14 Arenes

A Level Chemistry Topic 14: Arenes

What is a benzene ring called when it is treated as a substituent group? When is it used?

phenyl group

• when substituent group on benzene contains more than 6 carbon atoms (eg. 1-phenylheptane)

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

What is a benzyl group?

benzene–CH₂ (ie. C₆H₅CH₂)

(T/F) Benzene is soluble in water.

FALSE. Benzene is non-polar and insoluble in water.

(T/F) Benzene is soluble in all organic solvents.

TRUE

(T/F) Benzene is more dense than water.

FALSE. Less dense than water

Why does benzene burn with a smoky, luminous flame?

It has a relatively high carbon content.

Describe the resonance structure of benzene.

• Each carbon atom has 3 sp² hybrid orbitals that are arranged in a trigonal planar manner.

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

• Each carbon atom also has an unhybridised p orbital occupied by a single electron. Each singly-filled p orbital overlaps side-on with adjacent p orbital on either side. The continuous side-on overlap results in a cloud of delocalised π electrons above and below the plane of the ring, conferring a resonance structure.

Why are the carbon-carbon bond lengths in benzene an intermediate between the length of C-C bond and C=C bond?

Benzene has a resonance hybrid structure, where each carbon-carbon bond has partial double bond character, and are of equal lengths.

(T/F) The enthalpy change of hydrogenation of benzene is more exothermic than its expected value accroding to Kekule structure.

FALSE. It is less exothermic than predicted due to the delocalisation of 6 π electrons that confers extra stability on benzene and is reflected by its resonance energy.

Why does benzene undergo substitution rather than addition reactions?

This allows benzene to retain its aromaticity, as delocalisation of six π electrons in continuously overlapping p-orbitals confers additional stability.

Does benzene attract electrophiles or nucleophiles?

Electrophiles, as it is electron rich due to availability of 6 π electrons

List 4 examples of electrophilic substitution reactions benzene can undergo.

1. Nitration

2. Halogenation

3. Friedel-Crafts alkylation

4. Friedel-Crafts acylation

What are the steps in ES? Which is the fast step and slow step?

1. Generation of electrophile, E⁺

2. Electrophilic attack by E⁺ on the benzene ring to form a carbocation (slow)

3. Loss of proton from the carbocation to form the product (fast)

(T/F) The carbocation intermediate is aromatic.

FALSE. It is resonance-stabilised by not aromatic.

Explain how the electrophile attacks the benzene ring.

The electrophile attacks the electron-rich benzene ring. This involves the movement of 2 π electrons from the benzene ring to the electrophile, forming a sigma bond to one carbon atom of the benzene ring. A resonance-stabilised carbocation is formed.

Explain how substituted product is formed from carbocation.

The carbocation intermediate loses a proton (H⁺) from the carbon atom bearing the electrophile. The two electrons bonding this proton to carbon become part of the delocalised π-electron system. The aromatic character of the benzene ring is restored and the substituted product is formed.

What are the reagents and conditions required for nitration of benzene?

Reagents: conc. HNO₃, conc. H₂SO₄ catalyst

Conditions: 55˚C

What are the reagents and conditions required for halogenation of benzene?

Reagents: Br₂, FeBr₃ catalyst

Conditions: Nil

What are the reagents and conditions required for Friedel-Crafts alkylation/acylation of benzene?

Reagents: R–Cl OR R–(C=O)–Cl resp, AlCl₃ catalyst

Conditions: Nil

Write the equation for the generation of electrophile NO₂⁺

2H₂SO₄ + HNO₃ ←→ NO₂⁺ + 2HSO₄^- + H₃O⁺

Note: no state symbols (eg. aq) as reagents are concentrated.

What is the purpose of the second H₂SO₄ molecule in reaction to generate NO₂⁺?

It acts as a dehydrating agent, to immediately provide a proton for H₂O generated to form H₃O⁺ when NO₂⁺ is formed.

What is the geometry/hybrisation of the carbon atom bearing –NO₂ group in carbocation?

It has a tetrahedral shape with respect to this carbon atom, which is sp³ hybridised. The rest of the carbon atoms remain planar and sp² hybridised.

Draw the mechanism for nitration of benzene.

Refer to notes.

Remember to label fast/slow step, double-headed arrows, carbocation with + charge in centre, catalyst regenerated.

Write the equation for the generation of electrophile Cl⁺

Cl₂ + FeCl₃ ←→ Cl⁺ + [FeCl₄]^-

Compare the role of FeCl₃ and H₂SO₄ in halogenation and nitration of benzene respectively.

FeCl₃: Lewis acid catalyst

H₂SO₄: Bronsted Lowry catalyst

Write the equation for the generation of electrophile CH₃CO⁺ in FCAcyl.

CH₃COCl + AlCl₃ ←→ CH₃CO⁺ + [AlCl₄]^-

Explain how an activating group works.

• An activating group is electron-donating.

• It increases the electron density in the benzene ring and makes the ring more susceptible (more reactive) to electrophilic attack.

• It helps to disperse the positive charge in the intermediate carbocation and stabilise the carbocation.

Explain how a deactivating group works.

• A deactivating group is electron-withdrawing.

• It decreases the electron density in the benzene ring and makes the ring less susceptible (less reactive) to electrophilic attack.

• It helps to intensify the positive charge in the intermediate carbocation and destabilise the carbocation.

define: inductive effect

An inductive effect is the donation or withdrawal of electrons through sigma bonds due to the electronegativity difference between atoms

define: resonance effect

A resonance effect is the donation or withdrawal of electrons through π 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.

What reactions can benzene undergo?

1. Electrophilic substitution

2. Combustion

3. Reduction (Ni catalyst, high t&p)

What reactions can benzene not undergo?

1. Oxidation (KMnO₄, H₂SO₄, heat)

2. Bromination (Br₂ in CCl₄ in the dark)

3. Reduction of H₂ gas (Ni catalyst, rtp)

What are the two portions of methylbenzene / alkylbenzenes and what reactions can they undergo?

Aromatic portion (benzene): ES reactions

Aliphatic portion (alkyl side chain): FRS, oxidation

List the ways the alkyl side chain of benzene can be oxidised.

1. KMnO₄ (aq), H₂SO₄ (aq), heat - decolourisation of purple KMnO₄ and formation of white ppt of benzoic acid

2. KMnO₄ (aq), NaOH (aq), heat - decolourisation of purple KMnO₄ and formation of black-brown ppt of MnO₂, formation of white ppt of benzoic acid upon cooling and acidification

3. K₂Cr₂O₇ (aq), H₂SO₄ (aq), heat - orange K₂Cr₂O₇ does not turn green

(T/F) The length of the alkyl side chain on benzene determines the products formed after oxidation.

FALSE. Regardless of the length of the alkyl side chain, the entire side chain is destroyed during oxidation, with only –COOH remaining to indicate the position of the original side chain.

What are the criteria for oxidation of side chains on benzene?

Alkylbenzenes with a hydrogen or oxygen atom bonded to the benzylic carbon atom can undergo oxidation when treated with acidified KMnO₄

3˚ benzylic carbon C₆H₅C(CH₃)₃ cannot undergo oxidation.