25 aromatic compunds Flashcards
(20 cards)
define arene
aromatic compounds that contain a benzene ring as part of their structure
describe the structure of benzene
arene consisting of 6 C ([He]2S2 2P2) atoms
one electron bonds to a H atom and the other electron in the p-orbital is delocalised to form a central ring of pi bonds
C6H6
describe the thermochemical evidence for the structure of benzene
enthalpy change of hydrogenation for benzene is -360kJ/mol
(-120kJ/mol per each double bond)
the actual enthalpy change of hydrogenation of benzene is -208kJ/mol
the enthalpy change was less exothermic than expected so, benzene is more stable than expected
describe the length of C-C bonds in benzene
it is a length in-between a C-C bond and a C=C bond
this information was discovered by X-ray diffraction
describe the lack of reactivity of benzene
benzene does not decolourise bromine/undergo electrophilic addition reactions as it less reactive than expected
Why does benzene not undergo electrophilic addition?
would require breaking up stable delocalised ring of electrons
describe the electrophilic substitution on benzene
electrophile attacks electron ring and partially destroys it
the ring is restored from electrons from C-H bond which leads to the formation of mono-substituted products and H+ ion
what is the role of a halogen carrier in the halogenation of benzene
Halogen carrier eg FeBr3 (to generate the electrophile)
Br2 + FeBr3 -> FeBr4- + Br+
bromonium ion partakes in the reaction
H+ ion produces reacts with FeBr4 to regenerate FeBr4 (the catalyst)
what type of reaction is the halogenation of benzene?
electrophilic substitution
describe the process of nitration of benzene
- requires concentrated sulfuric acid and concentrated nitric acid to form electrophile; NO2(1+) ion
- sulfuric acid is a catalyst as it is not used up
H2SO4 + HNO3 -> H2NO3(1+) + H2SO4(1-)
H2NO3(1+) -> H20 + NO2(1+) - H2O is a by-product of the reaction
- NO2+ gets substituted
- H+ ion reacts with H2SO4- ion to regenerate the catalyst H2SO4
- OCCURS AT 55 DEGREES C FOR MONOSUBSTITUTION (if performed at higher temperatures than multiple substitutions occur)
Describe Friedel-Crafts Acylation
- delocalised electron ring acts as a nucleophile and attacks acyl chlorides
- aluminium chloride catalyst used
R-COCl + AlCl3 -> R-CO(+) + AlCl4(-) - R-CO+ subs onto benzene forming a phenylketone
- H+ ion reacts with AlCl4- to reform AlCl3 and gives off HCl gas
what uses are there for phenylketones
- industrial production of dyes
- pharmaceuticals
- explosives yum
why does benzene not decolourise bromine water?
- delocalised electron density of the pi system (relative to the localised pi system of alkenes) makes benzene relatively stable
- not reactive enough to decolorise bromine water
PHENOLS BABY
- contains benzene ring and OH alcohol group
- aromatic alcohols
- weak acids
- can be neutralised by NaOH (strong bases) but not carbonates
describe bromination of phenol
phenol + 3Br2 -> 2,4,6-tribromophenol + 3HBr
white precipitate
decolorises bromine water
no halogen carrier is required
describe the nitration of phenol
C6H5OH + HNO3 (dilute) -> 2-nitrophenol/4-nitrophenol + H2O
doesn’t req conc HNO3 or H2SO4 catalyst
- NO2 is electron withdrawing so electron density of ring decreases and only one sub happens at room temp
why is phenol more reactive than benzene?
lone pair of electrons from O are delocalised in benzene ring structure
electron density of ring increases
ring less stable
more susceptible to attack
Directing effects
can be used to predict substitution products
electron-withdrawing groups are 3-directing (meta)
electron donating/direct electrophiles are 2-/4- directing (ortho/para)
2-/4- directing
-NH2/-NHR
-OH
-OR
-R/-C6H5
-F/-Cl/-Br/-I (halogens)
3- directing
RCOR
-COOR
-SO3H
-CHO
-COOH
-CN
-NO2
-NR3(1+)
