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).

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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