What is Vinyl Carbocation?

A vinyl carbocation is a type of carbocation in which an alkene carbon is positively charged. The alkene carbon in a vinyl carbocation is a trivalent carbon and is sp hybridized. The empirical formula of the vinyl carbocation is ${\text{C}}_{\text{2}}{\text{H}}^{\text{+}}{}_{\text{3}}$. They play a significant role in the reaction mechanisms of organic chemistry.

Vinyl Group and Carbocation

Vinyl groups is the name given to the functional group of ${\text{-CH}}_{\text{2}}{\text{=CH}}_{\text{2}}$. It can be seen as an ethene molecule with one less hydrogen in number. Hence it is also called as ethynyl group at times.

Carbocations are positively charged carbon atoms.  It is also known as a carbonium ion. 

How to Generate a Vinyl Carbocation

Vinyl carbocations are found as reactive intermediates in solvolysis reaction.  Solvolysis is an elimination or nucleophilic substitution reaction where solvent acts as a nucleophile.  In photochemical solvolysis of halo styrene, light is used to generate vinylic carbocation by removing halide.  The chemical reaction is shown below.

The possibility of the generation of small cyclic vinyl carbocations is very less.  This is due to the fact that the three-membered cyclic ring is highly strained.

Structure and Reactivity of Vinyl Group       

The Vinyl group’s structure can be easily remembered as ethene with one less hydrogen. The hydrogen of ethene can be replaced with any other substituents such as alkyl groups, halogens, etc.       

The functional group as seen above contains two $s{p}^2$ hybridized carbon atoms with three hydrogens. One of the carbons is valence deficient and therefore can get bonded to many groups such as alkyl groups and halogen groups. These substituents can easily activate vinyl groups to give better reactivity. Electron releasing groups such as methyl groups and halogens can help to stabilize them via hyperconjugation, while electron withdrawing groups such as hydroxyl groups and sulfonyl groups etc. make them reactive and help them undergo addition reactions such as Michael Addition.

One of the popular reactive dyes formed by vinyl groups are vinyl sulfone dyes which have a formula of $-S{O}_{2}CH=C{H}_2$. It is popularly known as a reactive hook in dying industries. It is called a reactive hook because the sulfonyl group attached to vinyl makes the vinyl group electrophilic, which makes it a good attractant for base residues. This property makes it highly essential in the pharmacological and textile industries.

Structure of Vinyl Cation

The simplest vinyl cation ${\text{C}}_{\text{2}}{\text{H}}^{\text{+}}{}_{\text{3}}$ without any substitution in it has two possible structures such as non-traditional bridged structure and classical linear structure. According to Ab initio calculations the stability of bridged structure is more than linear by $\text{5}\text{.0 kcal/mol}$. However, linear structure is backed by ${}^{\text{13}}\text{C}$NMR and ${}^{\text{1}}\text{H}$NMR if it it is substituted by equivalent alkyl groups. A linear structure compound, beta-silyl vinyl cation gives single ${}^{\text{29}}\text{Si}$NMR signal in NMR spectroscopy which indicates the two silicon are equivalent and delocalize via hyperconjugation to the carbocation. Also, the bond angles between vinyl carbon and the substituted alkyl group is ${\text{180}}^{\text{o}}$. Therefore, vinyl cation exists as a linear structure.

How Carbocations are Stabilized

In an ${\text{S}}_{\text{N}}\text{1}$reaction, the rate determining step is the first step the formation of a carbocation intermediate. The bond between carbon and leaving group breaks fast if the carbocation is stabilized. A positively charged carbon atom is electron deficient and is ready to accept electrons. Therefore, anything that donates electrons to carbocation stabilizes it. Electron donating groups are group of atoms that donates electron to carbon with less electrons. Similarly, electron withdrawing groups destabilize carbocation. Alkyl groups such as methyl, propyl are weak electron donating groups. Carbocation with alkyl groups are stabilized. Therefore, carbocations with more electron withdrawing group are more stable. Tertiary carbocation having three electron donating substituents are stable than secondary carbocation and primary carbocation.

Resonance Effect

Resonance is defined as delocalization of electrons in a molecule.  A molecule with more resonance is more stable because of the delocalization of electron which leads to delocalization of positive charge.  In benzylic and allylic carbocations electrons are delocalized and positive charge is present in different carbon atoms. 

In vinylic carbocations, there is no resonance structure because the adjacent carbon atom has no double bond.  Also, the positive charge on double bonded carbon is unstable and less likely to form as intermediates.   Therefore, vinyl carbocations are highly unstable.

Hyperconjugation Effect

Carbocations are always stabilized when there is more hyperconjugation structures, this is because the positive charge on carbon can be accommodated better with the movement of electrons from nearby atoms and orbitals.  Tertiary carbocation with three alkyl groups is more stabilized than secondary carbocation and primary carbocation.  In vinyl, carbocation is a primary carbon and thereby hardly any hyperconjugation structures can be possible as well. This is why vinyl cation is highly unstable.

Hybridization

In general, $s{p}^2$ carbon atoms are less stable carbocations, as p-character is less in $s{p}^2$ hybridized atoms, as a result of which it will not have much electrons to donate to stabilize the positive charge on it. Vinyl cation has a positive charge on the double bonded carbon with $s{p}^2$ hybridization. This puts the positive charge in a very unstable position due to the double bond as there cannot be any possibility of resonance structures to this.

Compounds Generated from Vinyl Carbocation

• Vinyl acetate: It is an important homopolymerize and copolymer in industries and is mainly used in manufacturing of emulsifiers, paints, oils.
• Vinyl fluoride: It helps in manufacture of polyvinyl fluoride PVF which is used in metal sheets, solar panels, raincoats, parts of airplanes etc.
• Vinyl Bromide: This is known for being a fire inhibiting property. It is used to generate Grignard reagent and also used in manufacture of acrylate polymers.
• Polyvinyl nitrate: It is a popular polymer with very high energy and hence is explosives.

Common Mistakes    

Do not confuse between Allyl groups with vinyl groups. They are different because there is an extra $-C{H}_2$ group in allyls with which they bond to other substituents, whereas in vinyl group the bonding occurs directly to the carbon that is double bonded.

Practice Problem   

Explain why vinyl cations are unstable.

Solution

Vinyl carbocations are highly unstable due to the fact that they possess a positive charge on the $s{p}^2$ hybridized carbon atom and hence have no possibility for stabilizing effects like resonance and hyperconjugation.

Context and Applications

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

• B.Sc. in Chemistry
• Chemical Engineering
• M.Sc in Chemistry
• B.Tech Biochemistry.