A levels chemistry: arenes

what are arenes

hydrocarbons based on the benzene ring

what are aromatic compounds

compounds possessing benzene ring or other molecular structures that resemble benzene in structure and chemical behaviour

when does the nomenclature not end with benzene

• benzaldehyde (-CHO)

• phenol (-OH)

• phenylamine (-NH2)

• benzoic acid (-COOH)

• benzenesulfonic acid (-SO3H)

when is the benzene ring treated as a substituent

when

• the substituent on the benzene ring has more than 6 carbon atoms

• the highest priority functional group is not a substituent on the benzene ring

  • eg -OH is not on the benzene ring, then it is not a phenol but a phenyl alcohol

phenyl and substituted phenyl groups are called aryl groups

physical properties of benzene

• colourless liquid

• characteristic odour

• non-polar, insoluble in water, less dense than water

  • pre dominant IMF: id-id forces

• soluble in all organic solvents and is a good solvent for organic compounds

• burns with a smoky and luminous flame

  • due to its relatively high carbon content (C:H = 1:1)

  • undergoes incomplete combustion

explain the resonance structure of benzene

• to minimise electronic repulsion, the 3 regions of electron density about each C atom in a benzene molecule adopt a trigonal planar geometry

• Hence, all bond angles in the benzene ring are 120 degrees

• it is a planar molecule

• each of the 6 carbon atoms in benzene is sp2 hybridised, comprising 3 sp2 hybrid orbitals and 1 unhybridised p orbital

  • 2 sp2 hybrid orbitals are used to overlap head-on with the sp2 hybrid orbitals of 2 adjacent C atoms to form 2 C-C sigma bonds

  • 1 sp2 hybrid orbital is used to overlap head-on with the 1s orbital of the H atom to form the C-H sigma bond

  • each singly filled p orbital overlaps side-on with the adjacent p orbital on either side

    • this continuous side-on overlap of the p-orbitals results in a cloud of delocalised pi electrons above and below the plane of the ring

    • ie resonance is present

evidence for resonance in benzene

all the carbon-carbon bonds in benzene are identical and equal in length. This agrees with the description of benzene as a resonance hybrid of the two Kekulé structures

the measured carbon-carbon bond length in benzene is intermediate between the length of a C-C bond and that of a C=C bond

this indicates that the carbon-carbon bonds have partial double bond character

• pi electron density is evenly distributed

  • delocalisation of pi electrons (ie pi bond is averaged out across the 6 C atoms)

Why does benzene undergo substitution rather than addition reactions

If benzene undergoes addition reactions, the overall aromatic character is destroyed. The extra stability associated with the delocalisation of the 6 pi electrons is lost

Hence, the majority of reactions that benzene undergoes involve the substitution in the ring. The delocalisation of the six pi electrons in the continuously overlapping p-orbitals, and hence its aromatic character, is retained

explain the electrophilic substitution mechanism

• benzene ring is electron rich

  • due to the availability of 6 pi electrons

• benzene is a nucleophile, attracts electrophiles

Generation of electrophile, E+

• the electrophile E+ is first generated

Mechanism: electrophilic addition

Step 1

• the electrophile attacks the electron-rich benzene ring

• this involves the movement of 2 pi electrons from the benzene ring to the electrophile, forming a sigma bond to one carbon atom of the benzene ring

  • this carbon atom is sp3 hybridized, the rest remain sp2 hybridized

• a carbocation is formed.

  • the carbocation is resonance-stabilised but not aromatic

• this step is the slow step

  • ie the rate-determining step

  • because it involves the destruction of the aromatic character of the benzene ring.

  • the extra stability associated with the delocalisation of 6 pi electrons is lost

Step 2

• the carbocation intermediate loses a proton from the carbon atom that bears the electrophile

• the two electrons that bonded this proton to carbon become part of the delocalised pi-electron system

• the aromatic character of the benzene ring is restored

• the substituted product is formed

• this step is a fast step

reagents, conditions, product and mechanism of reaction of benzene with concentrated nitric acid

reagent: concentrated HNO3, concentrated H2SO4

• role of H2SO4: Bronsted-Lowry acid catalyst

Condition: 55-60 degrees celsius (or heat, depending on the substituents)

Product: nitrobenzene (a yellow oil)

mechanism: electrophilic substitution

what is a bronsted-lowry acid

a proton donor

what is a bronsted-lowry base

a proton acceptor

how is the electrophile generated during the nitration of benzene

2 H2SO4 + HNO3 ⇌ NO2+ + 2 H2SO4- + H3O+

• in the nitrating mixture, HNO3 acts as a Bronstead-Lowry base and accepts a proton from the Bronsted-Lowry acid H2SO4

  • H2SO4 + HNO3 ⇌ H2NO3+ + HSO4-

• the reaction leads eventually to the formation of the nitronium ion, NO2+

  • H2NO3+ ⇌ NO2+ + H2O

  • H2SO4 + H2O —> HSO4- + H3O+

adding up all 3 equations gives:

• 2H2SO4 + HNO3 ⇌ NO2+ + 2HSO4- + H3O+

explain the electrophilic substitution mechanism for the nitration of benzene

step 1: (electrophilic attack by NO2+) (rate-determining step)

• the NO2+ ion acts as an electrophile and attacks the electron-rich benzene ring.

