6.1.1 aromatic compounds Flashcards
aliphatic
straight or branched chain organic substance
aromatic/arene
includes one or more ring of six carbon atoms with delocalised bonding
benzene structure
simplest arene
C6H6
6 delocalised electrons over 6 bonds above & below plane of carbon atoms
has a lower electron density
so it cannot induce a dipole/polarise
kekule model
3 double bond alternating benzene
problems with kekules model
lacks reactivity
- if benzene contained the C=C bond, it should decolourise Br2 in electrophilic addition
- but, doesn’t undergo electrophilic addiction & doesn’t decolourise Br2 so no C=C
all bonds were found to be equal length, inbetween the value of the single and double bond length
enthalpy change of hydrogenation expected to be 3x of cyclohexenes (double bonds), -360 but only -208 so less exothermic
- more stable than thought
phenyl group
instead of benzene
C6H5-
e.g. phenylamine, phenylethene
alkenes reaction with bromine
react easily at room temperature
localised pi electrons above and below plane of 2 carbon atoms so has a higher electron density
can polarise/induce dipole in Br2
toxicity of benzene
carcinogen
methylbenzene is less toxic and reacts more readily than benzene as methyl side group releases electrons into delocalised system making it more attractive to electrophiles
nitration of benzene
benzene > nitrobenzene
conc nitric acid in presence of conc H2SO4
electrophilic substitution
electrophile: NO2+
50/60C as doing at higher temperatures results in further substitution
equation for formation of nitrification electrophile
HNO3 + 2H2SO4 > (NO2+)+ (2HSO4-) + H3O+
halogenation of benzene
benzene > bromobenzene or chlorobenzene
reagents: bromine or chlorine
conditions: FeBr3 or AlCl3 catalyst
electrophilic substitution
formation of electrophiles for halogenation
formation of catalyst
AlCl3 + Cl2 > AlCl4- + Cl+
FeBr3 + Br2 > FeBr4- + Br+
to reform AlCl3 catalyst
H+ + AlCl4- > AlCl3 + HCl
Friedel crafts alkylation
benzene > alkylbenzene
reagents: chloroalkane in presence of anhydrous aluminium chloride catalyst
conditions: heat under reflux
mechanism: electrophilic substitution
formation of electrophile in alkylation
AlCl3 + Ch3CH2Cl > CH3CH2+ AlCl4-
reform AlCl3 catalyst:
H+ + AlCl4- > AlCl3 + HCl
friedel crafts acylation
benzene > phenyl ketone
reagents: acyl chloride in presence of anhydrous aluminium chloride catalyst
conditions: heat under reflux (50C)
mechanism: electrophilic substitution
equation for formation of electrophile in acylation
AlCl3 + CH3COCl > CH3CO+ AlCl4-
reform catalst: H+ + AlCl4- > AlCl3 + HCl
phenol
OH group directly attached to benzene ring
lone pair of electrons on oxygen p-orbital is donated into pi system
- higher electron density so can polarise
weakly acidic - weaker than carboxylic acid
phenol + sodium
H from alcohol replaced by Na+ (sodium phenoxide) + 0.5H2
phenol + sodium hydroxide
H from alcohol replaced by Na+ (sodium phenoxide) + H2O
sodium phenoxide more soluble than phenol so solid phenol dissolves on addition of NaOH
phenol + bromine
Br2 reagent
room temperature
doesn’t need a FeBr3 catalyst like benzene and undergoes multiple substitutions whereas benzene will only add one Br
white solid formed (2,4,6-tribromophenol) and HBr
phenol use
production of plastics
antiseptics
disinfectants
phenol + nitric acid
4M HNO3 reagent - single substitution
room temp
forms 2-nitrophenol or 4-nitrophenol
effects of side groups on substitution (nitric and phenol )
position on ring of substitution reactions
electron-donating groups (OH, NH2) will force further substitutions to occur on 2nd and 4th positions of ring
electron-withdrawing (NO2) will have a 3-directing effect of in electrophilic substitution of aromatic compounds
effect of delocalisation of side groups with lone pairs
if a -OH group, Cl atom or NH2 group is directly attached to benzene ring, delocalisation in benzene ring will extend to include lone pairs on N, O and Cl > changes properties and reactions of side group
delocalisation on chlorobenzene
C-Cl bond is made stronger
halogenoalkane substitution and elimination reactions do not occur
electron rich benzene ring will repel nucelophiles
delocalisation on phenol
makes the C-O bond stronger and the O-H bond weaker
doesn’t act like an alcohol , it is more acidic and doesn’t oxidise
delocalisation on phenylamine
less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton
