aromatic compounds Flashcards
aromatic definition
a molecule which is stabilised by electron delocalisation in a ring
what is the molecular formula of benzene
C6H6
outline the structure and bonding in the kekule model of benzene
a 6 carbon ring with alternating single and double bonds, with 6 C-C sigma bonds and 3 C-C pi bonds formed by p-orbital overlap
in this model the double bonds have localised electrons giving them high electron densities
what is the difference between a sigma bond and a pi bond
sigma bonds are formed by the head on overlap of orbitals, creating a region of electron density diectly between atoms
pi bonds are formed by the sideways overlap of p-orbitals, creating regions of electron density above and below plane
outline the structure and bonding in the delocalised model of benzene
a 6 carbon ring with each C atom having a spare electron in a p-orbital, which overlap and spread around the carbon ring to form a ring of delocalised electron density in planes above and below the ring
what are the bond angles in benzene
120
what 3 pieces of evidence are used to disprove the kekule model
lack of reactivity - benzene does not decolourise Br(aq) in the dark at room temperature, less reactive compared to normal alkenes
thermodynamic stability - the enthalpy of hydrogenation of benzene is much less exothermic than 3x the enthalpy of hydrogenation of cyclohexene
bond lengths - despite C-C and C=C bonds having different lengths normally, all bonds in benzene are the same length, intermediate between C-C and C=C bonds
what makes benzene so stable
the delocalised ring of electrons spread out across 6 carbon ring
how does the delocalised model explain benzenes lack of reactivity
electron density between each C is lower than between a C=C in an alkene, so it has less attraction to electrophiles and is less polarising, therefore less reactive
how does the delocalised model explain benzene being less exothermic than expected
delocalised ring of electron stabilises benzene, this thermodynamic stability causes the hydrogenation of benzene to be less exothermic than expected when compared to other alkenes
how does the delocalised model explain benzenes bond lengths
no alternating C=C bonds so all bonds in benzene are the same length (intermediate between C-C and C=C bonds)
by what mechanism does benzene normally react + why
electrophilic substitution
benzene undergoes substitution rather than addition as this allows the stable system of electron delocalisation to be maintained
what is needed for benzene to react with most compounds
a catalyst
compare the reactivity of benzene and phenol
phenol is more reactive due to the OH group which is activating as lone pair on O is delocalised onto ring, this means it is more polarising ans to it has greater attraction to electrophiles and can react with electrophiles faster or without catalysts
what is needed for benzene to react with halogens, haloalkanes or acyl chlorides
halogen carrier catalysts
e.g. AlCl3, FeBr3
compare the reactivity of benzene and cyclohexene
electron density between each C in benzene is less than an alkene, whereas in the C=C in cyclohexene the electrons are localised, so it has a greater electron density making it more polarising and increasing its attraction towards electrophiles , meaning it is more reactive and can react with electrophiles faster or without catalysts
why are conditions in reactions involving benzene and nucleophiles often very harsh
this is to overcome the high activation energies of the reactions, as arenes normally repel nucleophiles
outline the physical properties of phenol
relatively high melting + boiling points, as it can form hydrogen bonds due to OH
phenol is very water soluble due to its OH group, which allows it to form hydrogen bonds with water
give the formula for phenol
C6H5OH
when phenol reacts with Br(aq) what is observed
the solution will decolourise and a white precipitate will be formed
this precipitate is 2,4,6-tribromophenol
when phenol reacts with Na what is observed
effervescence and bubbling
how can phenol act as an acid
due to its OH group, it is able to donate a proton, forming a phenoxide ion - C6H5O-
they are weak acids so they do not fully dissociate
in comparison to alcohols and carboxylic acids, how acidic is phenol
carboxylic acids are more acidic than phenols, which are more acidic than alcohols
- alcohols don’t readily react with NaOH - phenols can
- carboxylic acids readily react with Na2CO3 - phenols cannot
activating group definition
groups than can increase the reactivity of the benzene ring, as they are electron donating, as they often have a lone pair of atoms bonded to the ring which increases electron density
deactivating group definition
groups with decrease the reactivity of the benzene ring, as they are electron withdrawing, as they often have atoms with double bonds bonded to the ring
give examples of activating groups
NH2 NHR OH OR C6H5 R
give examples of deactivating groups
COR COOR SO3H CHO COOH CN NO2 NR3+ halogens
how do the reactions of benzene differ with activating or deactivating groups
if activating groups are present reactions will be faster, and conditions will be more mild compared to benzene, no catalyst is needed and multiple substitutions may occur
if deactivating groups are present reactions will be slower and conditions will be harsher compared to benzene, catalysts are often needed and multiple substitutions are less likely
which groups are 2,4 directing
NH2 NHR OH OR C6H5 R halogens
basically all activating groups + halogens
which groups are 3 directing
COR COOR SO3H CHO COOH CN NO2 NR3+
basically all deactivating groups except halogens
give the reagent and conditions needed to go from benzene to cyclohexene - hydrogenation
reagent - H2
conditions - Ni catalyst
give the reagent and conditions needed to go from benzene to C6H5-CO-R - acylation
then from C6H5COR to C6H5-CH(OH)-R - reduction to alcohol
reagent - acyl chloride
conditions - AlCl3 or FeCl3
- products are benzaldehyde or phenyl(methan..)one
- byproduct = HCl
reagent - NaBH4
conditions - aqueous
give the reagent and conditions needed to go from benzene to nitrobenzene or dinitrobenzene - nitration
reagent - conc HNO3
conditions - conc H2SO4 and temp of approx 55C
- byproduct = H2O
reagent - conc HNO3
conditions - conc H2SO4 and a temp of approx 70C
- byproduct = H2O
give the reagent to go from nitrobenzene to phenylamine - reduction
reagent - conc HCl and Sn and excess NaOH
- byproduct = H2O
give the reagent to go from phenylamine to C6H5-NHCO-R
reagent - acyl chloride
conditions - AlCl3 or FeCl3
- byproduct = HCl
give the reagent and conditions to go from benzene to methylbenzene - alkylation
reagent - haloalkane
conditions - AlCl3 or FeCl3
- byproduct = HCl
