arenes

Question 1

what is the molecular formula of benzene?

Answer 1

C6H6

Question 2

what are the properties of benzene?

mp, bp, solubility

Answer 2

physical properties: benzene is a colourless liquid at rtp. it is highly volatile and flammable, and is toxic and carcinogenic

melting & boiling points: melting point ⮕ 5.5C boiling point ⮕ 80C. as benzene is a non-polar molecule, only a small amount of energy is needed to overcome the weak dispersion forces between molecules, so benzee has low melting & boiling points

solubility: being a non-polar molecule, benzene is insoluble in polar solvents but soluble in non-polar solvents. benzene can also be used as a non-polar solvent

Question 3

describe the structure of benzene

Answer 3

sp² hybridisation in benzene
- each carbon is bonded to 2 other carbons and 1 hydrogen. (3 bond pairs)
- each carbon atom is sp² hybridised, as the mixing of 1 s orbital and 2 p orbitals gives 3 sp² hybrid orbitals oreinted at 120° to one another. one p orbital remains unhybridised, and is arranged perpendicular to the 3 sp² hybrid orbitals.
- the geometry around each carbon atom in benzene is trigonal planar with C-C-C bond angle of 120°

bonds in benzene
- there are 6 C-C sigma (σ) bonds in benzene, each of which is formed from the head-on overlap of one sp² hybrid orbital of one carbon atom with one sp² hybrid orbital of the adjacent carbon atom.
- there are also 6 C-H single bonds, each of which is an s-sp² σ bond formed from head-on overlap of the 1s atomic orbital of H and the sp² hybrid orbital of C.
- the remaining unhybridised p orbitals of the 6 carbon atoms are all parallel to one another due to the planarity of the benzene molecule. all 6 unhybridised p orbitals overlap side-on with each other equally to produce 2 continuous rings of π electrons above and below the plane of the benzene ring, also known as the delocalised π electron cloud.
- the 6 electrons found in this delocalised π electron cloud are free to move throughout the entire π electron cloud, so the electrons are delocalised.

Question 4

what are the 2 implications of the π electron cloud delocalisation?

Answer 4

1. the delocalised π electron cloud causes all carbon-carbon bond lengths to be equal, creating a planar, regular hexagonal shape
2. the delocalised π electron cloud prevents benzene from undergoing any of the typical addition reactions that alkenes show

for pt 1: all 6 carbon-carbon bonds of benzene are identical in length and intermediate between a C-C bond length and a C=C bond length. the carbon-carbon bond energy in benzene is also between that of a C-C bond and a C=C bond

Question 5

why does the delocalised π electron cloud prevent benzene from undergoing any of the typical addition reactions that alkenes show?

i.e. why is there a difference in reactivity between benzenes & alkenes?

Answer 5

• like electron-rich alkenes, the π electron cloud in benzene attracts electrophiles. however, the delocalisation of electrons in the π electron cloud in benzene results in extra stability of benzene.
• as a result, the π electron cloud in benzene is less susceptible to attack by electrophiles compared to the C=C double bonds in alkenes. hence, benzene requires stronger electrophiles to react as compared to alkenes.
• benzene undergoes electrophilic substitution instead of electrophilic addition, as electrophilic addition destroys the delocalised π electron cloud, which requires a significant amount of energy which is highly unfavourable. benzene preferentially undergoes electrophilic substitution reactons which preserves its aromaticity

Question 6

what are the reagents, conditions and observations for electrophilic substitution of benzene with chlorine (chlorination)?

Answer 6

reagents & conditions:
- Cl2 (g), FeCl3 (s) as Lewis acid catalyst, warm OR
- Cl2 (g), Fe (s), warm

observations:
- decolourisation of greenish-yellow Cl2 (g) AND
- white fumes of HCl (g)

Lewis acids accept electron pairs

Question 7

what are the reagents, conditions and observations for electrophilic substitution of benzene with bromine (bromination)?

Answer 7

reagents & conditions:
- Br2 (l), FeBr3 (s) as Lewis acid catalyst, warm
- Br2 (l), Fe (s), warm

observations:
- decolourisation of reddish-brown Br2 (l) AND
- white fumes of HBr (g)

Lewis acids accept electron pairs

Question 8

what are the reagents, conditons and observations for electrophilic substitution of benzene with concentrated nitric acid (nitration)?

