U4(a): Molecular Orbitals

U4:Organic Chemistry & Instrumental Analysis - (a) Molecular Orbitals

Formation of Molecular Orbitals

• Molecular orbitals form when atomic orbitals combine

• The number of molecular orbitals formed is equal to the number of atomic orbitals that 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

Basis of bonding between atoms

The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital is the basis of bonding between atoms

Bonding molecular orbital

Orbital where electrons spend most of their time between the two nuclei

Antibonding molecular orbital

Orbital where electrons spend most of their time away from the region between the two nuclei

Symmetry of bonding molecular orbital in non-polar covalent bond

In a non-polar covalent bond, the bonding molecular orbital is symmetrical about the midpoint between two atoms.

Symmetry of bonding molecular orbital in polar covalent bond

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.

Symmetry of bonding molecular orbital in ionic compound

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.

Sigma (σ) molecular orbitals or sigma bonds.

Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond

Pi (π) molecular orbitals or pi bonds.

Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond

Hybridisation

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.

Explain the bonding and shape of alkanes with reference to hybridisation

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.

Explain the bonding and shape of alkenes with reference to hybridisation

• 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 π bonds.

Explain the bonding and shape of alkynes with reference to hybridisation

• 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.

Explain the bonding and shape of benzene and other aromatic systems with reference to hybridisation

• 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.

Highest occupied molecular orbital (HOMO).

The highest bonding molecular orbital containing electrons

Lowest unoccupied molecular orbital (LUMO).

The lowest antibonding molecular orbital

Explain how organic molecules can be coloured.

• Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.

• Absorption of electromagnetic energy can cause electrons to be promoted from HOMO to LUMO.

• If the energy gap between HOMO and LUMO is small, then wavelength of light absorbed is in the visible region and the compound will exhibit the complementary colour.

Explain why most organic molecules appear colourless

• 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

Chromophore

Group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum

State when light can be absorbed by an organic molecule

When electrons in a chromophore are promoted from the HOMO to the LUMO

Chromophores exist in molecules containing a:

conjugated system

Conjugated system

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.

State the two kinds of molecules containing conjugated systems

• molecules with alternating single and double bonds

• aromatic molecules

The more atoms in the conjugated system the {smaller/larger} the energy gap between HOMO and LUMO. A {lower/higher} frequency of light is absorbed by the compound.

The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO. A lower frequency of light is absorbed by the compound.