Arenes Flashcards
[ARENES] What does aromatic compound mean?
cyclically conjugated
stability more than hypothetical localised structure due to delocalisation
its usually benzene
[ARENES] Formula of benzene ring?
C6H6
When the ratio of C:H is close to 1:1, its safe to assume a benzene ring is in the structure.
[ARENES] C-C bond length in benzene?
all same length 0.140nm
longer than C=C but shorter than C-C, hence partial double bond character
[ARENES] Enthalpy difference in theoretical and actual arenes
Hypothetical enthalpy is higher(less stable) than the actual of benzene.
Difference in energy is known as resonance energy.
[ARENES] Why does benzene not react like an alkene and undergo addition?
They undergo substitution reactions instead.
[ARENES] Resonance hybrid structure of Benzene?
Each C atom is sp2 hybridised
BONDS:
C has 3 sp2 hybrid orbitals in trigonal planar, hence 180°
2 overlap head on with other sp2s from other C atoms to make 2 ring C-C bonds
last 1 overlap head on with 1s in H
DELOCALISED pi e CLOUD: one more unhybridised p orbital, perpendicular to molecular plane (it just sticks out)
All the unhybridised p orbitals overlap side on with neighbouring p orbitals, forming a cyclic delocalised pi e cloud
[ARENES] What are the physical properties of arenes? (no explanation just yes/no answers)
- boiling/melting points
- Solubility in polar/non-polar solvents
- density compared to H2O
- Type of molecular attraction
- colour of flame
bp/mp are low, usually in liquid form
bp increase with Mr, more e, larger polarisibility or e cloud, stronger idid
mp trend irregular as it depends on molecular symmetry
Soluble in polar, not soluble in non-soluble
less dense than water, immiscible
idid foa
smoky, luminous orange flames
[ARENES] Bp/mp of arenes
Bp increases with Mr
more e
more polarisible e cloud
stronger idid
Mp depends on molecular symmetry but generally low
[ARENES] Solubilty of arenes in polar/H2O
Insoluble
energy formed by pd-id btwn water/polar and arene
insufficient to overcome
idid btwn arene and ion-dipole interaction/pdpd btwn H2O
[ARENES] Solubilty of arenes in non-polar
Soluble
energy formed by idid btwn water/non polar and arene
sufficient to overcome
idid btwn arene and non polar solvent
[ARENES] Density of arene
less dense than h2o and immiscible
[ARENES] Type of flame of burning arene
Smoky, luminous orange flame
due to high C:H
[ARENES] Reactivity and site of attack of benzene vs cyclohexene (Alkene)
C=C highly susceptible to electrophilic attack
pi e cloud is susceptible
Resonance stabilisation of benzene
less reactive towards electrophiles than alkene
[ARENES] What type of reaction and why?
Arene with halogen
Substitution to preserve resonance stabilised ring
[ARENES] Site of electrophilic attack when halogen is added to arene?
pi electron cloud
[ARENES] alkene vs arene
reactivity with halogens
arene less reactive with halogen
resonance stabilisation
delocalise pi e
less reactive to electrophiles
[ARENES] Types of electrophilic substitution reactions that can happen to an arene ring
Electrophilic substitution:
- Halogenation
- Nitration
- Friedel-crafts Alkylation
Friedel-crafts acylation (NOT TESTED DONT EVEN BOTHER)
[ARENES] Requirement for electrophilic substitution in Arenes
Strong electrophile
[ARENES] Electrophilic substitution in arenes (general eqn)
[ARENES] Slow and fast steps of electrophilic substitution in arenes
and why is that step the rate determining step (RDS)?
- Generate strong electrophile (fast)
- Electrophilic attack of E+ on benzene ring forms resonance-stabilised arenium ion (slow)
- Deprotonation and regen benzene ring (fast)
RDS (step 2) due to destruction of aromaticity of benzene ring
[ARENES] Steps of electrophilic substitution in arenes and fast/slow steps
- Generation of strong electrophile (slow)
- Electrophilic attack on benzene ring to from resonance-stable arenium ion (slow/RDS)
- Deprotonation and regeneration of benzene ring
[ARENES] Reagent and conditions for Nitration of arenes
conc HNO3
conc H2SO4 catalyst
kept under 55c, exothermic, must cool
[ARENES] Eqn/mechanism for Nitration of arenes
conc HNO3
conc H2SO4 catalyst
kept under 55c, exothermic, must cool
[ARENES] Reagent and conditions for Halogenation of arenes
X2 (Cl/Br)
Lewis acid catalyst ANHYDROUS AlX3, FeX3
(if there is water present it will (hydrolyse) incapacitate the catalyst so dont do that)
room temp
[ARENES] Mechanism for Halogenation of arenes
Electrophilic sub
[ARENES] Reagent and conditions for Friedel-crafts alkylation of arenes
RX, excess benzene
Lewis acid catalyst AlX3, FeX3
room temp
[ARENES] Difference between 3 types of electrophilic substitution in arenes
Nitration: Nitrate added to benzene ring
Halogenation: Halogen added to benzene ring
Friedel-crafts alkylation: R added to benzene ring while X (halogen) joins the removed H to form HX
[ARENES] Requirement of halogenation of arene
Lewis acid acting as a halogen carrier
anhydrous FeX3, AlX3,
or finely divided Fe which converts to FeX3 anyway lmao
[ARENES] Nitration in arenes eqns
[ARENES] Summary of electrophilic substitution reactions of benzene
[ARENES] Substituent on benzene ring determines
- reactivity
- orientation
[ARENES] Substituents effect on reactivity
ACTIVATING GROUPS
increase rate of electro sub
DEACTIVATING GROUPS
decrease rate of electro sub
[ARENES] Substituents effect on reactivity
ACTIVATING GROUPS
increase rate of electro sub
DEACTIVATING GROUPS
decrease rate of electro sub
[ARENES] Effect on reactivity for activating groups
increase electo sub
activating groups
donate e density to benzene ring
increase avail of pi electron cloud
stabilises arenium intermediate
by dispersing pos charge
lowering EA
more susceptible to electro attack
[ARENES] Effect on reactivity for deactivating groups
decrease electo sub
deactivating groups
withdraw e density from beznene ring
decrerase avail of pi electron cloud
destabilises arenium intermediate
by intensify pos charge
raising EA
less susceptible to electro attack
[ARENES] Types of reactions on benzene (aromatic vs aliphatic portion)
Aromatic portion (benzene ring): electrophilic sub Aliphatic (side group): FRS, side chain [O]
[ARENES] Compare products of benzene vs methylbenzene for halogenation and nitration
Benzene just adds the NO2 or X anywhere
Methylbenzene NO2/X attaches to 2-/4- position
RELATIVE TO CH3
aka adjacent or opposite
[ARENES] Side chain FRS mechanism
UV light
Limited Cl
Excess of methylbenzene
[ARENES] Side chain oxidation
Acidified KMnO4, heat
methybenzene to
benzoic acid (-CO2H)
benzaldehyde (-CHO)
