AH Chemistry - Organic Chemistry and Instrumental Analysis - (a) - Molecular Orbitals

Since VSEPR cannot explain the bonding in all compounds, what can provide an explanation for more complex molecules?

VSEPR cannot explain the bonding in all compounds. Molecular orbital theory can provide an explanation for more complex molecules.

When do molecular orbitals form?

Molecular orbitals form when atomic orbitals combine.

What is the number of molecular orbitals formed equal to?

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

What does the combination of two atomic orbitals result in?

The combination of two atomic orbitals results in the formation of a bonding molecular orbital and an antibonding orbital.

What does the bonding molecular orbital encompass?

The bonding molecular orbital encompasses 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 is the basis of bonding between atoms.

How many electrons can each molecular orbital hold?

Each molecular orbital can hold a maximum of two electrons.

In a non-polar covalent bond, what is the bonding molecular orbital?

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

What do polar covalent bonds result from?

Polar covalent bonds result from bonding molecular orbitals that are asymmetric about the midpoint between two atoms.

What does the atom with the greater value for electronegativity have?

The atom with the greater value for electronegativity has the greater share of the bonding electrons.

What are ionic compounds an extreme case of?

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

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. add pic

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

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. add pic

What can the electronic configuration of an isolated carbon atom not explain?

The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules.

How can the bonding and shape of molecules of carbon be explained?

The bonding and shape of molecules of carbon can be explained by hybridisation.

What is 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.

In alkanes, what can the 2s orbital and the three 3 2p orbitals of carbon do?

In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerative sp³ hybrid orbitals. These adopt a tetrahedral arrangement. The sp³ hybrid orbitals overlap end-on with other atomic orbitals to form σ bonds.

How can the bonding in alkenes be described?

• The bonding in alkenes 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.

How can the bonding in benzene and other aromatic systems be described?

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.

How can the bonding of alkynes be described?

The bonding of 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 orbital overlap end-on end 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.

How can molecular orbital theory be used to explain why organic molecules are colourless or coloured?

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 molcular orbital containing electrons is called the highest occupied molecular orbital (HOMO). The lowest antibonding molecular orbital is called the lowest unoccupied molecular orbital (LUMO).

What can the absorption of electromagnetic energy cause?

The absorption of electromagnetic energy can cause electrons to be promoted from HOMO to LUMO.

Why do 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.

Some organic molecules contain chromophores. 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 HOMO to LUMO.

Where do chromophores exist?

Chromophores exist in molecules containing a conjugated system. A conjugated system is 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.

The more atoms in the conjugated system, the smaller the what?

The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO. A lower frequency of light (longer wavelength, lower energy) is absorbed by the compound. When the wavelength of light absorbed is in the visible region, the compound will exhibit the complementary colour.