Allylic Carbon

What is Allylic Carbon?

The double-bonded carbon atoms can be classified as vinylic and allylic carbon atoms. The general formula for the vinyl group is R-CH=CH2 in which both carbon atoms are bonded with a double bond and R is attached at vinylic position.

The Modern Periodic table in which all known elements are arranged in increasing order of atomic number. There are 18 vertical columns called groups and 7 horizontal rows called periods.

The bottom part of the periodic table contains two series of 14 elements which are called an f-block elements. The left side of the Modern Periodic Table mainly contains metals whereas the right side is the non-metallic region. Some elements show intermediate properties of metals and non-metals. These elements are called metalloids and are located in between metals and non-metals as a zigzag line.

Table of Contents

• Allylic Carbon Meaning
• Allylic Carbon Atoms
• Hydrocarbons
• Allylic Carbocation
• Frequently Asked Questions – FAQs

Metals can be identified as elements which are highly reactive and electropositive in nature. Unlike metals, non-metals are electronegative in nature. Carbon is one of the most common non-metals. It is the basis of all organic compounds.

Allylic Carbon Meaning

The double-bonded carbon atoms can be classified as vinylic and allylic carbon atoms. The general formula for vinyl group is R-CH=CH₂ in which both carbon atoms are bonded with a double bond and R is attached at vinylic position.

• Since both carbon atoms form a double covalent bond so both are sp² hybridized. The allylic position is also like a vinylic position. The allylic carbon is bonded to a carbon atom which is doubly bonded to another carbon atom.
• The general formula for allyl is R-CH₂-CH=CH₂ in which the asterisk carbon atom is an allylic carbon atom. Unlike vinyl group, the allylic carbon atom is sp³ hybridized as it bonded with CH=CH₂ through a single covalent bond.
• It imparts unique chemical properties to the allylic group and the presence of this group in different compounds forms allylic compounds which are used for the preparation of different natural products like natural rubber, terpenes etc.

Allylic Carbon Atoms

The allylic carbon atom is the sp³ hybridized carbon atom in the allylic group RCH₂-CH=CH₂ that is bonded with the -CH=CH₂ group.

For example in propene, the highlighted carbon atom is the allylic carbon atom (CH₃-CH=CH₂). Similarly, in cyclohexene, the carbon atoms next to the double bond are allylic carbon atoms.

Hydrocarbons

Organic compounds are composed of different elements with a parent carbon chain. Hydrocarbons are the most common organic compounds. They are composed of carbon and hydrogen mainly.

• Carbon exhibits tetravalency so it can form four covalent bonds with the same or different elements.
• Due to tetravalency, carbon shows catenation and can form different organic compounds.
• Catenation is the property of carbon or other elements to form covalent bonds with the same element.
• On the basis of a number of carbon atoms bonded with a carbon atom, it can classify as a primary, secondary and tertiary carbon atom.
• A carbon atom which is bonded with one another carbon atom is called a primary carbon atom.

For example, in ethane molecule (CH₃-CH₃) both carbon atoms are bonded with one other carbon atom so both carbon atoms are the primary carbon atom here. The secondary carbon atom is bonded with the other two carbon atoms whereas the tertiary carbon atom is bonded to three other carbon atoms.

Allylic Carbocation

Carbocations are ionic species which carry a positive charge on the carbon atom of the molecule. They usually form as an intermediate during various chemical reactions. The stability of carbocations determines by steric hindrance and +I effect of alkyl groups attached to C⁺ of the carbocation.

As the +I effect increases due to the positively charged carbon atom of the carbocation, it reduces the positive charge on the carbocation. So we can say that as the number of alkyl groups increases on C⁺, the stability of carbocation also increases.

Therefore, the stability order of carbocation can be written as

Tertiary Carbocation > Secondary Carbocation > Primary Carbocation

• In the allylic group, if the allylic carbon atom carries a positive charge, it forms an allylic carbocation. The allylic carbocation is stable due to the delocalisation of electrons on carbon atoms.
• Similarly, in the case of carbocation of cyclohexene, the formal charge on allylic carbon is +1 and it stabilizes by resonance with pi-bond.
• If the allylic carbon atom is attached to one carbon atom and carries a +1 charge, it is called primary allylic carbocation. Here formal charge of +1 is on the primary carbon atom so it is named primary allylic carbocation.
• In the case of the secondary allylic carbocation, the +1 formal charge is distributed on the secondary carbon atom as in cyclohexene cation.
• A tertiary allylic carbocation has a +1 charge on a tertiary carbon atom of the cation.