Allylic Carbon

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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.

FAQ (Frequently Asked Questions)

1. Explain Primary, Secondary, and Tertiary Allylic Carbocations?

Primary Carbocation

If in one of the two resonance forms of the allylic carbocation, the regular price of +1 is on one carbon level, the allylic carbocation is referred to as the level one (1°) allylic carbocation.


Secondary Carbocation

If there is a formal charge of +1 on the secondary carbon inside the more strong of two allyl carbocation resonance groups, the allyl carbocation is referred to as secondary (2°) allyl carbocation; if there is a formal charge of +1 on the secondary carbon in each resonance carbocation, it is also referred to as secondary allyl carbocation.


Tertiary Carbocation

If the formal price of +1 is on tertiary carbon in either one or both of the resonance styles of the allylic carbocation, the allylic carbocation is referred to as tertiary (3°) allylic carbocation.

2. Explain the Factors Affecting the Stability of Carbocation?

Generally, carbocations are unstable due to not having eight electrons to satisfy the octet rule.

The three foremost factors to increase the stability of carbocations are, 

Increasing the adjacent carbon atom count

Methyl (the least stable carbocation) < primary < secondary < tertiary (the most stable carbocation)

The tertiary carbocation is the most stable one, as it is surrounded by other three carbon atoms that share its positive charge burden. Primary and methyl carbocations especially are seen rarely in organic reactions except under special circumstances such as in the case of benzylic or allylic cations.