Vinyl Cation

A carbocation where the positive charge is on the alkene carbon is known as the vinyl carbocation or vinyl cation. The empirical formula for vinyl cation is C₂H₃⁺. In the vinyl carbocation, the positive charge is on the carbon atom with the double bond therefore it is sp hybridized. It is known to be a part of various reactions, for example, electrophilic addition of alkynes and solvolysis as well. It plays the role of a reactive intermediate in these reactions.

The Father of Vinyl Cation

During the progress of organic chemistry, the solvolysis reaction conducted by Grob and Cseh formed the vinyl cation and therefore Grob is now known as the father of vinyl cation. Further many scientists reported the formation of vinyl cation in their work. One such example is where the phenylporopiolic acid undergoes hydration through acid catalysis and the intermediate formed was the vinyl cation. This was reported by Noyce and her colleagues. The way to detect the formation of vinyl cation was therefore introduced as the benzylic carbon that tends to possess positive charge in huge amounts during the rate limiting step.

Structure of Vinyl Cation

The proposed structures for vinylic carbocation in organic chemistry include the classical and the non classical structure. The classical structure refers to the linear structure and the bridged structure is known as the non classical structure. Various calculations and research done have collectively agreed that the stability of the bridged structure is more than that of the linear structure. The difference between their energies is 5 Kcal/mol.

Although when it was studied with substituents, the vinyl carbocation was found to be stable with the linear structure. This was backed up by the ¹³C and ¹H NMR. When silylvinyl carbocation was studied with the help of NMR spectroscopy, it was observed that the linear structure is being stabilized through the hyperconjugation. This hyperconjugation was the result of two equivalent Si atoms and therefore the carbocation is stable.  

Generation of Vinyl Cation

The formation of the vinylic carbocation can be understood by the feasibility of this intermediate formation.

• The addition of an electrophile to the alkyne moiety gives a vinyl cation. The electrophile gets attached to the sp hybridized carbon atom and a positive charge is formed on it. The intermediate is further converted to form products.
• The addition of an electrophile to alkene. The terminal carbon atom is attacked by the electrophile which is positively charged, to give an intermediate of the vinyl cation. But if the alkene is substituted and could possibly stabilize the electrophilic attack on the terminal carbon then the attack occurs at the central carbon atom.
• A vinyl carbon atom undergoing heterolytic fission. A vinyl carbon compound containing a C-X bond can undergo fission heterolytically. The feasibility of this reaction under mild conditions depends upon the nature of X and vinyl structure.
• One of the ways to generate vinyl cation is also by abstracting an electron from a neutral species. 

Reactions of Vinylic Carbocations

The vinyl cation forms as an intermediate in various reactions as mentioned earlier. Further these undergo transformation to give the products which will be studied ahead.

• The vinyl cation reacts with electrons of the anions or neutral species that are non-bonded. 
• The vinyl cation reacts with the π electrons of cumulated double bonds or triple bonds. This is observed in the reaction involving cyclodimerization of alkene and propene using halide. 
• When there is availability of β-hydrogen, it is possible to undergo Elimination type reactions.

The Geometry of Vinyl Cation

The linear and the bridged geometry of the vinyl cation are studied widely in organic chemistry. The nature of the entity that will be bridged and the substituents present on the vinyl carbon determine the geometry of the cation. The various researches resulted in the conclusion that the cycloalkanol moiety size would not affect the stability of the bridgehead with H and therefore it will not form the bridged species with H. Also, as the chances of linear arrangement attainment increases, the vinyl cation stability also increases.

Saturated Carbocations v/s Vinylic Carbocations

Since the theoretical calculations of organic chemistry could not provide ground reasons for the stability of either of the carbocations, the attention was turned towards the rate of solvolysis. The energy required for the generation of vinyl cation, more or less falls in the range of formation of the methyl and ethyl cations. This was further debated to settle on the assumption that the ethyl cation is more stable than vinyl cation in the gas phase. The value of difference is roughly around 10-20 kcal/mol. 

Although this information can be changed according to the type of reaction and the reaction conditions involved. However, after studying this comparison under various conditions, it was observed that the involvement of substituents also played a major role in the stability of the cations. In conclusion, the ethyl cation was prominently found to be more stable than the vinyl cation under most conditions.

The Rearrangement of Vinyl Cation Intermediate

The tendency to rearrange is observed in vinyl cation intermediates. The classification of these rearrangements is done as follows:

Migrating into double bonds

Here, the allyl carbocation is formed through 1,2- shifts. 

Rearranging through the double bonds

In this, an isomer of the vinyl cation is formed.

Although the allyl stabilized carbocation is thermodynamically stable and the  formation is feasible through 1,2- hydride shifts, in vinyl cation this is rare. The transition state undergone by this rearrangement is viewed to be non-planar and therefore the rearrangement is complicated. 

The high activation barrier is also viewed for the 1,2 methyl shifts in vinyl cation in comparison with the alkyl carbocation equivalent. 

The process of forming an isomer in the second type of rearrangement depends upon various factors. The factors involve the species involved, nature of the nucleophile and the solvent in use. It is not feasible to undergo 1,2-hydride shift in primary vinyl cation since the stability of the molecule is low. This is because hydrogen is not capable of donating electron. 

Vinyl cations undergo [2+2] cycloaddition reaction via concerted manner under thermal conditions due to orthogonally placed pi orbitals similar to the ketenes and alkenes [2+2] cycloaddition. Both the pi orbitals also overlap with the orbitals of the dienophile in a similar way.

Context and Applications

The study of vinyl carbocation is significant in the intermediate formation studied in organic chemistry. The reaction and its conditions required can be understood well if the carbocation formation is studied. The reactivity of the carbocation group is the determining factor for many reactions.

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.