topic 18a: benzene Flashcards
aliphatic
straight or branched
aromatic/arene
contains a ring
descrieb the bonding in benzene
- each c atom is bonded to 2 other c atoms and one H atom, by SINGLE SIGMA BONDS
- this leaves one unused electron on each c atom in a p orbital
- the 6 p electrons are in a delocalised ring structure above and below the plane of carbon atoms, forming pi bonds by overlapping
problem with kekule model
- c-c bonds should all be the same legnth
bond angle
120
describe the enthalpies of hyration and how this disproves the double bond model?
- cyclohexene + h2 = cyclohexane with delta h -120
- 1,3,5 cyclohexatriene + 3H2 gives 3x the delta h
- BUT FOR BENZENE, the delta h is less negative
- shows that the 6 pi electrons are DELOCALISED and not arranged in 3 double bonds
benzene stability?
- v stable
- delocalisation energy
why doesnt benzene undergo addition but alkenes do?
- for benzene this would involve breaking the delocalised electron structure which makes it stable
- whereas in alkenes, the pi bond is LOCALISED
what has priority a halogen or alkyl group
HALOGEN
Benzene safety
toxic
methyl benzene vs benzene
- less toxic
- reacts more readily, as methyl group has positive inductive effect; pusehs electrons into the delocalised ring system, increasing electron density so more attractive to electrophiles
combustion of benzene equation
C6H6 + 7.5 O2 = 6CO2 + 3H2O
benzene state
liquid
what kind of flame does benzene combust with and why
- very sooty
- LOW C:H RATIO
benzene + bromine reagent and conditions
- Br2 reagent
- FeBr3 catalyst
- FeBr3 +Br2 = FeBr4- + Br+
benzene and bromine formula
c6h6 + br2 = c6h5Br + HBr
benzene and chlorine reagent and condition
- AlCl3 + Cl2 = AlCl4- + Cl+
- OR FeCl3
nitration of benzene reagent and condition
- conc nitric acid and conc sulfuric acid (CATALYST)
- 60 degrees
name of mechanism for halogenation and nitration of benzene
electrophilic substitution
equation of formation of electrophile for nitration
HNO3 + 2H2SO4 = NO2+ + 2HSO4- + H3O+
reformation of AlCl3 catalyst
H+ + AlCl4- = AlCl3 + Hcl
reformation of sulfuric acid catalyst
H+ + HSO4- = H2SO4
equaion of nitration of benzene
C6H6 + NO2+ = C6H5NO2 + H+
hydrogenation of benzene
- benzene to cyclohexane
- h2
- nickel catalyst
friedel crafts alkylation reagents and conditions
- haloalkane
- ANHYDROUS AlCl3 catalyst
- heat under refluc
formation of electrophile eg ethyl group for freidel crafts alkylation
Ch3Ch2Cl + AlCl3 = CH3CH2+ + AlCl4-
regenratin of catalyst after friedel crafts alkylation
H+ + AlCl4- = AlCl3 + HCl
what does friedel crafts ACTYLATION do
adds a c=och3
friedel crafts acylation conditions and reagents
- ACYL CHLORIDE
- anhydrous AlCl3 catalyst
- heat under reflux
formation of electrophile in friedel crafts ACYLATION
CH3COCl + AlCl3 = AlCl4- + CH3CO+
in nitration, which of the 2 acids acts as a base
- HNO3 acts as a base as H2SO4 is a stronger acid so protonates HNO3
why can phenol act as a weak acid
can donate the H from the OH
why is phenol much more reactive than benzene
- lone pairs on oxygen atom
- delocalises into ring, interacting with delocalised pi system
- increases electron density of ring
- more susceptible to attack by electrophile
why is the phenol ion very stable and therefore a good acid
- when the h+ leaves you get the o-
- the charge can delocalise into the delocalised pi electron structure so the ion is more stable
phenol + bromine
- bromine water decolourises
- white ppt formed if 2,4,6 tribromophenol
phenol vs benzene, bromonation (6)
-both electrophilic substitution
-benzene forms bromobenzene (monosub) phenol forms 2,4,6 tribromophenol (triple sub)
- benzene needs FeBr3 catalyst, phenol doesnt
- phenol room temp, benzene heat under refluc
- benzene less reactive than phenol
- lone pair on o dissociates into ring, into delocalised pi system
- increases electron density in ring so more susceptible to attack by electrophiles
proton NMR of benzene
- 1 peak as all protons equivalent
- delta 7-8
important to note in mechanisms
- draw arrow frmo the ring to the POSITIVE ATOM (eg the c+ for alkylation)
alcohol to alkene
concentrated phosphoric acid
aldehyde/ketone to alcohol
- LiAl4 in dry ether
if you form intramolecular forces then…
- weaker IMFs
- lower bp
why must fume cupboard stay above safety line (2)
- above is when exhaust system not strong enough to draw in fues
- to toxic fumes escape into lab
what combusts with a black smoky flame
- arenes
- alkenes
- low h:c ratio
why is phenyl methanol less reactive than phenol (4)
- lone pair on O delocalises into ring
- increases e- density of ring
- in phneol the -OH is directly attached to ring so directly overlaps with pi electrons
- but in phenylmethanol is too far away
