Allylic Carbon

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 element. 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.

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 basic 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 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 allylic group and presence of this group in different compounds form 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 allylic group RCH2-CH=CH2 that is bonded with -CH=CH2 group.

For example in propene, the highlighted carbon atom is allylic carbon atom (CH3-CH=CH2). Similarly, in cyclohexene, the carbon atoms next to 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 compound.
  • 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 primary carbon atom here. The secondary carbon atom is bonded with other two carbon atoms whereas tertiary carbon atom is bonded to three other carbon atoms.

Allylic Carbocation

Carbocations are ionic species which carry 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 delocalization 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 with one carbon atom and carries a +1 charge, it is called as primary allylic carbocation. Here formal charge of +1 is on primary carbon atom so it is named as primary allylic carbocation.
  • In case of the secondary allylic carbocation, the +1 formal charge is distributed on secondary carbon atom as in cyclohexene cation.
  • A tertiary allylic carbocation has +1 charge on a tertiary carbon atom of the cation.

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