BENZENE Flashcards
name some qualities of benzene
colourless
aromatic (sweet smelling)
highly flammable
carcinogen
Describe Kekulé’s structure of benzene
Planar
cyclic
alternating double and single bonds
what type of reactions would we expect benzene to undergo based on Kekulé’s model
- electrophilic addition w/ alkenes
- decolourise Br water rapidly
*but does not do this
describe the thermodynamic evidence for stability that disproved Kekulé’s theory
when unsaturated hydrocarbons are reduced to corresponding saturated compound energy is released
when one C=C is broken, 120 KJ/mol is released
therefore if benzene had 3 separate C=C we would expect (3x120) 360 KJ/mol to be released
benzene actually only released 208 KJ/mol when reduced .:. lower down energy scale
it is 152 KJ more stable than expected
–> this is known as the resonance energy
what kind of C to C bods does benzene have
no true C=C as does not readily go under electrophilic addition
all 6 C-C bond lengths are similar
(C=C are shorter than C-C)
how much energy per mol would be released if 1 C=C brond where to break in cyclohexane
120 kJ mol-1
how much energy per mol would we expect benzene to be release based off kekulé’s model and how much is actually released
kekulé= 360 kJ mol-1
actual= 208 kJ mol-1
what is the actual structure of benzene
6 π e-
that are delocalised around the benzene ring
by overlapping P orbitals
no double bonds + all bond lengths are equal
planar structure
- to draw: “squashed benzene” with halo above + below
halo = 6 overlapping P orbitals above + below the plane (π bond)
naming benzene derivatives
monosubstitution of halogen / NO2 / simple alkyl, they become the prefeix (e.g cholor benzene / nitrobenzene / ethylbenzene
if more reactive functional group / large alkyl (+7 C), PHENYL becoms the prefix, e.g, phenylethanone
exlcusions: benzoic acid (benzene w/ COOH) phenylamine (benzene w/ NH2) benzaldehyde (benzene w/ CHO) namphthalene (two benzenes)
halogenation of benzene
- type of reaction
- type of reaction: ELECTROPHILIC SUB
halogenation of benzene
- electrophile?
+Cl
halogenation of benzene
- reagents + conditions
- reagents: HALOGEN + HALOGEN CARRIER
- conditions: REFLUX W/ HALOGEN CARRIER
halogenation of benzene
- equation using Cl2
- equation using Cl2:
C6H6 + Cl2 -(FeCl3)-> C6H5Cl + HCl
halogenation of benzene
- draw mechanism
1.
arrow from benzene ring to +Cl
2.
electron ring breaks around the C the CL bonds to
arrow from H+ on benzene –> positive ring
3.
forms halobenzene + H+ (which regenerates the catalyst)
halogenation of benzene
- electrophile formation (w/ Cl2 + FeCl3)
Cl2 + FeCl3 –> +Cl + FeCl4-
halogenation of benzene
- catalyst regeneration
FeCl4- + H+ –> FeCl3 + HCl
Nitration of Benzene
- type of reaction
electrophilic substitution
Nitration of Benzene
reagents + conditions
reagents: CONC H2SO4 + CONC HNO3
conditions: reflux @ 50ºC
any higher than 50 –> polysubstitution
Nitration of Benzene
equation
C6H6 + HNO3 –> C6H5NO2 + H2O
Nitration of Benzene
- mechanism
1.
arrow from benzene ring to +NO2
2.
electron ring breaks around the C the NO2 bonds to
arrow from H+ on benzene –> positive ring
3.
forms nitrobenzene + H+ (which forms the H2O)
Nitration of Benzene
- electrophile
+NO2
Nitration of Benzene
- electrophile formation
H2SO4 + HNO3 –> (+)NO2 + HSO4(-) +H2O
electron density in benzene vs alkene
benzene = delocalised alkene = localised
benzene vs alkene reactivity
benzene = more stable
requires halogen carrier to create an electrophile
alkene = less stable
no halogen carrier required, as it can induce a dipole in the halogen itself
which groups are e- donating and how does this affect the e- density of the ring.
when these groups are attached what are the substitution positions
OH, CH3, NH2
increases e- density as they are e- donating
take up C 2 4, 6
*activate benzene
which group(s) are e- withdrawing and how does this affect the e- density of the ring.
when these groups are attached what are the substitution positions
NO2
decreases e- desnity as they are e- withdrawing
C 3 + 5
*deactivate benzene
FRIEDAL-CRAFT ACYLATION
reagents + conditions
reagents: acyl chloride + anhyderous AlCl3
conditions: dry inert solvent + relfux @ 50ºc
FRIEDAL-CRAFT ACYLATION
electrophile
R-C(+)=O
FRIEDAL-CRAFT ACYLATION
regenerating the catalyst
H(+) + AlCl4(-) –> AlCl3 + HCl
FRIEDAL-CRAFT ACYLATION
generating electrophile
R-C(=O)Cl + AlCl3 –> R-C(+)=O + AlCl4(-)
FRIEDAL-CRAFT ALKYLATION
reagents + conditions
reagents: haloalkane + anhydrous AlCl3
conditions: room temp + dry inert solvent
FRIEDAL-CRAFT ALKYLATION
electrophile
+carbocation (e.g, +CH3)
FRIEDAL-CRAFT ALKYLATION
regenerating catalyst
H(+) + AlCl4(-) –> AlCl3 + HCl
FRIEDAL-CRAFT ALKYLATION
generating electrophile
e.g, CH3CH2Cl
CH3CH2Cl + AlCl3 –> CH3C(+)H + AlCl4(-)
