Organic - Molecular Orbitals

Can VSEPR explain the bonding in all compounds?

No VSEPR cannot examples the bonding in all compounds. Molecular orbital theory can provide an explanation for more complex molecules.

What forms when atomic orbitals combine?

Molecular orbitals

What does the number of molecular orbitals formed equal to?

The number of atomic orbitals that combine

What does the combination of two atomic orbitals result in?

the formation of a bonding molecular orbital and an anti bonding orbital.

What does the bonding molecular orbitals formed encompass?

Both nuclei

What is the basis of bonding between atoms?

The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital.

What is the maximum each molecular orbital can hold?

A maximum of two electrons

What happens to the bonding molecular orbitals formed in a non-polar covalent bond?

Symmetrical about the midpoint between two atoms

What happens to the bonding molecular orbitals formed in a polar covalent bond?

Asymmetric about the midpoint between two atoms.

What is the case for ionic compounds?

Extreme case of asymmetry —> resulting in the formation of ions

What are molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bonds called?

Sigma (o-) molecular orbitals or sigma bonds

What are molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bonds called?

Pi (n-) molecular orbitals or pi bonds

Can the electron configuration of an isolated carbon atom explain the number of bonds formed by carbon atoms in molecules?

No.

What can explain the bonding and shape of molecules of carbon?

By hybridisation

What is the process of hybridisation?

Process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.

What are these hybrid orbitals?

Degenerate

What can the bonding in alkanes be described in terms of and what shape does it result in?

Sp3 hybridisation. These adopt a tetrahedral arrangement.

What can the bonding in alkenes (also benzene and other aromatic systems) be described in terms of and what shape does it result in?

Sp2 hybridisation. These adopt a trigonal planar arrangement.

What can the bonding in alkynes be described in terms of and what shape does it result in?

Sp. These adopt a linear arrangement.

What can be used to explain why organic molecules are colourless or coloured?

Molecular orbital theory.

What do electrons do with bonding and anti-bonding orbitals?

Electrons fill bonding molecular orbitals, leaving higher energy anti-bonding orbitals unfilled.

What is the highest bonding molecular orbital containing electrons called?

Highest occupied molecular orbital (HOMO)

What is the lowest anti-bonding molecular orbital called?

Lowest unoccupied molecular orbital (LUMO)

What can the absorption of electromagnetic energy cause for electrons?

Can cause electrons to be promoted from HOMO to LUMO

Why does most organic molecules appear colourless?

Because the energy difference between HOMO and LUMO is relatively large.

What does this result in?

In absorption of light from the ultraviolet region of the spectrum.

What do some organic molecules contain?

Chromophores.

What are chromophores?

A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible part of the spectrum.

What happens when electrons in a chromophore are promoted from the HOMO to LUMO?

Light can be absorbed

Chromophores exist in molecules containing a conjugated system. What does conjugated system mean?

A system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms.

What type of electrons are there within this conjugated system?

Delocalised

What other types of molecules have conjugated systems?

Molecules with alternating single and double bonds, and aromatic molecules.

The more atoms in the conjugated system the …?

The smaller the energy gap between HOMO and LUMO

What is absorbed by the compound?

A lower frequency of light (longer wavelength, lower energy)

What happens when the wavelength of light absorbed is in the visible region?

The compound will exhibit the complementary colour.