Organic Chemistry 1.3 - Molecular Orbitals - Molecular Orbital Colour Theory

Question 1

Molecular Orbital (MO) colour theory can be used to explain why ___ molecules are colourless or coloured.

Answer 1

organic

Question 2

The fact that bonding MOs have ___ ___ than antibonding MOs is the basis of this colour theory.

Answer 2

different energies

Question 3

Bonding MOs have ___er energies than antibonding* MOs, and contain ___.

Answer 3

lower, electrons

Question 4

Antibonding* MOs have ___ energies than bonding MOs, and do not contain ___.

Answer 4

higher, electrons

Question 5

What is the highest energy bonding MO called?
What is the lowest energy antibonding MO?

Answer 5

HOMO.
LUMO.
*(The highest of the lowest and the lowest of the highest)

Question 6

What does HOMO and LUMO stand for?

Answer 6

Highest Occupied MO
Lowest Unoccupied MO
*occupied with electrons

Question 7

Which has higher energy: HOMO or LUMO?

Answer 7

LUMO

Question 8

When is electromagnetic radiation absorbed?

Answer 8

whilst promoting electrons from HOMO to LUMO

Question 9

Why do certain organic molecules appear colourless?

Answer 9

because no EMR from the visible region is absorbed (and therefore it is all reflected - and all colours together = clear)

Question 10

The majority of organic molecules appear colourless because the ___ difference between ___ and ___ is so large that EMR from the __ region is absorbed.

Answer 10

energy, HOMO, LUMO, UV

Question 11

Coloured molecules have a HOMO-LUMO energy difference which corresponds to…

Answer 11

visible light

Question 12

The absorption of a wavelength of visible light means that the ___ ___ is observed/reflected.

Answer 12

complementary colour
*colour wheel in db page 19

Question 13

Some organic molecules contain…

Answer 13

chromophores

Question 14

What is a chromophore?

Answer 14

a group of atoms within an molecule that is responsible for the absorption of a photon/wavelength of light from the visible region of the EM spectrum

Question 15

Light is absorbed when ___ in a ____ are promoted from HOMO to LUMO.

Answer 15

electrons, chromophore

Question 16

Chromophores exist in molecules which have a…

Answer 16

conjugated system

Question 17

What is a conjugated system?

Answer 17

a conjugated system is a chain of alternating double single bonds (with delocalised electrons).
c-c=c-c=c-c=c

Question 18

Conjugated systems can only exist between atoms of which element?

Answer 18

carbon

Question 19

A conjugated system is system of adjacent ___ p orbitals between carbon atoms which overlap __-on to form a ___ orbital which spans a number of atoms

Answer 19

unhybridised, side, molecular.

Question 20

It is said that ‘molecules with ___ of alternating double-single bonds between carbon atoms have a ___ ___.’

Answer 20

chains, conjugated system.

Question 21

The more carbon atoms in a conjugated system, the…

Answer 21

smaller the energy difference between HOMO and LUMO, therefore the lower energy photons of light absorbed

Question 22

To reiterate: the larger the conjugated system, the ___ the energy of light absorbed.

Answer 22

lower

Question 23

‘What structural feature allows molecule x to absorb visible light?’

Answer 23

a conjugated system made up of alternating double and single bonds

Question 24

Explain how a conjugated system gives rise to the blue colour of molecule x.’

Answer 24

the wavelength of light which is complementary to blue is absorbed whilst promoting an electron from HOMO to LUMO.
or other words to that affect.

Question 25

What are the key words in this answer?
‘the wavelength of light which is complementary to blue is absorbed whilst promoting an electron from HOMO to LUMO’

Answer 25

light, complementary, blue, absorbed, promoting, electron, HOMO, LUMO

Question 26

What is the difference between a conjugated system and a chromophore??

Answer 26

a conjugated system is a series of alternating double and single bonds between carbon atoms

a chromophore is a group of atoms within a molecule that is capable of absorbing a photon of visible light.

Question 27

Explain fully why this dye molecule will absorb a shorter wavelength of light than the other two dye molecules. (it is smaller than another ones and has less carbons in its conjusated system)

Answer 27

There is less conjugation
OR
a shorter sequence of alternating double and single bonds
OR
a smaller chromophore. (1)

Larger gap/greater energy
(absorbed). (1)

Question 28

Smaller conjugated system = (smaller/larger) HOMO-LUMO energy difference.

Answer 28

LARGER