CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.1.8 Hybridisation in Organic Molecules

Hybridised Atoms: Shapes & Bond Angles in Molecules

  • Each carbon atom has four electrons in its outer shell (electronic configuration: 1s22s22p2)
  • Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a full outer shell configuration
  • These electrons are found in orbitals within the respective atoms
  • When forming a covalent bond, the orbitals overlap in such a way to form two types of bonds
    • Sigma bonds (σ)
    • Pi bonds (π)

Hybridisation: sp3

  • The electron pair in a σ bond is found in a region of space between the nuclei of the two atoms that are sharing the electrons
  • The electrostatic attraction between the electrons (negatively charged) and the two nuclei (positively charged) holds the two atoms together
  • Carbon atoms that form four σ bonds are said to be sp3 hybridised
  • The four pairs of electrons around each carbon repel each other forcing the molecule to adopt a configuration in which the bonding pairs of electrons are as far away from each other as possible
    • The molecule adopts a tetrahedral arrangement with bond angles of 109.5 o

 

An Introduction to AS Level Organic Chemistry Sp3 Hybridisation, downloadable AS & A Level Chemistry revision notes

The diagram shows a molecule of ethane in which each carbon atom forms four σ bonds to adopt a tetrahedral configuration and minimise the repulsion between the bonding pairs of electrons

Hybridisation: sp2

  • When carbon atoms use only three of their electron pairs to form a σ bond, they are said to be sp2 hybridised
    • Each carbon atom will have a p orbital with contains one spare electron
  • When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)
  • The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap
  • The three bonding pair of electrons are in the plane of the molecule and repel each other
  • The molecule adopts a planar arrangement with bond angles of 120 o

 

An Introduction to AS Level Organic Chemistry Sp2 Hybridisation, downloadable AS & A Level Chemistry revision notes

The overlap of the two p orbitals results in the formation of a π bond in ethene (sp2 hybridised molecule) in which the bonding pair of electrons repel each other to force the molecule into a planar configuration with bond angles of 120 o

Hybridisation: sp

  • Carbon atoms can also use only one of their electron pair to form a σ bond, in which case the carbon atoms are said to be sp hybridised
    • Each carbon atom will have two p orbitals with one spare electron each
  • When the four p orbitals of the carbon atoms overlap with each other, two π bonds are formed (each π bond contains two electrons)
  • The two orbitals that form the π bond lie above and below the plane of the carbon atoms
  • The two orbitals of the other π bond lie in front and behind the plane of the atoms
    • This maximises the overlap of the four p orbitals
  • The molecule adopts a linear arrangement with bond angles 180 o

 

An Introduction to AS Level Organic Chemistry Sp Hybridisation, downloadable AS & A Level Chemistry revision notes

The overlap of the p orbitals results in the formation of two π bonds in ethyne (sp hybridised molecule) which adopts a linear arrangement with bond angles of 180

Exam Tip

A double bond is a combination of a σ and π bond and a triple bond is a combination of one σ and two π bonds.

The strength of the bonds increases as follows: single < double < triple bond

This is due to the increased electron density around the C-C atom, making the bond stronger and more difficult to break.

Hybridised Atoms: σ and π Bonds in Molecules

σ bonds

  • Sigma bonds are formed from the end-on overlap of atomic orbitals
    • S orbitals overlap this way as well as p orbitals

 

An Introduction to AS Level Organic Chemistry Bond Overlap in Sigma Orbitals, downloadable AS & A Level Chemistry revision notes

Sigma orbitals can be formed from the end-on overlap of s or p orbitals

  • The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
    • The pair of electrons is found between the nuclei of the two atoms
    • The electrostatic attraction between the electrons and nuclei bonds the atoms to each other
  • The diagram below shows the arrangement of the σ bond in sp3, sp2 and sp hybridised carbon atoms

 

An Introduction to AS Level Organic Chemistry Arrangement of Sigma Bonds, downloadable AS & A Level Chemistry revision notes

The σ orbitals are formed from the end-on overlap of the atomic orbitals resulting in symmetrical electron density on the atoms

π bonds

  • Pi (π) bonds are formed from the sideways overlap of p orbitals
  • The two lobes that make up the π bond lie above and below the plane of the atoms
    • This maximises overlap of the p orbitals

 

An Introduction to AS Level Organic Chemistry Bond Overlap in Pi Orbitals, downloadable AS & A Level Chemistry revision notes

π orbitals can be formed from the end-on overlap of p orbitals

  • In triple bonds, there is an additional overlap of p orbital
  • The two lobes of the π bond lie in front of and behind the plane of the atoms in the molecule
    • This maximises overlap of the p orbitals
  • The diagram below shows the arrangement of the π bond in sp3, sp2 and sp hybridised carbon atoms

 

An Introduction to AS Level Organic Chemistry Arrangement of Pi Bonds, downloadable AS & A Level Chemistry revision notes

The π orbitals are formed from the sideway overlap of the atomic orbitals

Exam Tip

π bonds are drawn as two electron clouds, one arising from each lobe of the p orbitals.

The two clouds of electrons in a π bond represent one bond consisting of two electrons (one from each orbital).

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