Answer 8

reagent:
- concentrated HNO3

conditions:
- concentrated H2SO4 as Bronsted-Lowry acid catalyst
- maintained at 55C

observation:
- pale yellow oily liquid formed

Bronsted-Lowry acids donate protons

Question 9

what are the reagents, conditions and observations for electrophilic substitution of benzene with halogenoalkanes (Friedel-Crafts alkylation)?

Answer 9

reagent:
- chloroalkane

conditions:
- AlCl3 as Lewis acid catalyst
- warm

observations:
- whwite fumes of HCl (g)

Lewis acids accept electrons

Question 10

what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with chlorine?

Answer 10

reagents & conditions:
- Cl2 (g), FeCl3 (s), room temperature, absence of UV

observations:
- decolourisation of greenish-yellow Cl2 (g) AND
- white fumes of HCl (g)

FeCl3 (s) acts as a Lewis acid catalyst

room temp & absence of UV prevents free radical sub on alkyl side chain

Question 11

what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with bromine?

Answer 11

reagents & conditions:
- Br2 (l), FeBr3 (s), room temperature, absence of UV

observations:
- decolourisation of reddish-brown Br2 (l) AND
- white fumes of HBr (g)

FeBr3 (s) acts as a Lewis acid catalyst

room temp & absence of UV prevents free radical sub on alkyl side chain

Question 12

what are the reagents, conditions and observations for electrophilic subsitution of methylbenzene with concentrated nitric acid?

Answer 12

reagent:
- concentrated HNO3

conditions:
- concentrated H2SO4 as Bronsted-Lowry acid catalyst
- maintained at 30C

observation:
- yellow oily liquid formed

Bronsted-Lowry acids donate protons

Question 13

what are the reagents, conditions and observations for electrophilic substitution of methylbenzene with halogenoalkanes? (friedel-crafts alkylation)

Answer 13

reagent:
- chloroalkane

conditions:
- AlCl3 (s) as Lewis acid catalyst
- room temperature

observation:
- white fumes of HCl (g)

Lewis acids accept electrons

Question 14

what are the 2 ways substituents on a benzene ring affect the reactivity of the benzene ring towards electrophilic substitution?

Answer 14

1. inductive effect
2. delocalisation

electron-donating groups increase the electron density in the benzene ring, while electron-withdrawing groups decrease the electron density in the benzene ring.

Question 15

how does the inductive effect arise?

Answer 15

inductive effect arises from the polarisation of electron density in a bond due to the electronegativity of nearby atoms.

electron-withdrawing groups via inductive effect:
- electron-withdrawing groups like -OH, -NH2, -Cl, inductively withdraw electron density from the benzene ring via the σ bond as O, N and halogens are more electronegative than the C atom in the benzene ring.

electron-donating groups via inductive effect:
- electron-donating alkyl groups like -CH3, CH2CH3, inductively donate electron density into the benzene ring via the σ bond.

Question 16

how does delocalisation arise?

Answer 16

delocalisation occurs when p orbitals on 3 or more adjacent orbitals overlap, forming a π electron cloud. for substituents with a p orbital no te atom joined to the benzene ring, the p orbital can overlap with the π electron cloud of the ring.

electron-donating groups via delocalisation
- electron-donating groups like -OH, -NH2, -Cl, have a lone pair of electrons in the p orbital of th atom directly joined to the benzene ring, and can thus donate the lone pair of electrons into the benzene ring via delocalisation

electron-withdrawing groups via delocalisation
- in electron-withdrawing groups like -CHO, COOH, NO2, the atom directly joined to the benzene ring forms double/triple bonds to electronegative atoms like O or N, thus electron density in the benzene ring is drawn away by these electronegative atoms via delocalisation

Question 17

how does delocalisation and inductive effect affect the reactivity of the benzene ring?

