CD 1, 2

Question 1

What is a conjugated system

Answer 1

a system with alternating double and single bonds that allows the overlap of p-orbitals

Question 2

what are delocalised electrons

Answer 2

electrons that are not associated with a particular pair of atoms, but are able to spread over several atoms

Question 3

why do white opaque solids appear white in sunlight (or white light)

Answer 3

• because none of the wavelengths of incident of light are absorbed by the surface of the object, they are all reflected
• our brains perceive a picture of all the wavelengths of visible light as white

Question 4

why so substances appear coloulress

Answer 4

• none of the wavelengths of incidence of light are absorbed
• instead they are transmitted (they pass through the object)

Question 5

what are the complementary colours of the following
1. red
2. orange
3. yellow

Answer 5

1. green
2. blue
3. violet

Question 6

define complementary colours

Answer 6

• two colours which when combined together produce white light

Question 7

what is the relationship between wavelength and frequency

Answer 7

wavelength x frequency = speed of light (3.0 x 10^8)

Question 8

what happens when substances absorb radiation from the visible light region

Answer 8

• the energy absorbed causes a change in electronic energy
• electrons are excited from their ground state to an excited state

Question 9

what electronic transitions occur for colourless compounds

Answer 9

• the compound absorbs ultraviolet radiation but NOT VISIBLE LIGHT
• so it appears colourless

Question 10

what electronic transitions occur in colourless compounds

Answer 10

• the compound does not absorb light in the visible light region
• they require greater energy
• they absorb ultraviolet radiation
• electrons are excited from its ground state to an excited state

Question 11

how does delocalisation work in a conjugated system

Answer 11

• consists of alternating double and single bonds
• electrons in the p-orbitals from the pi bond in the double bond spread out all over all the atoms in the system

Question 12

how do conjugated systems affect energy gaps

Answer 12

• the presence of a conjugated system (e.g benzene) decreases the energy gap between ground state and excited state
• the more delocalised electron sin a conjugated system, the smaller the energy gap
• smaller energy gaps result in the absorption of longer wavelengths of light

Question 13

what is the modern model of benzene based off of, what features of benzene did it reveal

Answer 13

• X-ray diffraction that produced a contour map of electron density in a benzene molecule
• benzene ring is a regular planar hexagon
• all the bond angles are 120 degrees
• all the carbon-carbon bonds are the same length, shorter than a double carbon bond but shorter than a single one

Question 14

how do you know that benzene is unreactive

Answer 14

• it does not decolourise bromine water

Question 15

in terms of electron and structure, explain why benzene is a stable molecule

Answer 15

• each carbon has 4 outer electrons and uses 3 (1s2, 2s2) of them to make single sigma bonds to carbon or hydrogen, this leaves one p orbital electron
• instead of forming 2 pi bonds, these electrons delocalise and spread out evenly amongst the molecule to be shared by each carbon atom
• this reduces the electron density of the molecule
• these electrons form a delocalised charge cloud above and below the molecule
• the more delocalised a structure, the more stable it is

Question 16

what type of reactions does benzene undergo and which does it NOT undergo

Answer 16

• it undergoes electrophilic substitution
• it cannot undergo electrophilic addition because the molecule is too stable

Question 17

what a piece of experimental evidence proving that benzene is a stable molecule

Answer 17

• hydrogenation
• when one C=C bond is present in a 6 membered ring, the enthalpy pf hydrogenation is -120 KJ/mol
• when 3 C=C bonds are present, Kekule’s structure (no delocalisation), the expected outcome is -360 KJ/mol
• but the actual enthalpy change of benzene is -208
• this is what is predicted by the delocalised model because it’s more stable, so more energy is needed for hydrogenation, so less energy is released

Question 18

why isn’t Kekule’s model of benzene a regular hexagon

Answer 18

• because it consists of alternating double and single bonds
• double bonds are shorter than single bonds, so it wouldn’t be regular because the sides would be of different lengths

Question 19

briefly state the similarities and differences between the Kekule model and the modern model of benzene

Answer 19

Kekule
1. asymmetrical hexagon (planar)
2. bond angles 120
3. alternating short and long bond lengths
4. enthalpy change of hydrogenation -360 kJ/mol

Delocalised model
1. symmetrical hexagon (planar)
2. bond angles 120
3. bond lengths are equal
4. enthalpy change of hydrogenation -208 kJ/mol (or just much less negative)