Key Difference – Allylic vs Vinylic Carbons
Functional groups are very important in understanding the different physical and chemical properties of organic molecules. The terms allylic and vinyl carbons indicate whether the carbon atom is bonded directly or indirectly to a double bond in a molecule. The key difference between allylic and vinylic carbon is that allylic carbon is the carbon atom adjacent to the double-bonded carbon atom whereas vinylic carbon atom is one of the two atoms that share the double bond.
CONTENTS
1. Overview and Key Difference
2. What is Allylic Carbon
3. What is Vinylic Carbon
4. Side by Side Comparison – Allylic vs Vinylic Carbons in Tabular Form
5. Summary
What is Allylic Carbon?
Allylic Carbon can be described as the carbon atom that is adjacent to the double bond. This carbon atom is the nearest to the double bond, but it is not a part of the double bond. In other words, this carbon atom is bonded to a carbon atom that in turn is doubly bonded to another carbon atom. The carbon atoms in the double bond are sp2 hybridized. But the allylic carbon is sp3 hybridized. It is bonded to the sp2 hybridized carbon atom through a single bond. The electron density around this carbon atom is lower than that of carbon atoms in the double bond. The general formula can be given as CH3-CH=CH2. As it is not directly bound to the double bond, this carbon is not affected by reactions that occur on double bond such as electrophilic additions. Hydrogen atoms bonded to this allylic carbon are called allylic hydrogen. Allylic carbon can act as a bridge which combines a carbon chain and a double bond. Here, the C-H bond is weaker than ordinary C-H bonds. That is because the electrons around this carbon are displaced by the double bond. Thus, these places are very reactive.
Figure 01: The atom in red is an Allylic carbon.
What is Vinylic Carbon?
Vinylic carbon is a carbon that is involved in a double bond with another carbon. It is sp2 hybridized. Vinylic carbon makes a double bond with another carbon which is also sp2 hybridized. Both carbons involved in this bond are vinylic carbons. Electron density around these atoms is higher than the density around allylic carbon atoms. The general formula can be given as CH2=CH2.
Vinylic carbon is a type of alkenyl functional group because carbon is in alkene functional group. The vinylic group is derived from the corresponding alkene. Therefore, this carbon is also called alkenyl carbon. Sometimes this carbon can be bonded to other carbons via double bonds from its both sides. Then all three carbon atoms are called vinylic carbons. This formula can be given as, CH2=C=CH2. Since these carbons are directly bonded to the double bond, they undergo the reactions such as electrophilic addition.
Figure 02: Allylic and Vinylic Carbons
What is the difference between Allylic and Vinylic Carbon?
Allylic vs Vinylic Carbon |
|
| Allylic carbon is a carbon atom bonded to a carbon atom that in turn is doubly bonded to another carbon atom. | Vinylic carbon is a carbon that is involved in a double bond with another carbon. |
| Hybridization | |
| Carbon atom in an allylic group is sp3 hybridized. | Vinylic carbon is sp2 hybridized. |
| Bond Length | |
| C-H bond length in allylic carbon is higher. | Vinylic C=H bond is lower. |
| Bond Type | |
| Allylic carbon only forms a single bond. | Vinylic carbon can have either two double bonds in its sides or one double bond. It forms at least one double bond. |
| Number of Hydrogen Atoms | |
| Allylic carbon can have a maximum of 3 hydrogen atoms. | Vinylic carbon can have only two carbon as the maximum number. |
| Carbon Atom and Double Bond | |
| Allylic carbon act as a bridge to combine the double bond with the rest of the molecule. | Vinylic carbon makes the double bond. |
Summary – Allylic vs Vinylic Carbon
The difference between Allylic and Vinylic carbon depends on whether the carbon atom is bonded directly or indirectly to a double bond. Allylic carbon indirectly relates to a double bond whereas vinylic carbon is directly involved in a double bond. Another important difference between Allylic and Vinylic carbon is that allylic carbon is sp3 hybridized where vinylic carbon is sp2 hybridized.
References:
1.Chang, R., 2010. Chemistry. 10th ed. New York: McGraw Hill.
2.Gunawardana, G., n.d. OChemPal. [Online] Available here. [Accessed 30 05 2017].
Image Courtesy:
1.”Vinyl-ether-3D-balls” By Jynto (talk) – Own work (CC0) via Commons Wikimedia
2. “AlkeneGroups” By Walkerma – Own work (Public Domain) via Commons Wikimedia