(10) Aromatic Flashcards
what does aromatic mean?
particular sort of delocalised bonding
what are arenes?
aromatic hydrocarbons; based on benzene rings
benzene state and where it comes from
colourless liquid at RT and pressure
obtained from petroleum (crude oil)
three different bonding compounds of arenes
- cyclohexene (one C=C)
- cyclohexane (all C-C)
- benzene (3 C=C, every other)
what is the kekulé structure
benzene has 3 C-C and 3 C=C
problems with kekule structure
- alternating bonds, C-C and C=C have different lengths —> xray shows bonds are equal lengths
- would be known as cyclohexa-1,2,3-triene; can hydrogenate = enthalpy -120 kJ mol-1 —> enthalpy = -208 kJ mol-1
- undergoes electrophilic substitution (hydrogen replaced) —> undergoes electrophilic addition (pi-bond breaks = new bond)
what is the difference in enthalpy due to between kekule structure and benzene
the stability from the system of delocalised electrons
how many electrons are free to bond in a benzene ring and what are they used for
- 30 (H 1 each, C 4 each)
- 12 C electrons used for 6 C-H sigma bonds
- 12 C electrons used for 6 C-C sigma bonds
explain the pi bonding in a benzene ring
- each C has an electron not used in sigma bonds
- p-orbitals overlap to form pi-bond delocalised* system of electrons above and below plane of hydrocarbon skeleton
*pi-bonds are spread over several atoms
what does delocalisation of electrons do in a benzene ring
- lowers energy if structure (makes more stable)
- difference in energy values = delocalisation energy
- unique electronic structure has dominant influence on properties of molecule
structure of benzene ring
planar, hexagonal molecule with bond angle = 120°
what does the circle in a benzene ring represent ?
the delocalised pi-electron cloud
what type of reaction will alkenes readily undergo? and an example
electrophilic addition; eg. ethene and bromine (test for unsaturation)
why does benzene not undergo electrophilic addition?
unsaturated, doesn’t react with bromine due to stability of pi-delocalised system of electrons
what type of reaction does benzene undergo
electrophilic substitution where pi-electron cloud is preserved
substituted benzene compounds:
- phenol
- methylbenzene
- phenylamine
- iodobenzene
- bromobenzene
- benzoic acid
- methyl benzoate
- methyl-3-nitrobenzoate
- benzene diazonium chloride
- C6H5OH
-C6H5(CH3)
-C6H5NH2
-C6H5I
-C6H5Br
-C6H5COOH
-C6H5COOCH3
C6H4(NO2)COOCH3
-C6H5N2+Cl-
what are the two mechanisms for reactions of benzene
- monohalogenation
- mononitration
main monohalogenation of benzene studied
monobromination of benzene using bromine in presence of iron/iron bromide (due to stability of delocalised system of electrons in benzene catalyst reuqired)
explain monobromination mechanism reaction
- Br2 is polarised as it approaches benzene
- FeBr3 acts as halogen carrier
- δ- Br atom transfers pair of electrons to iron in FeBr3// other Br atom accepts pair of electrons from delocalised cloud of elections in benzene
- temporilary, delocalised pi-electron system stability lost
- electrons in C-H go back to ring to reform electron cloud losing H+ ion
- FeBr4- reacts with H+ —> FeBr3 (catalyst) and HBr
when does mononitration of benzene occur
when one (or more) H atoms on benzene is replaced by nitrogen group (NO2)
what mechanism is mononitration of benzene and how is the ion needed produced
electrophilic substitution, electrophile (nitronium ion) produced through reaction of conc nitric acid and conc sulfuric acid
HNO3 + 2H2SO4 —> NO2+ + H3O+ + 2HSO4-
what is the nitro group and describe its appearance and use when attached to benzene
NO2
nitrobenzene = yellow oily liquid
dyes and explosives
what happens when benzene reacts with halogenoalkanes and acyl chlorides and what is this called
alkylated and acylated benzene products (called friedel-crafts reactions)
catalyst in friedel-crafts reactions
aluminium halides
what is alkylation
substituting H atom for alkyl group
describe alkylation reaction between chloroethane and ethyl benzene
- occurs in presence of aluminium chloride (halogen carrier/catalyst)
- aluminium chloride reacts with halogenoalkane to form electrophile
- electrophile reacts with benzene
- catalyst regenerated when AlCl4- reacts with H+
what is acylation
substituting a H atom for an acyl group
describe acylation reaction between ethanoyl chloride and phenylethanone
- occurs in presence of aluminium chloride (catalyst)
- aluminium chloride reacts with acyl chlorides to form electrophile
- electrophile reacts with benzene
- catalyst regenerated
monobromination equation
FeBr3 + Br2 —> FeBr4- + Br+
mononitration equation
HNO3 + 2H2SO4 —> NO2+ + H30+ + 2HSO4-
monoalkylation equation
CH3CH2Cl + AlCl3 —> CH3CH2+ + AlCl4-
monoacylation equation
CH3COCl + AlCl3 —> CH3CO+ + AlCl4-
benzene ring we use in school (normal benzene toxic)
methyl-3-nitrobenzoate, prepared from methyl benzoate (ester of benzoic acid)
describe prep of methyl 3 nitribenzoate
- dissolve methyl benzoate in conc H2SO4
- cool in ice
- add conc H2SO4 to conc HNO3 (=NO2+)and cool
- drop by drop nitrstion mixture to solution and keep below 10°
- leave at room temp for 15mins
- pour into crushed ice and stir until melted and crystalline methyl 3 nitrobenzoate fromed
- filter using buchner filtration, wash w/ cold water
- recrystallise from ethanol, filter off and dry
purification of methyl 3 benzoate
- recrystallise from min vol of hot ethanol
- warm ethanol about 50° by beaker of hot water (no naked flames)
- dissolve crystals in hot ethanol
- filter using fluted filter paper
- cool and crystallise
- immerse in ice to complete crystallisation
-suction filter and wash crystals with ice cold solvent - dry between filter paper and weigh
- record mass
- observe melting point (77.5°) - cream solid
why is the reaction mixture kept below 10° (methyl-3-benzoate)
greater chance of multiple nitrations at higher temp (explosive)
why is suction filtration used (methyl-3-benzoate)
faster and drier product
why is fluted filter paper used in recrystallisation (methyl3benzoate)
minimises contact with cold funnel - could cause cause crystallisation
