unit 3)molecular orbitals -colour

Question 1

molecular orbital theory can be used to explain why organic molecules are ____ or _____
most organic molecules are ______

Answer 1

coloured
colourless
most organic molecules are colourless

Question 2

end on overlap of atomic orbitals along the axis of the bond results in….
side on overlap of atomic orbitals along the axis of the bond results in….

Answer 2

end on- sigma bonding orbital, sigma antibonding orbital

side on- pi bonding orbital, pi antibonding orbital

Question 3

bonding molecular orbitals contain _____ and non bonding orbitals contain ____ _____ of electrons

Answer 3

electrons

lone pairs

Question 4

the sigma and pi bonding orbitals contain normal bonding pairs of ______
the sigma and pi antibonding orbitals contain no ______
why?

Answer 4

electrons
no electrons , so are empty
- electrons fill the bonding molecular orbitals first as they have lower energy than the antibonding molecular orbitals, so antibonding orbitals will be empty

Question 5

several transitions are possible between the molecular orbitals, each with an electron being ______ from a full orbital into an empty one

Answer 5

excited

Question 6

how do transitions between molecular orbitals occur

Answer 6

• energy from photon is used to promote electrons from bonding or non bonding orbitals, into the higher energy antibonding orbitals
• so when absorption occurs , electrons are excited and promoted from the filled orbital(eg electron from pi or sigma bonding orbital/lone pair from non bonding orbital) into higher energy antibonding orbital

Question 7

colour or colourless ?

• saturated compounds with only sigma bond
• unsaturated compounds with pi bond

Answer 7

• colourless

- colourless

Question 8

in saturated compounds containing only a sigma bond, what is the HOMO and what is the LUMO

Answer 8

• sigma bonding orbital is the HOMO(highest occupied molecular orbital)
• sigma antibonding orbital is the LUMO (lowest unoccupied molecular orbital)

Question 9

the transition between these orbitals (sigma bonding orbital and sigma antibonding orbital) is ____ ______

Answer 9

quite large (requires high energy)

Question 10

so why are saturated compounds with only sigma bonds colourless

Answer 10

the transition (between sigma bonding and antibonding orbitals) is high energy and corresponds to the ultraviolet part of the spectrum, no visible light is absorbed so the compound is colourless

Question 11

explain why compounds containing simple pi bonds are also colourless

Answer 11

eg ethene , butene
- there is still a large transition required to promote an electron from the HOMO (pi bonding orbital to the LUMO( sigma antibonding orbital) so these compounds also absorb in the ultraviolet part of the spectrum and are colourless.

Question 12

what does homo and lumo actually mean

Answer 12

• the orbital containing electrons with the highest energy is known as the highest occupied molecular orbital (HOMO)
• the name given to lowest energy molecular orbital that is empty is the lowest unoccupied molecular orbital (LUMO)

Question 13

what is conjugation/ a conjugated system

Question 14

what features do organic molecules with colour have

what dp these features allow?

Answer 14

• large numbers of carbon atoms, must contain alternating pi and sigma bonds (single and double bonds) or benzene rings.
• these features allow the molecule to have electrons that are delocalised across a number of carbon atoms. this is known as a conjugated system

Question 15

organic molecules must contain a large degree of ______ to be coloured

Answer 15

conjugation

Question 16

explain how colour appears in organic molecules

Answer 16

• molecule has alternating pi and sigma bonds or a benzene ring allowing electrons to be delocalised among a number of carbon atoms
• the greater the number of ,molecular orbitals containing delocalised electrons the smaller the energy difference between the HOMO and the LUMO
• so to excite delocalised electrons , it will require less energy , and falls within the visible light part of the electromagnetic spectrum so molecule appears coloured

Question 17

summary: the ______ the conjugation in the molecule , the _____ the HOMO-LUMO gap and therefore the lower the absorbed energy

Answer 17

greater

smaller

Question 18

what is a chromophore

Answer 18

a section within a molecule which is responsible for its colour
(group of atoms within the molecule that is responsible for the absorption of light in the visible range and therefore its colour )(so alternating single and double bonds in the conjugated system)

Question 19

what can be found out by comparing chromophores

Answer 19

by comparing chromophores it is possible to find out about the energy of light being absorbed

Question 20

molecules with _____ conjugation absorb lower energy light (____ wavelength)

Answer 20

greater

longer

Question 21

how do the colours we observe show up in an organic molecule

Answer 21

• if the chromophore absorbs light of one colour then the complementary colour is observed

Question 22

what colours are likely to be observed with

• great conjugation
• less conjugation

Answer 22

great conjugation : red

less conjugation: yellow

Question 23

organic compounds absorb photons of energy from the electromagnetic spectrum these compounds can by analysed using……

Answer 23

a UV-VIS Spectrophotometer