Molecular Orbitals

Question 1

Hybridisation

Answer 1

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.

Question 2

Hybridisation in Alkanes

Answer 2

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.

Question 3

Hybridisation in Alkenes

Answer 3

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.

Question 4

Hybridisation in alkynes

Answer 4

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.

Question 5

HOMO

Answer 5

The highest bonding molecular orbital containing electrons is called the
highest occupied molecular orbital

Question 6

LUMO

Answer 6

The lowest antibonding molecular orbital is
called the lowest unoccupied molecular orbital

Question 7

Most organic molecules appear colourless because…?

Answer 7

the energy difference between HOMO
and LUMO is relatively large. This results in absorption of light from the ultraviolet region of the spectrum.

Question 8

Chromophores

Answer 8

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.

Question 9

Chromophores exist in molecules containing…

Answer 9

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.

Question 10

The more atoms in the conjugated system the…

Answer 10

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.

Question 11

Hybridisation in benzene

Answer 11

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.