Answer 17

• inductive effect and delocalisation do not necessarily act in the same direction. when 2 effects are opposing, the stronger effect dominates
• eg the -OH group is electron-withdrawing via inductive effect but electron-donating via delocalisation.
• substituents that are overall-electron-donating towards the benzene ring will increase the electron density of the benzene ring, making it more reactive towards electrophilic attack. they are called activating groups. eg -OH, -NH2, alkyl groups
• substituents that are overall electron-withdrawing from the benzene ring will decrease the electron density of the benzene ring, making it less reactive towards electrophilic attack. they are called deactivating groups. eg halogens, -CHO, -COOH, -NO2, -CN

Question 18

why is there a difference between conditions for the same electrophilic substitution reactions for benzene and methylbenzene?

Answer 18

• since the -CH3 group is activating, the conditions for it to undergo electrophilic substitution are milder compared to those for benzene
• activating groups like -CH3 increase the electron density of the benzene ring, making it more reactive towards electrophilic attack, hence milder conditions are needed

Question 19

how do substituents on a benzene ring affect the position of the incoming electrophile of monosubstitued arenes?

Answer 19

the position of the incoming group is determined by the nature of the substituent/group already bonded to the ring, NOT by the nature of the incoming group!!!

activating groups are 2,4-directing groups while deactivating groups are 3-directing groups.

if there are 2 groups already on the benzene ring,
- if both groups direct to the same position, substitution will occur there
- if each group directs the incoming electrophile to a different position, the major product follows the directing effect of the more strongly activating group
- further subsitution rarely occurs between 2 groups in 1,3-disubstituted benzene rings as the site is too sterically hindered

the data booklet contains a table that summarises the effects and position of substitution by he group alreaedy on the benzene ring (don’t have to memorise!)

Question 20

what are the reagents, conditions & observations for side chain free radical substitution of methylbenzene with chlorine?

Answer 20

reagents & conditions:
- Cl2 (g), UV light/heat

observation:
- greenish-yellow Cl2 (g) decolourises slowly

Question 21

what are the reagents, conditions & observations for side chain free radical substitution of methylbenzene with bromine?

Answer 21

reagents & conditions:
- Br2 (l), UV light/ heat

observation:
- reddish-brown Br2 (l) decolourises slowly

Question 22

what are the reagents, conditions & observations for side chain oxidation of methylbenzene?

Answer 22

oxidation to benzoic acid

reagents & conditions:
- KMnO4 (aq), dilute H2SO4, heat

observations:
- purple KMnO4 is decolourised
- white precipitate of benzoic acid is formed

oxidation to benzoate salt

reagents & conditions:
- KMnO4 (aq), dilute NaOH, heat

observations:
- purple KMnO4 is decolourised
- brown precipitate of manganese dioxide, MnO2 (s) is formed

Question 23

how can bromine in tetrachloromethane be used to differentiate between alkenes and arenes?

Answer 23

procedure: add bromine in tetrachloromethan dropwise with shaking to 1cm3 of each compound in separate test tubes
observations: for benzene, Br2 in CCl4 remains reddish-brown. for the alkene, reddish-brown Br2 in CCl4 decolourises.

Question 24

how can aqueous bromine be used to distinguish between alkenes and arenes?

Answer 24

procedure: add aqueous bromine dropwise with shaking to 1cm3 of each compound in separate test tubes.
observations: for benzene, BR2 (aq) remains yellow-orange. for the alkene, yellow-orange Br2 (aq) decolourises

note that for aqueous bromine, if qns ask for equation, must include H2O!! so reactants would be Br2 + H2O

Question 25

how to distinguish betwen certain alkenes and alkylbenzenes?

Answer 25

eg: hexene, benzene, methylbenzene, ethylbenzene
procedure: KMnO4 (aq), dilute H2SO4, heat
observations: for hexene (oxidative cleavage), effervescence of CO2 and decolourisation of purple KMnO4 is observed. no white ppt. for benzene, purple KMnO4 not decolourised, no white ppt, no effervescence. for methylbenzene, purple KMnO4 decolourises and white ppt of benzoic acid forms. no effervescence. for ethylbenzene, purple KMnO4 decolourises, white ppt of benzoic acid forms, effervescencec of CO2 gas is observed.