Allylic carbon

1.0What is Allylic Carbon

An allylic carbon is a carbon atom that is directly attached to a carbon-carbon double bond (C=C bond) but is not part of the double bond itself. This unique position next to a double bond gives the allylic carbon distinctive properties and reactivity patterns in organic chemistry.

2.0Structure and Properties of Allylic Carbon

• The allylic position refers to the carbon(s) directly attached to the double bond. If we look at a simple molecule like propene (CH₂=CH-CH₃), the CH₃ group is attached to the CH=CH₂ double bond. In this case, the CH₃ carbon is called the allylic carbon.
• Allylic carbons are adjacent to double bonds and have higher reactivity due to resonance, while vinylic carbons are part of the double bond itself and have limited reactivity.

• The allylic carbon has sp³ hybridization since it is single-bonded to other atoms, but it is uniquely reactive due to its proximity to the π-electrons of the double bond.

3.0Difference between Allylic Carbon and Vinylic Carbon

4.0Resonance and Stability

• One of the importance of the allylic position is the resonance stabilization it provides. When an allylic carbon loses a hydrogen to form an allylic carbocation, radical, or anion, the resulting intermediate is resonance-stabilized. This means that the charge or radical can be delocalized across the adjacent double bond, spreading it over multiple atoms, which increases stability. For example:

CH₂=CH−CH₂⁺ → CH₂⁺−CH=CH₂

• Allylic carbocations (positively charged intermediates), allylic radicals (radicals), and allylic anions (negatively charged intermediates) are all stabilized by this resonance.

Stability order of Allylic carbocation

As we know Stability is related to +I effects and steric hindrance from alkyl groups attached to the positively charged carbon (C⁺).

More alkyl groups reduce the positive charge, increasing stability.

Stability Order: Tertiary > Secondary > Primary carbocation.

5.0Reactions Involving Allylic Carbons

• Allylic Substitution Reactions: Allylic carbons can participate in substitution reactions due to the stability provided by resonance. In these reactions, a substituent on the allylic carbon is replaced by another group.

Example Reaction:

• Allylic Rearrangement: Certain reactions, such as S_N1 reactions involving allylic carbocations, can cause rearrangements because of resonance. This can lead to the formation of different isomers.

• Conjugation in Dienes: Allylic carbons are significant in conjugated systems, especially in compounds with alternating double and single bonds (dienes). This conjugation increases stability and reactivity, particularly in Diels-Alder reactions.