What is a Vinyl Group?
Vinyl group is the name given to the functional group of -CH=CH2. It can be seen as an ethene molecule with one less hydrogen in number. Hence it is also called as ethenyl group at times.
Commercial Use of Vinyl Groups
The most popular term which familiarizes vinyl group to us is the compound polyvinyl chloride, which is the chemical compound from which plastic is made. Plastic undoubtedly is almost used in many things that we use in our day to day life these days, but the first person to commercialize plastic by discovering polvinyl chloride (PVC) was a scientist named Eugen Baumann. It is in fact interesting to know that Eugen Baumann accidentally discovered PVC when he left vinyl chloride in a beaker exposed to sunlight. He found that vinyl chloride polymerized to form the PVC, which was then widely used for materializing several things.
Plastic, as we know now, is a polymer of vinyl chloride and is widely used in manufacturing many amenities such as cutleries, bottles, floor tiles, solid tiles, rubbers, sheets, insulation wires, buttons and even in textiles and fabric industries.
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 sp2 hybridized carbon atoms with three hydrogens. One of the carbons is electron 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 stabilizing them via hyperconjugation, while electron-withdrawing groups such as hydroxyl groups and sulfonyl groups etc. makes them reactive and helps them undergo addition reactions such as Michael addition.
One of the popular reactive dyes formed by vinyl groups is vinyl sulfone dyes which have a formula of -SO2CH=CH2. 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.
Stability of Vinyl Carbocation
Vinyl carbocation in organic chemistry is considered one of the most unstable and reactive intermediates in solvolysis reactions. It has a chemical formula of C2H3+ and structure as shown below.
The reason behind the instability of vinyl cation is due to two reasons, less hyperconjugation, and less resonance. Carbocations are always stabilized when there are more hyperconjugation structures and resonance structures, this is because the positive charge on carbon can be accommodated better with the movement of electrons from nearby atoms and orbitals. In general, sp2 carbon atoms are less stable carbocations, as p-character is less in sp2 hybridized atoms, as a result of which it will not have many electrons to donate to stabilize the positive charge on it. Vinyl cation has a positive charge on the double-bonded carbon with sp2 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. Also as carbon in a 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.
Reactions where they are formed as intermediates
Vinyl carbocations being reactive intermediates are part of various sets of reactions such as electrophilic additions, pericyclic reactions, and rearrangements. Generating vinyl cations were dependent on the ring size, smaller being the ring harder it is to generate a vinyl carbocation due to steric strain
Polyvinyl Chloride: Uses and Hazards
Preparation of PVC
Here n moles of vinyl chloride react in the presence of high pressure and peracid as a catalyst to get polymerized and form PVC or polyvinyl chloride. This is a classic example of addition polymerization in polymer chemistry.
Vinyl Chloride in Plastics
Plastic is widely used everywhere as we know, but plastic is considered as a huge pollutant and these days are being replaced by other alternatives because of the hazardous and toxic nature of vinyl chloride in it. Vinyl chloride is not only used in manufacture of plastic, but is also used as a petrochemical. Vinyl chloride is highly toxic to human and other animals as it is carcinogenic and when consumed it is known to cause lung and brain tumors along with mutation in genetic level.
Vinyl chloride exists as a gas with characteristic sweet smell, but is highly inflammable when exposed to air or moisture due to its reactive nature.
Plastic is categorized as a toxic substance due to the leaching out of vinyl chloride in it which is known to have such adverse effects of life and environment in general.
Other Important Chemicals Generated from Vinyl Groups
- Vinyl acetate: It is an important homopolymer and copolymer in industries and is mainly used in manufacturing of emulsifiers, paints, oils.
- Vinyl fluoride: It helps in the 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 the manufacture of acrylate polymers.
- Polyvinyl nitrate: It is a popular polymer with very high energy and hence is explosive.
Do not confuse Allyl groups with vinyl groups. They are different because there is an extra -CH2 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.
Are vinyl carbocations stable? Explain your answer.
Vinyl carbocations are highly unstable due to the fact that they possess a positive charge on the sp2 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 Chemistry and Biochemistry.
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