• 2 of the 6 pi electrons in the benzene ring are used to form the C-N bond.

• Hence the aromatic character of the ring is destroyed.

• this is the slow step/rate-determining step of the mechanism

• a carbocation is formed.

  • the C atom bearing the -NO2 group is sp3 hybridized

  • shape wrt to this atom is tetrahedral

  • the rest of the C atoms remain sp2 hybridized

Step 2: (loss of proton from carbonation) (fast step)

• the unstable carbocation loses a proton to the HSO4- ion to form nitrobenzene, thus regaining the aromatic character.

• H2SO4 is regenerated

• H2SO4 acts as a Bronsted-Lowry acid catalyst

reagents, conditions, product and mechanism of reaction of benzene with concentrated nitric acid to form 1,3-dinitrobenzene

reagent: concentrated HNO3, concentrated H2SO4

Condition: heat

Product: 1,3-dinitrobenzene (major product)

• minor products: 1,2-dinitrobenzene, 1,4-dinitrobenzene

mechanism: electrophilic substitution

reagents, conditions, product and mechanism of reaction of benzene with concentrated nitric acid to form 1,3,5-trinitrobenzene

reagent: concentrated HNO3, fuming H2SO4

Condition: very high temperatures for several days

Product: 1,3,5-trinitrobenzene (major product)

mechanism: electrophilic substitution

* yield of the reaction is only about 40%

what is a lewis acid

electron pair acceptor

what is a lewis base

electron pair donor

how is electrophile generated in the halogenation of benzene

AlCl3/FeCl3/AlBr3/FeBr3 accepts a lone pair of electrons from the halogen molecule, generating the Cl+/Br+ electrophile

• AlCl3/FeCl3/AlBr3/FeBr3 functions as a lewis acid

FeCl3 can be generated in situ from Fe and Cl2 and FeBr3 from Fe and Br2

• 2Fe + 3Cl2 —> 2FeCl3

• 2Fe + 3Br2 —> 2FeBr3

Note

• AlCl3 reacts readily with water

  • AlCl3 (s) + 6H2O(l) —>[Al(H₂O)₆]³⁺ (aq) + 3Cl- (aq)

  • [Al(H₂O)₆]³⁺ (aq) + H2O(l) ⇌ [Al(H₂O)₅(OH)]²⁺ (aq) + H3O+ (aq)

• hence the reaction with AlCL3 can only proceed under anhydrous condition

• I2 is too unreactive and F2 reacts too violently

reagents, conditions, product and mechanism of reaction of benzene with halogen (Br2 or Cl2) to form chloro/bromobenzene

reagents: FeCl3 or AlCl3 (anhydrous) or FeBr3 or AlBr3 or Fe

• role of AlCl3/FeCl3/AlBr3/FeBr3: lewis acid catalyst

condition: room temperature, anhydrous condition if using AlCl3

product: chlorobenzene or bromobenzene (and HCl/HBr)

mechanism: electrophilic substitution

• In the presence of a suitable lewis acid catalyst such as AlCl3 or FeCl3, benzene undergoes electrophilic substitution reaction with chlorine at room temperature

• Similarly, benzene reacts with bromine to form bromobenzene and hydrogen bromide in the presence of AlBr3 or FeBr3

what is friedel-crafts alkylation

alkylation with a halogenoalkane (RX) and a trace amount of anhydrous AlCl3 as catalyst

• role of AlCl3: lewis acid catalyst

what is friedel-crafts acylation

acylation is similar to that of alkylation except that an acid halid (RCOX or ArCOX) instead of a halogenoalkane (RX) is used. The acid halide provides the acyl group (-COR) needed for the reaction

• role of AlCl3: lewis acid catalyst

reagents and conditions for nitration of substituted benzene rings

compound	reagents	conditions
nitrobenzene (least reactive; -NO2 group is deactivating)	conc HNO3	conc H2SO4 catalyst, heat (»55°C)
chlorobenzene (-Cl group is weakly deactivating)	conc HNO3	conc H2SO4 catalyst, heat (> 55°C)
benzene ring	conc HNO3	conc H2SO4 catalyst, 55-60°C
methylbenzene (-CH3 group is weakly activating)	conc HNO3	conc H2SO4 catalyst, 30°C
phenol (most reactive; -OH group is activating)	DILUTE HNO3	room temperature

explain what an activating group is

• electron-donating

• makes the benzene ring more reactive towards electrophilic substitution reactions

• increases the electron density in the benzene ring and makes the ring more susceptible to electrophilic attack

• helps to disperse the positive charge on the intermediate carbocation and stabilise the carbocation

  • Ea for step 1 is smaller —> faster rate of reaction for slow step —> faster rate of reaction

explain what a deactivating group is

• electron-withdrawing

• makes the ring less reactive than benzene towards electrophilic substitution reactions

• decreases the electron density in the ring and makes the ring less susceptible to electrophilic attack

