What is an Allylic Carbon Definition Structure Stability and Reactions
What is Allylic Carbon?
Allylic carbon definition can be given as a carbon atom bonded to another carbon atom, which in turn is bonded doubly to another carbon atom, in the Modern Periodic table, where all the known elements are arranged in increasing order considering the atomic number. There are 18 vertical columns known as groups and seven horizontal rows, known as periods.
The periodic table’s bottom part contains two series of 14 elements, which are known as an f-block element. The left side of the Modern Periodic Table contains metals mainly, whereas the right side contains the non-metallic region. A few elements show the intermediate properties of metals and non-metals. Such elements are called metalloids and are located in between metals and non-metals in the form of a zig-zag line.
Metals are identified as elements that are highly reactive and electropositive in nature. The non-metals are electronegative in nature, unlike metals. Carbon is one of the most commonly used non-metal. It is a basic of all other organic compounds.
Allylic Carbon Meaning
The double-bonded carbon atoms are further classified as vinylic and allylic carbon atoms. The general chemical formula for the vinyl group is R-CH=CH2, where both the carbon atoms are bonded with a double bond, and R is attached at the vinylic position.
Since both the carbon atoms form a double covalent bond, so both of them are sp2 hybridized. The allylic position is also similar to a vinylic position. It is bonded to a carbon atom that is bonded doubly to another carbon atom.
The general formula of allyl is given as - R-CH2-CH=CH2, where the asterisk carbon atom is an allylic carbon atom. Unlike the vinyl group, the allylic carbon atom is sp3 hybridized as it bonded with CH=CH2 via a single covalent bond.
The allylic carbon imparts unique chemical properties to the allylic group, and the presence of this group in different compounds form allylic compounds, used to prepare various natural products like terpenes, natural rubber, and many more.
Allylic Carbon Atoms
The allylic carbon atoms are sp3 hybridized carbon atoms in the allylic group, RCH2-CH=CH2, that is bonded with the -CH=CH2 group.
For example, in propene, the highlighted atom is the allylic carbon atom (CH3-CH=CH2). Likewise, in cyclohexene, the carbon atoms that are next to the double bond are known as the allylic carbon atoms.
Hydrocarbons
Organic compounds that are composed of different elements with a parent carbon chain are referred to as hydrocarbons. These are the most common organic compounds which are composed of mainly hydrogen and carbon.
Carbon exhibits the tetravalency. So, it can form four covalent bonds either with the same or different elements.
Because of its tetravalency, carbon exhibits catenation and can form different organic compounds.
Catenation is the property of either carbon or other elements that help to form covalent bonds with the same element.
Based on the carbon atoms bonding count with a carbon atom, we can classify these as primary, secondary, and tertiary carbon atoms.
A carbon atom that is bonded with one other carbon atom is called a primary carbon atom.
For example, in the ethane molecule (CH3-CH3), both the carbon atoms are bonded with one other carbon atoms. So, both carbon atoms are the primary carbon atom here. The secondary carbon atom is bonded with the other two carbon atoms, and the tertiary carbon atom is bonded to the other three carbon atoms.
Allylic Carbocation
The allylic carbocations are ionic species that carry a positive charge on the carbon atom of the molecule. Usually, they form as an intermediate during various chemical reactions.
The stability of the carbocations is determined by the steric hindrance and +I effect of alkyl groups attached to C+ of the carbocation.
As the +I affects the increases of the positively charged carbon atom of the carbocation, it reduces the positive charge that exists on the carbocation. So, as the number of alkyl groups increases on C+, the stability of carbocation increases accordingly.
Thus, the stability order of carbocation can be represented in the following method.
Tertiary Carbocation > Secondary Carbocation > Primary Carbocation
If the allylic carbon atom is carried by a positive charge in the allylic group, it forms an allylic carbocation. The allylic carbocation is stable because of the delocalization of electrons on carbon atoms.
Likewise, in the carbocation of cyclohexene case, the formal charge on allylic carbon is +1, and it stabilizes by resonance with a pi-bond.
