Molecular Orbitals Flashcards
What are molecular orbitals ?
-Molecular orbitals form when atomic orbitals combine; the combination of two atomic orbitals results in the formation of a bonding molecular orbital and an antibonding orbital.
-The bonding molecular orbital encompasses both nuclei.
-The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital is the basis of bonding between atoms.
-Each molecular orbital can hold a maximum of two electrons
Describe molecular orbitals in non-polar covalent bonds
In a non-polar covalent bond, the bonding molecular orbital is symmetrical about the midpoint between two atoms.
Describe molecular orbitals in polar covalent bonds
Polar covalent bonds result from bonding molecular orbitals that are asymmetric about the midpoint between two atoms. The atom with the greater value for electronegativity has the greater share of the bonding electrons.
Describe molecular orbitals in ionic bonds
Ionic compounds are an extreme case of asymmetry, with the bonding molecular orbitals being almost entirely located around just one atom, resulting in the formation of ions
What are sigma bonds ?
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond are called sigma (σ ) molecular orbitals or sigma bonds.
What are pi bonds ?
Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond are called pi (π ) molecular orbitals or pi bonds.
What is hybridisation ?
-The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules. The bonding and shape of molecules of carbon can be explained by hybridisation.
-Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals. These hybrid orbitals are degenerate.
Describe hybridisation in alkanes
-In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals.
-These adopt a tetrahedral arrangement.
-The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form σ bonds.
Describe hybridisation in alkenes
-The bonding in alkenes can be described in terms of sp2 hybridisation.
-The 2s orbital and two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals. These adopt a trigonal planar arrangement. The hybrid sp2 orbitals overlap end-on to form σ bonds. The remaining 2p orbital on each carbon atom of the double bond is unhybridised and lies perpendicular to the axis of the σ bond. The unhybridised p orbitals overlap side-on to form πbonds.
Describe hybridisation in alkynes
-The bonding in alkynes can be described in terms of sp hybridisation.
-The 2s orbital and one 2p orbital of carbon hybridise to form two degenerate hybrid orbitals. These adopt a linear arrangement. The hybrid sp orbitals overlap end-on to form σ bonds. The remaining two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the σ bond. The unhybridised p orbitals overlap side-on to form two π bonds.
Describe hybridisation in benzene
-The bonding in benzene and other aromatic systems can be described in terms of sp2 hybridisation.
-The six carbon atoms in benzene are arranged in a cyclic structure with σ bonds between the carbon atoms.
-The unhybridised p orbitals on each carbon atom overlap side-on to form a π molecular system, perpendicular to the plane of the σ bonds.
-This π molecular system extends across all six carbon atoms. The electrons in this system are delocalised.
Lack of CC double bonds = addition reactions, aromatic = electrol deloca
What is the HOMO and the LUMO ?
-Molecular orbital theory can be used to explain why organic molecules are colourless or coloured.
-Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.
-The highest bonding molecular orbital containing electrons is called the highest occupied molecular orbital (HOMO).
-The lowest antibonding molecular orbital is called the lowest unoccupied molecular orbital (LUMO).
Why are most organic compounds colourless ?
-Absorption of electromagnetic energy can cause electrons to be promoted from HOMO to LUMO.
-Most organic molecules appear colourless because the energy difference between HOMO and LUMO is relatively large. This results in absorption of light from the ultraviolet region of the spectrum.
What is a chromophore ?
-Some organic molecules contain chromophores.
-A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum.
-Light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO.
Where do chromophores exist ?
-Chromophores exist in molecules containing a conjugated system —a system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms.
-Electrons within this conjugated system are delocalised.
-Molecules with alternating single and double bonds, and aromatic molecules have conjugated systems.
