in stability than the methyl free radical. Therefore it depends on the substrate which has 66-72 Kcal more stability than the methyl cation. 

• Forms between the reaction takes place
• Short life span approximately of (10− 6 sec)
• Which decides final product
• Examples: Carbocation, Carbanion, Free Radical, Carbenes, Nitrenes, Benzyne etc. Carbocation Characteristics:
• 6 electrons in valence shell/ Electron deficient- It is an electrophile
• SP² Hybridized with trigonal planar shape.

Inductive Effect of Carbocation Stability

• The inductive effect is caused by the shifting of sigma electrons as a result of the popularity of the bond.
• The polarity is the electronegativity difference.
• There are two types of inductive effects. Positive inductive effect and negative inductive effect
• Positive inductive effect occurs due to the relocation of sigma electron towards carbon atom as a result of the presence of electron releasing group
• Negative inductive effect is due to shifting of sigma electrons away from carbon atoms due to presence of electron withdrawing group.

Hyper-Conjugation Effect on Carbocation Stability

• This is a stabilising impact caused by the letter bond within the C-H bond once there's associate degree empty or partly crammed or pi bond adjacent to the C-H bond.
• Hyperconjugation primarily involves delocalisation of letter electrons through overlapping p-orbitals of a covalent bond with letter orbital of the adjacent single bond.
• Since one H+ is free and conjointly the actual fact that it will happen in a very resonant manner, provides it a reputation of no bond resonance. 
• The number of hyperconjugative structures and its stability is proportional to each other.
• Tertiary carbocation is additional stable than secondary carbocations
• Secondary carbocation is area unit additional stable than primary carbocations
• Primary carbocation may be a rue stable than alkyl group carbocations
• Stability order-“ Tertiary C> Secondary C> Primary C”
• In carbocation, it's the immediate Carbon connected to the carbocation that undergoes this hyperconjugation

Resonance Effect

• It is outlined as ‘the polarity created within the molecule by the interaction of 2 pi-bonds or between a pi-bond and lone combine of electrons gift on the adjacent atom’
• The impact is transmitted to chain
• There area unit 2 kinds of resonance or mesomeric impact selected as R or M impact

1. Positive Resonance impact
2. Negative Resonance impact

• Positive Resonance Effect( +R Effect): Electrons area unit transferred faraway from the conjugated system's atom or substituent cluster.
• Negative Resonance Effect( -R Effect): Electrons area unit transferred towards the conjugated system's atom or substituent cluster.
• More resonant Structure will increase stability

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Things to Remember

• Carbocation stability is a part of CBSE class 11 first term chemistry syllabus.
• It comes under unit 7 Organic Chemistry: Some basic Principles and Techniques carrying a total of 10 periods and 9 marks. 
• Carbocation has a greater influence on the final product because it forms as an intermediate product between reactions
• Precautions must be taken ahead of time because these are extremely unstable.
• 6 electrons in valence shell/ electron deficient is known as an electrophile.
• SP2 Hybridization and its shape is trigonal planar in carbocation.
• Carbocation can be found in elimination reaction, electrophilic addition reaction and substitution nucleophilic unimolecular reaction.