Arenes

Question 1

[ARENES] What does aromatic compound mean?

Answer 1

cyclically conjugated
stability more than hypothetical localised structure due to delocalisation
its usually benzene

Question 2

[ARENES] Formula of benzene ring?

Answer 2

C6H6
When the ratio of C:H is close to 1:1, its safe to assume a benzene ring is in the structure.

Question 3

[ARENES] C-C bond length in benzene?

Answer 3

all same length 0.140nm
longer than C=C but shorter than C-C, hence partial double bond character

Question 4

[ARENES] Enthalpy difference in theoretical and actual arenes

Answer 4

Hypothetical enthalpy is higher(less stable) than the actual of benzene.
Difference in energy is known as resonance energy.

Question 5

[ARENES] Why does benzene not react like an alkene and undergo addition?

Answer 5

They undergo substitution reactions instead.

Question 6

[ARENES] Resonance hybrid structure of Benzene?

Answer 6

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

Question 7

[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

Answer 7

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

Question 8

[ARENES] Bp/mp of arenes

Answer 8

Bp increases with Mr
more e
more polarisible e cloud
stronger idid

Mp depends on molecular symmetry but generally low

Question 9

[ARENES] Solubilty of arenes in polar/H2O

Answer 9

Insoluble
energy formed by pd-id btwn water/polar and arene
insufficient to overcome
idid btwn arene and ion-dipole interaction/pdpd btwn H2O

Question 10

[ARENES] Solubilty of arenes in non-polar

Answer 10

Soluble
energy formed by idid btwn water/non polar and arene
sufficient to overcome
idid btwn arene and non polar solvent

Question 11

[ARENES] Density of arene

Answer 11

less dense than h2o and immiscible

Question 12

[ARENES] Type of flame of burning arene

Answer 12

Smoky, luminous orange flame
due to high C:H

Question 13

[ARENES] Reactivity and site of attack of benzene vs cyclohexene (Alkene)

Answer 13

C=C highly susceptible to electrophilic attack

pi e cloud is susceptible
Resonance stabilisation of benzene
less reactive towards electrophiles than alkene

Question 14

[ARENES] What type of reaction and why?
Arene with halogen

Answer 14

Substitution to preserve resonance stabilised ring

Question 15

[ARENES] Site of electrophilic attack when halogen is added to arene?

Answer 15

pi electron cloud

Question 16

[ARENES] alkene vs arene
reactivity with halogens

Answer 16

arene less reactive with halogen
resonance stabilisation
delocalise pi e
less reactive to electrophiles

Question 17

[ARENES] Types of electrophilic substitution reactions that can happen to an arene ring

Answer 17

Electrophilic substitution:

1. Halogenation
2. Nitration
3. Friedel-crafts Alkylation

Friedel-crafts acylation (NOT TESTED DONT EVEN BOTHER)

Question 18

[ARENES] Requirement for electrophilic substitution in Arenes

Answer 18

Strong electrophile

Question 19

[ARENES] Electrophilic substitution in arenes (general eqn)

Answer 19

Question 20

[ARENES] Slow and fast steps of electrophilic substitution in arenes
and why is that step the rate determining step (RDS)?

Answer 20

1. Generate strong electrophile (fast)
2. Electrophilic attack of E+ on benzene ring forms resonance-stabilised arenium ion (slow)
3. Deprotonation and regen benzene ring (fast)

RDS (step 2) due to destruction of aromaticity of benzene ring

Question 21

[ARENES] Steps of electrophilic substitution in arenes and fast/slow steps

Answer 21

1. Generation of strong electrophile (slow)
2. Electrophilic attack on benzene ring to from resonance-stable arenium ion (slow/RDS)
3. Deprotonation and regeneration of benzene ring

Question 22

[ARENES] Reagent and conditions for Nitration of arenes

Answer 22

conc HNO3
conc H2SO4 catalyst
kept under 55c, exothermic, must cool

Question 23

[ARENES] Eqn/mechanism for Nitration of arenes

Answer 23

conc HNO3
conc H2SO4 catalyst
kept under 55c, exothermic, must cool

Question 24

[ARENES] Reagent and conditions for Halogenation of arenes

Answer 24

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

Question 25

**[ARENES]** Mechanism for Halogenation of arenes

Answer 25

Electrophilic sub

Question 26

**[ARENES]** Reagent and conditions for Friedel-crafts alkylation of arenes

Answer 26

RX, excess benzene Lewis acid catalyst AlX3, FeX3 room temp

Question 27

**[ARENES]** Difference between 3 types of electrophilic substitution in arenes

Answer 27

_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**

Question 28

**[ARENES]** Requirement of halogenation of arene

Answer 28

Lewis acid acting as a halogen carrier anhydrous FeX3, AlX3, or finely divided Fe which converts to FeX3 anyway lmao

Question 29

**[ARENES]** Nitration in arenes eqns

Answer 29

Question 30

**[ARENES]** Summary of electrophilic substitution reactions of benzene

Answer 30

Question 31

**[ARENES]** Substituent on benzene ring determines

Answer 31

* reactivity * orientation

Question 32

**[ARENES]** Substituents effect on reactivity

Answer 32

ACTIVATING GROUPS increase rate of electro sub DEACTIVATING GROUPS decrease rate of electro sub

Question 33

**[ARENES]** Substituents effect on reactivity

Answer 33

ACTIVATING GROUPS increase rate of electro sub DEACTIVATING GROUPS decrease rate of electro sub

Question 34

**[ARENES]** Effect on reactivity for activating groups

Answer 34

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

Question 35

**[ARENES]** Effect on reactivity for deactivating groups

Answer 35

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

Question 36

**[ARENES]** Types of reactions on benzene (aromatic vs aliphatic portion)

Answer 36

``` Aromatic portion (benzene ring): electrophilic sub Aliphatic (side group): FRS, side chain [O] ```

Question 37

**[ARENES]** Compare products of benzene vs methylbenzene for halogenation and nitration

Answer 37

Benzene just adds the NO2 or X anywhere Methylbenzene NO2/X attaches to 2-/4- position **RELATIVE TO CH3** aka adjacent or opposite

Question 38

**[ARENES]** Side chain FRS mechanism

Answer 38

UV light Limited Cl Excess of methylbenzene

Question 39

**[ARENES]** Side chain oxidation

Answer 39

Acidified KMnO4, heat methybenzene to benzoic acid (-CO2H) benzaldehyde (-CHO)