Carbenium Ion or Carbocations

 

A group of organic species having a positively charged carbon atom bearing only six bonded electrons is called carbenium ion or carbocation. For example, CH₃⁺, CH₃CH₂⁺, etc. are considered as carbenium ions.

The electronic configuration of methyl cation is as follows:

Heterolytic cleavage of a bond gives rise to a carbenium ion. Heterolysis is carried out by several methods:

1) Direct ionisation method: In this method a group leaves the substrate with a pair of electrons. In this case a highly polar solvent medium having high dielectric constant is required. Since the reaction is reversible, the anion formed needs to be removed. Ag⁺, Zn²⁺, Hg²⁺, etc. can readily do the job.

CH₃Br + Ag⁺ ⇋ AgBr + CH₃⁺

2) Addition of acids to multiple bonds: Acids may be used in presence of alkenes and alkynes to form carbenium ions. Acids in this case may be protic acid or Lewis acid. Addition of cations to a multiple bond also generates a carbenium ion.

3) Abstraction of an atom or group using protic or Lewis acids: In this case a lone pair of electrons of the substrate co-ordinates either with the proton or with the Lewis acid and then decomposes to a carbenium ion.

4) Decomposition reaction:  In such cases organic species decomposes to produce a carbenium ion. Example: diazonium ions decompose to form carbenium ions.

CH₃-N≡N ⇋ CH₃⁺ + N₂

5) Rearrangement of carbenium ion: In this case a less stable carbenium ion rearranges itself to form a more stable carbenium ion. Neopentyl carbenium ions rearranges readily.

Carbenium ions are short-lived species. They can accept electrons and hence may be looked upon as Lewis acid. They undergo four types of reaction:

1) Combination with an anion: In this case one often gets a stable product. Thus, When HCl is added to ethylene, first ethyl cation is formed which combines with Cl^- ion to form ethyl chloride.

2) Elimination of a proton: In this case a stable unsaturated compound is formed; for instance, isopropyl cation gives up a proton to form propylene.

3) Addition to a multiple bond: In this case another cation is formed which then produces a stable compound by another reaction.

4) Rearrangement reaction: It has already been discussed how carbenium ions rearrange themselves.

The three bond axes of a cationic carbon have been found to have a plannar triogonal orientation like those of boron in BF₃ molecule. So the C atom carrying the positive charge is considered to be sp³ hybridised in a carbenium ion; one of its p A.O.s remains vacant.

Stability of a carbenium ion may be ascertained by considering:

a) Inductive effect,

b) Conjugation effect,

c) Hyperconjugation or resonance effect,

d) Solvation effect,

e) Steric effect.

The stability of alkylcarbenium ions and radicals is well explained by considering inductive and hyperconjugative effects and the order are found to be:

 (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃-CH₂⁺ > CH₃⁺

(CH₃)₃C^̇ > (CH₃)₂CH^̇ > CH₃-CH₂^̇ > CH₃^̇

Several effects need to be considered for explaining the given order:

Hyperconjugation effect: We know that the more the number of contributing structures of comparable energy in a resonance hybrid, the greater is the stability. In tert-butyl cation, there are nine α C-H σ-bonds and hence its resonance hybrid consists of ten resonating forms, nine of which are uncharged structures; we shall have six similar resonating forms in isopropyl cation, Three such in ethyl cation and none in methyl cation. Therefore, the order of stability of the carbenium ions is as given in the question.

Inductive effect: This can be explained by +I effect of the methyl groups. Thus, due to the +I effect of the three methyl groups attached to the positively charged carbon atom of tert-butyl cation, its charge is neutralized to a greater extent than that of isopropyl cation which possesses only two methyl groups for such an effect; this charge neutralization effect is still days in the case of ethyl cation as it contains only one methyl group and it is least in the methyl cation because it does not have any electron releasing group attached to the positively charged carbon atom.

The order for the stability of the radicals can be explained by hyperconjugative effect.