If the allylic carbon atom is associated with one carbon atom carrying a +1 charge, it is referred to as a primary allylic carbocation. Since the formal charge of +1 is on primary carbon atom here, it is named as primary allylic carbocation.
In the case of the secondary allylic carbocation, the +1 formal charge is distributed on the secondary carbon atom the same as in cyclohexene cation.
A tertiary allylic carbocation has a +1 charge on a cation’s tertiary carbon atom.
FAQs on Allylic Carbon in Organic Chemistry
1. What is an allylic carbon?
An allylic carbon is a carbon atom that is directly attached to a carbon–carbon double bond (C=C). In organic chemistry, this means the carbon is adjacent to an alkene functional group.
- It is not part of the double bond itself.
- It is single-bonded to one of the double-bonded carbons.
- Example: In CH2=CH–CH3 (propene), the CH3 carbon is the allylic carbon.
2. What is the difference between allylic and vinylic carbon?
The key difference is that an allylic carbon is adjacent to a double bond, while a vinylic carbon is directly part of the double bond.
- Vinylic carbon: One of the carbons involved in the C=C bond.
- Allylic carbon: A carbon single-bonded to a vinylic carbon.
- Example: In CH2=CH–CH3, the first two carbons are vinylic, and the third carbon is allylic.
3. Why is the allylic position more reactive?
The allylic position is more reactive because intermediates formed there are stabilized by resonance.
- When a carbocation or radical forms at the allylic carbon, the positive charge or unpaired electron can delocalize over the double bond.
- This resonance stabilization lowers the energy of the intermediate.
- As a result, allylic substitution reactions occur more readily than at ordinary alkyl positions.
4. What is an allylic carbocation?
An allylic carbocation is a positively charged carbon adjacent to a carbon–carbon double bond, stabilized by resonance.
- General form: CH2=CH–CH2+
- The positive charge is delocalized between two carbons through resonance structures.
- This makes allylic carbocations more stable than simple primary carbocations.
5. What is allylic halogenation?
Allylic halogenation is a reaction in which a hydrogen at the allylic position is replaced by a halogen, usually under radical conditions.
- Common reagent: N-bromosuccinimide (NBS) in the presence of light (hν).
- The reaction proceeds via a resonance-stabilized allylic radical.
- Example: CH2=CH–CH3 + Br2 → CH2=CH–CH2Br + HBr (radical conditions).
6. How do you identify an allylic carbon in a structure?
To identify an allylic carbon, locate a carbon–carbon double bond and find the carbon directly attached to it by a single bond.
- Step 1: Identify the C=C bond in the molecule.
- Step 2: Look at the carbons next to the double-bonded carbons.
- Step 3: Any such adjacent carbon is an allylic carbon.
7. What is an allylic radical?
An allylic radical is a free radical located on a carbon adjacent to a carbon–carbon double bond and stabilized by resonance.
- Example: CH2=CH–CH2•
- The unpaired electron is delocalized over the π system.
- This makes allylic radicals more stable than typical alkyl radicals.
8. What is the difference between allylic and benzylic carbon?
An allylic carbon is adjacent to a C=C bond, whereas a benzylic carbon is adjacent to a benzene ring.
- Allylic example: CH2=CH–CH3
- Benzylic example: C6H5–CH3
- Both positions are stabilized by resonance.
9. Can allylic carbons undergo oxidation?
Yes, allylic carbons can undergo oxidation to form allylic alcohols, aldehydes, or ketones depending on conditions.
- Common oxidizing agents include SeO2 (selenium dioxide).
- Example: CH2=CH–CH3 + [O] → CH2=CH–CH2OH (allylic alcohol).
- The reaction selectively oxidizes the allylic C–H bond.
10. Why are allylic hydrogens more acidic than alkane hydrogens?
Allylic hydrogens are more acidic than alkane hydrogens because the resulting carbanion is stabilized by resonance.
- Removal of an allylic hydrogen forms an allylic carbanion.
- The negative charge is delocalized over the adjacent double bond.
- This stabilization lowers the energy of the conjugate base.
