27 aromatic chemistry
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what does the ring in benzenes skeletal formula represent?
cloud of delocalised electrons
what is the name for kekule’s model?
cyclohexatriene
what is the difference in reactions with Br2 between benzene and cyclohexatriene?
kekule model: orange to colourless benzene: only undergoes substitution
what is the enthalpy of hydrogenation of cyclohexatriene and benzene?
kekules model: 360 kj mol -1 benzene: 208 kj mol -1
using the enthalpy of hydrogenation, how is benzene more stable than cyclohexatriene?
because the difference in enthalpy change of benzene and cyclohexatriene is 152 kj mol-1 more stable benzene being less exothermic
how long are the bonds in benzene?
length in between single and double bond
why can kekule’s model undergo addition but benzene can’t?
benzene is less reactive than alkenes
why does benzene undergo substitution reactions rather than addition reactions?
to maintain the ring of delocalised electrons
what do the overlapped p orbitals form?
extended pi electron system
what does benzene look like at room temp?
colourless liquid
is benzene’s boiling point similar or different to hexane’s? why?
similar
is benzene’s melting point similar or different to hexane’s? why?
benzene is planar so can stack when solid- harder to break VDW forces
what mechanism does benzene undergo?
electrophilic substitution
what are the two types of electrophilic substitution that benzene undergoes?
acylation nitration
describe the overall bonding in benzene
• each carbon has 3 covalent bonds • electrons in p orbitals overlap to form a pi cloud • delocalisation of electrons
describe the overall shape of benzene
• planar • hexagonal, 6 carbons, 180 bond angle • C-C equal length, between length of single and double bond
describe the overall stability of benzene
• expected enthalpy change of hydrogenation of cyclohexatriene -360 • benzene less exothermic at -208 • benzene more stable
what is the catalyst used in acylation?
AlCl3/aluminium chloride
what is the electrophile in acylation?
RC+O
how do you form RC+O for acylation?
RCOCl + AlCl3 -> RC+O AlCl4-
what replaces a H on benzene during acylation?
RCO
how many curly arrows are used in acylation and where do they go to and from?
2 curly arrows 1- from benzene ring to C+ (RC+O) 2- from H to + in benzene ring
how do you reform the catalyst in acylation?
AlCl4- + H+ -> AlCl3 + HCl (g)
what is the electrophile in nitration reaction?
NO2+