(CH₃)₃Ċ has nine resonating forms without odd electron in its resonance hybrid, (CH₃)₂CḢ has six such structures, CH₃CH₂̇ possesses only three structures without odd electron, and methyl radical has none. Therefore, the order of contributing structures in the resonance hybrid of the radicals is

The stabilities of allyl and benzyl cations are found to be high; conjugative effect can explain their stability:

CH₂=CH-CH₂⁺ ⇋ ⁺CH₂-CH=CH₂

Resonance: The resonance hybrid of allyl cation consists of two equivalent resonating structures and hence they have equal contributions to the hybrid. Thus, it is a stable cation.

Resonance is a major factor influencing the stability of carbenium ions. When the positive carbon of a carbenium ion is α to a double bond, effective charge delocalisation with consequent stabilization occurs in allyl and benzyl cations, for example, are found to be highly stabilized by resonance.

Steric effect: Steric effect causes an increase in stability of tertiary carbenium ions having bulky alkyl groups. For example, the substituents in triisopropyl cation (having planar arrangement with 120^oC angles) are far apart from each other and so there is no steric interference among them. However, if this carbenium ion is added to a nucleophile, then a change of hybridization of the central carbon atom from sp² (trigonal) to sp³ (tetrahedral) takes place and the bulky isopropyl groups are pushed together. This will result in a steric strain (B strain) in the product molecule. Because of this, the carbenium ion is much reluctant to react with a nucleophile, that is, its stability is enhanced due to steric reason.

Lesser steric crowding to larger steric crowding

Solvent effect: carbenium ions are species with a positively charged carbon atom. Their stability in solvents depends on the nature of the solvent. In polar solvents such as water or alcohol, carbenium ions are more stable because these solvents can stabilize the positive charge through interactions with their negative ends. Non-polar solvents like hexane or benzene, however, don't provide such stabilization making carbenium ions less stable in these environments.

In a nutshell, the following rules are to be remembered for comparing of carbenium ions (carbenium ions).

1) The more the +I effect on the carbon atom bearing the positive charge, the more stable is the carbenium ion.

2) The more the –I effect on the carbon atom possessing the positive charge, the less stable is the carbenium ion.

3) The more the delocalisation of positive charge through conjugation, the more stable is the ion.

4) The more polar the solvent, the more stable is the carbenium ion through solvation, provided there is no chemical reaction between them.

Carbenium ions discussed so far are called classical carbenium ions which contain two-electron two-centre bonds. There is a type of carbenium ion which the positive charge does not remain on a single carbon atom but spreads over atleast three atoms and those three atoms form a cyclic cation and are thus called bridged carbenium ions or non-classical carbocations.  β-phenethyl cation is an example of non-classical carbenium ion:

Bridged cations with delocalisation bonding σ electrons are called non-classical carbenium ions which contain two-electron three centre bonds. These are now called carbonium ions.

Non-classical norbornyl cation forms as an intermediate when exo- and endo-norbornyl brosylates (brosyl group is p-BrC₆H₄SO₂) are separately subjected to acetolysis in the acetic acid medium containing potassium acetate. Norbornyl cation then gives racemic norbornyl acetate though the two substrates, the exo- and endo-forms, are diastereomeric. However, the former one reacts much faster than the latter and direct non-classical carbenium ions formation by the neighbouring group participation is said to be the cause for the very fast rate of the reaction. The endo-isomer, under the same set of reaction conditions, undergoes the reaction but slowly. It is supposed that a classical carbenium ion forms first which then changes slowly to the non-classical carbenium ion before the reaction with acetic acid and potassium acetate.

Norbornyl carbenium ion

Bridge-head carbocations or carbenium ions are different from bridged or non-classical carbenium ions. Carbon atoms at the junction of a bridged ring system are called bridged-head carbons. When a bridge-head carbon bears a positive charge, it is called a bridge-head carbenium ions as shown below:

Bridge-head carbenium ions are highly unstable since the cationic carbon cannot assume its usual planner trigonal orientation of its bonds. However, (b) is less strained and hence less unstable than (a); perhaps a six membered ring being more stable than a five membered ring, (b) is more stable than (a).

Understanding the structures and reactions of carbenium ions is crucial part of organic chemistry as it opens up several avenues to explain reaction properties, stability of organic molecule and transition state, reaction rate etc. 

 Reference

1) Jonathan Clayden, Nick Greeves, Stuart Warren, organic chemistry book second edition.