Study Guide For Allylic Carbon: Its Structure and Reactions

An allylic carbon atom is an sp3 hybridised carbon atom in the allylic group RCH2-CH=CH2 and is bonded to the -CH=CH2 group. For example, the exposed carbon atom in Propene is an allylic carbon atom (CH3-CH=CH2). Comparably, in cyclohexene, the carbon atoms adjacent to the double bond become allylic carbon atoms.

Location of Allylic Carbon in Propene (Zone)

H₂C=CH–CH₃
              ↑ allylic carbon

Zone	Hybridisation	Carbon Type
Two carbons of the C=C	sp2	vinylic
CH3 carbon beside the C=C	sp3	allylic

An allylic carbon is sp³-hybridised but sits one σ-bond away from a C=C π-system. When a radical, carbocation or carbanion forms at that carbon, the charge or unpaired electron can delocalise over the adjacent double bond, lowering the activation barrier for many otherwise sluggish reactions. 

1. Radical halogenation (NBS or X₂ / hν)

Use-case: Industrial allyl bromide production; late-stage bromination without touching the double bond.

Do remember that entrance exams like JEE Main and IIT JAM ask questions based on these reactions.

2. SN1 and SN2′ (allylic substitution)

Pharma & fragrance: An Easy way to install new groups while migrating the double bond.

3. Allylic oxidation

4. Pd-catalysed π-allyl substitutions (Tsuji–Trost)

1. Oxidative addition of an allylic carbonate/halide to Pd⁰ → π-allyl–Pd²⁺ complex.
2. Nucleophilic attack regenerates Pd⁰ and forms an allylated product.
   
   

5. Allylic rearrangements and shifts

6. Polymer initiation via allylic radicals

Monomers such as chloroprene form allylic radicals during chain initiation; resonance delocalisation tempers their reactivity and gives controllable polymer growth (e.g., neoprene rubber).

7. Coupling reactions of allylic halides

Allylic halides are softer electrophiles than vinylic halides, so oxidative addition is smoother.

The following table gives the details related to the structure of Allylic carbon:

Compounds of halogen atoms bound to sp3-hybridized carbon to carbon-carbon double bond (C=C) i.e., to allylic carbon.

Allylic carbonation is actually a resonance-stabilized carbonium ion that has two resonance structures in which the formal charge of +1 is on an allylic carbon.

An allylic rearrangement or an allylic transition is an organic reaction in which the double bond of an allyl chemical compound moves to the next carbon atom. It is used for nucleophilic substitution.

The intermediate is a carbohydrate for which several resonance structures are possible under the reaction conditions that support the SN1 reaction method. This demonstrates the distribution (or spread) of the sample after recombination with nucleophilic Y. This step is called the substitution of SN1.

Alternatively, nucleophiles may strike directly at the allylic position, displacing the leaving group in a single stage, in a process referred to as SN2 substitution. This is likely in situations when the allyl compound is unhindered and a good nucleophile is used. The goods would be identical to those used with the replacement of SN1. Thus, the interaction of 1-chloro-2-butene with sodium hydroxide results in a mixture of 2-buten-1-ol and 1-buten-3-ol.

Here is a simple yet useful guide for those who plan to take exams like IISER and NEET in the near future.

H₂C = CH – CH₃
          ↑
     Allylic Carbon