module six: organic chemistry and analysis Flashcards
kekulé’s model of benzene
alternating double and single bonds
delocalised model of benzene
delocalised electron ring
main features of delocalised benzene model
planar, cyclic, hexagonal hydrocarbon with 6 carbons, 6 hydrogens
carbon uses 3/4 electrons to bond to 2 carbons, 1 hydrogen
1 electron in p-orbital at right angles to plane
p-orbitals overlap sideways above + below plane of carbon atoms, forming a ring of electron density
overlapping creates system of 𝝅-bonds, across all 6 carbons in ring
electrons in 𝝅-bonds are delocalised
molecular and empirical formula of benzene
molecular: C6H6
empirical: CH
bond angles in benzene
120
evidence that does not support kekulé’s model
all carbon bonds are the same length, even though double and single bonds have different lengths
benzene is less reactive than alkenes, benzene does not undergo electrophilic addition or decolourise bromine under normal conditions
enthalpy change of hydrogenation is lower than expected - more stable molecule
technique to find bond lengths of benzene
x-ray diffraction
4th electron in the p-orbital of each carbon atom in benzene
delocalises to form rings above and below the hexagon
forms rings of delocalised electron density above and below the hexagon
why is benzene attacked by electrophiles
high electron density above/below the ring due to the delocalised electrons
type of reaction: nitration of benzene
electrophilic substitution
how is the NO2 + ion created
conc. H2SO4, conc. HNO3
H2SO4 + HNO3 –> HSO4 - + H2O + NO2 +
how is H2SO4 catalyst regenerated after electrophilic subsitution
H+ + HSO4 –> H2SO4
hydrogen ion from benzene
why does benzene need a halogen carrier
benzene does not react directly with halogens as aromatic ring is too stable
nitration of benzene
reagents: conc. HNO3, conc. H2SO4 (catalyst)
conditions: 50°C - monosubstitution
equation: C6H6 + HNO3 –> C6H5NO2 + H2O
nitration of benzene above 50°C
produces dinitrobenzene
below 50°C ensures monosubstitution
nitration of benzene mechanism
**step one: ** NO2 + ion produced
step two: electron pair leaves delocalised system to form a bond to NO2
an unstable intermediate is formed
pair of electrons in C-H bond moves back into ring
step three: regenerate H2SO4 catalyst
halogenation of benzene
reagents: halogen (Cl, Br) and a halogen carrier (catalyst) - produces electrophile
conditions: reflux in the presence of halogen carrier
equation: C6H6 + Cl2 –> C6H5Cl + HCl
halogenation of benzene mechanism
step one: halogen ion produced
step two: ion accepts pair of electrons from benzene ring - forms dative covalent bond
organic intermediate is unstable - breaks down to become stable and relase a H+ ion
step three: hydrogen ion reacts with halogen carrier to regenerate catalyst
production of halogen ion: halogenation of benzene
halogen + halogen carrier –> halogen ion + halogen carrier ion
e.g. Br2 + FeBr3 –> Br+ + FeBr4 -
regeneration of catalyst: halogenation of benzene
H+ + halogen carrier ion –> halogen carrier + acid
e.g. H+ + FeBr4 - –> FeBr3 +HBr
alkylation of benzene
increases no. of C atoms by forming carbon-carbon bonds
reagents: haloalkane, AlCl3 (catalyst) - produces electrophile
substitution of H atom by alkyl group
acylation of benzene
reacts with acyl chloride
presence of AlCl3 (catalyst)
forms aromatic ketone
why is a halogen carrier req. to react bromine with benzene
benzene = delocalised pi-electrons spread above and below the plane of ring
low electron density
Br2 = non-polar, insufficient pi-electron density around 2 carbon atoms to polarise Br2
explain resistance to bromination of benzene compared with alkenes
benzene: electrons delocalised
alkenes: electrons localised
benzene has a lower electron density than alkenes
benzene attracts bromine less
benzene induces weaker dipole in bromine
