What Makes Allylic Carbons Unique in Organic Chemistry?
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 Explained: Key Concepts & Uses
1. What is an allylic carbon?
An allylic carbon is a saturated, sp³-hybridised carbon atom that is directly bonded to a carbon atom of a carbon-carbon double bond (C=C). The position itself, being one carbon away from the double bond, is referred to as the allylic position.
2. How do you identify an allylic carbon in a chemical structure?
To identify an allylic carbon, first locate a carbon-carbon double bond (C=C) within the molecule's structure. Then, look for a saturated carbon atom (one with only single bonds) that is immediately adjacent to either of the carbons in the double bond. That adjacent, sp³-hybridised carbon is the allylic carbon. For example, in propene (CH₂=CH-CH₃), the carbon of the methyl (-CH₃) group is the allylic carbon.
3. What is the key difference between an allylic carbon and a vinylic carbon?
The primary difference between an allylic and a vinylic carbon lies in their position relative to the double bond and their hybridisation state.
- An allylic carbon is sp³-hybridised and is adjacent to a C=C double bond.
- A vinylic carbon is sp²-hybridised and is one of the two carbon atoms directly participating in the C=C double bond.
4. Why are allylic carbocations more stable than typical primary or secondary carbocations?
Allylic carbocations exhibit enhanced stability due to resonance. The positive charge is not confined to a single carbon atom. Instead, it is delocalised, or spread out, across two carbon atoms through the adjacent π-electron system of the double bond. This distribution of positive charge over a larger area significantly stabilises the molecule, making it more stable than a simple carbocation where the charge is localised on a single carbon.
5. What are the main types of allylic compounds, with examples?
Allylic compounds are molecules containing a functional group bonded to an allylic carbon. Common types include:
- Allylic Halides: Where a halogen is attached to the allylic carbon. Example: Allyl chloride (CH₂=CH-CH₂Cl).
- Allylic Alcohols: Where a hydroxyl (-OH) group is attached. Example: Allyl alcohol (CH₂=CH-CH₂OH).
- Allylic Ethers: Where an alkoxy (-OR) group is attached. Example: Allyl methyl ether (CH₂=CH-CH₂OCH₃).
6. What is the importance of the allylic position in organic reactions?
The allylic position is a highly reactive site in organic molecules for two main reasons. First, the C-H bonds at an allylic position are weaker than standard alkane C-H bonds, making them susceptible to reactions like free-radical substitution (e.g., allylic bromination with NBS). Second, reactions that form intermediates like carbocations, radicals, or carbanions at the allylic position are highly favoured because these intermediates are stabilised by resonance. This leads to unique and synthetically useful reactions like allylic rearrangements.
7. How does the stability of primary, secondary, and tertiary allylic carbocations compare?
Just like simple carbocations, the stability of allylic carbocations increases with the degree of substitution on the carbon bearing the charge. All of them benefit from resonance, but the additional stabilising effect of alkyl groups (inductive effect and hyperconjugation) still applies. Therefore, the order of stability is: Tertiary (3°) allylic > Secondary (2°) allylic > Primary (1°) allylic. A tertiary allylic carbocation is the most stable because it benefits from both resonance and the electron-donating effects of three alkyl groups.