• intensifies the positive charge on the intermediate carbocation and destabilises the carbocation

  • Ea for step 1 is larger —> slower rate of reaction for slow step —> slower rate of reaction

explain why methylbenzene requires a lower temperature to carry out nitration

Methylbenzene is more reactive than benzene towards electrophilic substitution as the -CH3 group is an electron-donating group and hence an activating group. It increases the electron density in the benzene ring and makes it more susceptible to electrophilic attack

what is inductive effect

donation or withdrawal of electrons through sigma bonds due to the EN difference between atoms

• more EN atom will attract the bonding electrons closer to itself —> shift in electron density towards the more EN atom

explain electron-donating via inductive effect

eg - CH3

• alkyl groups have a sp3 carbon

• compared to the sp2 carbon in benzene ring, sp3 orbitals have less s character and its electrons are less tightly held and more easily donated to the benzene ring

  • higher s character of sp2 C atoms: stronger attraction for bonding e-

• the substituent is said to be electron donating by inductive effect

explain electron-withdrawing via inductive effect

eg -Cl

• atoms such as O, N and Cl are more EN than C atom (on benzene ring)

• they pull electron density away from C though the sigma bond

• the substituent is said to be electron-withdrawing by inductive effect

explain what resonance effect is

a resonance effect is the donation or withdrawal of electrons through pi bonds due to the continuous side-on p-orbital overlap of the substituent and the benzene ring.

This results in the delocalisation of electrons, either towards or away from the benzene ring

explain electron-donating via resonance effect

eg -OH

• if a substituent has a lone pair of electrons on the atom that is directly attached to the benzene ring

  • the lone pair is usually in a p-orbital and

  • can be delocalised into the ring

  • net shift of electrons into the benzene ring since the substituent p orbital has >1 pi electron for that atom attached to the benzene ring

• the substituent is said to be electron-donating by resonance effect

explain electron-withdrawing via resonance effect

eg -C=O, -NO2

• if a substituent is directly attached to the benzene ring by an atom that is doubly or triply bonded to a more EN atom, the pi electrons of the benzene ring can be delocalised onto the substituent

  • in -C=O, the pi electrons in the delocalised pi electron cloud shift towards O (EN of O > EN of C)

  • in -NO2, the pi electrons in the delocalised pi electron cloud shift to O (EN of O > EN of N)

• Each substituent is unsaturated and does not have an atom with a lone pair bonded directly to the benzene ring

  • C in -C=O only has one electron in its unhybridised p orbital, so do its O atoms

  • N in -NO2 only has one electron in its unhybridised p orbital, so do its O atoms

• the substituent is said to be electron-withdrawing by resonance effect

In chlorobenzene, is the electron-withdrawing, inductive effect of Cl greater or the electron-donating, resonance effect of Cl stronger

inductive effect of Cl is stronger

• Cl is in Period 3 and its p-orbitals are more diffuse, leading to a less effective side-on overlap with the p-orbitals of the benzene ring.

• Hence, the overall effect of the -Cl group in chlorobenzene is electron-withdrawing (by inductive effect)

• The -Cl group decreases the electron density in the benzene ring and makes the ring less susceptible to electrophilic attack

  • it is said to be a deactivating group

In phenol, is the electron-withdrawing, inductive effect of Cl greater or the electron-donating, resonance effect of Cl stronger

The resonance effect is stronger than the inductive effect

• the ability to donate e- back through the pi bond > the e- withdrawing effect through sigma bond

• hence, the overall effect of the -OH group in phenol is electron-donating

• the -OH group increases the electron density in the benzene ring and makes the ring more susceptible to electrophilic attack

• it is said to be a strongly activating group

combustion reaction

benzene ring + 15/2 O2(g) —> 6 CO2(g) + 3 H2O(l)

reagents, conditions and product of reduction of benzene ring

Reagents: H2(g)

conditions: Ni catalyst, high temperature, high pressure

product: cycloalkane

reactions that benzene does not undergo

• oxidation

• bromination

• hydrogenation at room temperature

what reactions can methylbenzene undergo

• electrophilic substituition reactions

• FRS

• oxidation

reagents, conditions, product and observations for oxidation of alkyl side chain of alkylbenzene

Acidic conditions

reagent: KMnO4 (aq), H2SO4 (aq)

condition: heat/heat under reflux

product: -CH3 change to -COOH

• -CH2Ch3 change to -COOH + CO2

observations: purple KMnO4 decolourises (if it is LR), formation of white ppt of benzoic acid upon cooling

Alkaline conditions

reagent: KMnO4 (aq), NaOH (aq)

condition: heat/heat under reflux

product: -CH3 change to -COO-Na+

• -CH2Ch3 change to -COO-Na+ + CO2

observations: purple KMnO4 decolourises (if it is LR), formation of brown-black ppt of MnO2, formation of white ppt of benzoic acid upon cooling

critieria needed for oxidation of alkyl side chain on benzene

the benzylic carbon atom must be directly bonded to H or O atom

ie the C atom directly bonded to the benzene ring must be directly bonded to a H or O atom as well