A2 organic chemistry Flashcards
Benzene
C6H8
colourless, sweet smelling and highly flammable liquid
carcinogen
kekule model
6 ring of carbons joined by alternate single and double bounds
3 reasons why the Kekule model is wrong
- lack of reactivity of benzene
- lengths of carbon-carbon bonds in benzene
- hydrogenation enthalpies
how does lack of reactivity of benzene prove the Kekule model wrong
if benzene contained c=c bonds it would decolourise bromine in electrophilic addition reaction
benzene does not undergo electrophilic addition
benzene does not decolourise bromine under normal conditions
therefore benzene cannot have any C=C bonds.
how does the length of carbon carbon bonds in benzene prove the kekule model wrong
using x ray diffraction to measure bonds
all bonds of benzene were 0.139 nm in length
bond length between single and double bonds
how does hydrogenation enthalpies prove the kekule model wrong
we would expect the enthalpy change of hydration 🛆hydH that is 3 times of cyclohexene. because it contains 3 double bonds
when cyclohexene is hydrogenated 1 double bond reacts with hydrogen.
120 KJ mol-1 for cyclohexene. benzene should be 360 KJ mol-1 but is actually -208 KJ mol-1
what is the delocaised model of benzene
- benzene is a planar, cyclic, hexagonal hydrocarbon containing carbon atoms and 6 hydrogen atoms
- each carbon atom uses 3 of its available 4 electrons in bonding to 2 other carbon atoms and 1 hydrogen atom
- each carbon has 1 electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
adjacent p orbitals overlap sideways in both directions above and below the plane of carbon atoms to form a ring of electron density
overlapping of p orbitals creates a system of pi bonds which spread over all 6 carbon atoms in the ring structure.
6 electrons occupying this system of pi bonds are said to be delocalised
naming aromatic compounds with 1 substitute group
aromatic compounds with one substitute group are monosubsituted additions are prefixes to benzene. when benzene rings attached to alkyl chain with functional group or long alkyl chain. the prefix phenyl is used.
what are the exceptions to naming aromatic compounds with 1 substitute group
benzoic acid C6H5COOH
phenlyamine C6H5NH2
benzaldehyde C6H5CHO
what type of reactions do benzene undergo
electrophilic substitution
where 1 hydrogen atom is replaced. reacts with electrophiles
nitration of benzene
benzene + nitric acid --> nitro-benzene catlaysed by H2SO4 50oc electrophile = NO2+ HNO3 + H2SO4 --> NO2+ _ HSO4- + H2O H+ + HSO4- --> H2SO4
halogenation of benzene
cataylst = halogen carrier e.g AlCl3, FeBr3 etc
benzene + halogen —> halidebenzene + HBr
why does halogenation of benzene require a halogen catalyst
unlike alkenes benzene does not react with bromine unless a halogen carrier catalyst is present. this is because benzene has delocaliosed pi electrons spread above and below the plane. Electron density around any 2 carbon atoms is less than that in C=C double bond in an alkene.
when a non polar molecule such as bromine approaches benzene there is insufficient pi electron density around any 2 carbon atoms to polarise the bromine molecule. this prevents any reaction taking place
what is phenol
phenol are a type of organic chemical containing a hydroxyl -OH function group directly bonded to an aromatic ring
phenol is the simplest member C6H5OH
compounds that contain an -OH group bonded to a carbon side chain rather than the aromatic ring. classified as alcohols. q
why is phenol a weak acid
phenol is less soluble in H2O than alcohols due to non polar benzene ring. when dissolved in water, phenol partially dissociated forming plenoxide ion and a proton
it is more acidic than alcohol but less acidic than carboxylic acids.
