CD 1, 2

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

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

• 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

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

• green • blue • violet

• two colours which when combined together produce white light

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

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

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

• 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

• 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

• 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

• 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

• it does not decolourise bromine water

• 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

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

• 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

• 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

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)