What is the Difference Between Allylic and Vinylic Carbons

Allylic and vinylic carbons are important concepts in organic chemistry. These positions influence the reactivity and stability of molecules in different ways, especially during chemical reactions like substitution reactions and elimination reactions.

What is the difference between allylic and vinylic carbons? The allylic carbon atom is a carbon atom directly adjacent to the vinylic carbon atom, whereas vinylic carbon is one of the two carbon atoms that form the double bond.

Key Areas Covered

1. What is Allylic Carbon
      – Definition, Features 
2. What is Vinylic Carbon
      – Definition, Features 
3. Similarities Between Allylic and Vinylic Carbons
      – Outline of Common Features
4. Difference Between Allylic and Vinylic Carbons
      – Comparison of Key Differences
5. FAQ: Allylic and Vinylic Carbons
      – Answers to Frequently Asked Questions

Key Terms

Allylic, Allylic Carbon, Vinylic, Vinylic Carbon

What is Allylic Carbon

Allylic carbon is a carbon atom adjacent to a carbon-carbon double bond. The reactivity of the allylic carbon atom is more than that of the regular alkane carbons due to the electron-withdrawing effect of the double bond. Allylic carbons lose electrons and form allylic radicals. They are stable because of resonance delocalization. Allylic carbons participate in various different chemical reactions, including electrophilic addition, nucleophilic substitution, and radical reactions.

Stated below are some of the examples of allylic carbon atoms.

• Propene: The carbon atom bonded to the methyl group is allylic.
• Cyclohexene: Any carbon atom directly attached to the double bond is allylic.
• Benzene: All six carbons in benzene are allylic due to the delocalized pi-electron system.

Allylic carbons are used as starting material for the synthesis of alkenes by reactions such as dehydrohalogenation and dehydration. Moreover, allylic monomers are used in polymer production. Natural products like terpenes and steroids contain allylic carbon.

What is Vinylic Carbon

Vinylic carbons are those that form part of a carbon-carbon double bond. sp2 hybridization, planar geometry, and higher reactivity are the characteristics of vinylic carbons. Vinylic carbons have three sigma bonds and one pi bond; hence, they are sp2 hybridized. This hybridization allows for the formation of a double bond, which is essential for the stability and reactivity of many organic compounds.

Due to sp2 hybridization, the compound is a part of planar molecular geometry, meaning that the atoms of the compound are of the same plane. Vinylic carbons are generally more reactive than saturated carbons. This increased reactivity is due to the presence of the pi bond, which can be easily broken to form new bonds. Alkanes, alkynes, and aromatic compounds are examples of molecules that have vinylic carbon atoms.

Similarities Between Allylic and Vinylic Carbons

1. Both types of carbon atoms are sp² hybridized.
2. Allylic and vinylic carbons are involved in double bonds.
3. Both allylic and vinylic carbons are involved in double bonds.
4. They show improved reactivity compared to saturated carbons due to the presence of double bonds.

Difference Between Allylic and Vinylic Carbons

Definition

• Allylic is a carbon atom directly adjacent to a double bond, whereas vinylic is one of the carbon atoms that form the double bond.

Hybridization

• Allylic is typically sp³ hybridized, while vinylic is always sp² hybridized.

Reactivity

• Allylic carbons are more reactive than vinylic carbons.

Stability

• Allylic carbons are more stable than vinylic radicals and carbocations.

Conclusion

Allylic and vinylic carbons are important concepts in organic chemistry. The main difference between allylic and vinylic is their proximity to double bonds. Vinylic carbons are directly attached to a double bond, while allylic carbons are one carbon away.

FAQ: Allylic and Vinylic Carbons

1. Which is more stable, allylic or vinyl?

Allylic carbocations are more stable than vinyl carbocations. This is due to the resonance stabilization provided by the adjacent double bond.

2. What is the difference between vinylic and allylic hydrogen?

Vinylic hydrogen is directly attached to a carbon atom that is part of a double bond. Allylic hydrogen is attached to a carbon atom adjacent to a double bond. This is the structural difference between vinylic and allylic hydrogen.

3. What is the difference between allylic and vinylic carbocation?

Allylic carbocations are more stable than vinylic carbocations due to resonance delocalization of the positive charge onto the adjacent double bond. Vinylic carbocations have a positive charge directly on a carbon atom involved in the double bond, making them less stable.

4. Why is vinyl carbocation not stable?

A vinyl carbocation is not stable because the positive charge is directly on a carbon that’s part of a double bond. This type of carbon doesn’t hold onto the positive charge well since it doesn’t have enough electrons around it to spread out the charge, making it very unstable.

5. Which is more reactive, allylic or vinylic?

Allylic carbons are more reactive than vinylic carbons. This is due to the resonance stabilization of the intermediate formed during reactions.

Reference:

1. “Allyl Group.” Wikipedia. Wikipedia Foundation.
2. “Vinyl Cation.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Allyl” By User:Innerstream – Own work (Public Domain) via Commons Wikimedia
2. “Nomenc of vinyl” By Vinylcation – Own work (CC BY-SA 4.0) via Commons Wikimedia