CD 1, 2 Flashcards
What is a conjugated system
a system with alternating double and single bonds that allows the overlap of p-orbitals
what are delocalised electrons
electrons that are not associated with a particular pair of atoms, but are able to spread over several atoms
why do white opaque solids appear white in sunlight (or white light)
- 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
why so substances appear coloulress
- none of the wavelengths of incidence of light are absorbed
- instead they are transmitted (they pass through the object)
what are the complementary colours of the following
1. red
2. orange
3. yellow
- green
- blue
- violet
define complementary colours
- two colours which when combined together produce white light
what is the relationship between wavelength and frequency
wavelength x frequency = speed of light (3.0 x 10^8)
what happens when substances absorb radiation from the visible light region
- the energy absorbed causes a change in electronic energy
- electrons are excited from their ground state to an excited state
what electronic transitions occur for colourless compounds
- the compound absorbs ultraviolet radiation but NOT VISIBLE LIGHT
- so it appears colourless
what electronic transitions occur in colourless compounds
- 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
how does delocalisation work in a conjugated system
- 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
how do conjugated systems affect energy gaps
- 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
what is the modern model of benzene based off of, what features of benzene did it reveal
- 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
how do you know that benzene is unreactive
- it does not decolourise bromine water
in terms of electron and structure, explain why benzene is a stable molecule
- 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
what type of reactions does benzene undergo and which does it NOT undergo
- it undergoes electrophilic substitution
- it cannot undergo electrophilic addition because the molecule is too stable
what a piece of experimental evidence proving that benzene is a stable molecule
- 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
why isn’t Kekule’s model of benzene a regular hexagon
- 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
briefly state the similarities and differences between the Kekule model and the modern model of benzene
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)
