Chapter 11: Arenes

Chapter 11: Arenes

Chapter 11
Arenes

In this chapter, we will deal with the properties and reactions of arenes, which are essentially aromatic hydrocarbons based on the benzene ring as a structural unit. The chemistry of arenes will be highlighted through the reactions of benzene and methylbenzene.

Benzene has a unique structure. Each of the 6 carbon atoms in benzene is sp² hybridised. There are 3 sp² hybrid orbitals and 1 unhybridized p orbital for each carbon atom. Benzene is hence said to have a resonance structure.

Three sp² hybrid orbitals

One unhybridized p orbital

Each C atom has 3 sp² hybrid orbitals that are arranged in a trigonal planar manner 2 sp² hybrid orbital are used to overlap head-on with the sp² hybrid orbitals of 2 adjacent C atoms to form 2 C-C σ bonds 1 sp² hybrid orbital is used to overlap head-on with the 1s orbital of the H atom to form the C-H σ bond The benzene molecule is entirely planar and all bond angles in the molecule is 120° (trigonal planar shape)

Each carbon atom also has an unhybridized p orbital occupied by a single electron Each singly-filled p orbital overalls sideways ( π bonding) with the adjacent p orbital on either side This continuous overlap of the p-orbitals results in a cloud of cyclic delocalised π electrons above and below the plane of the ring (hence resonance is present in benzene) The delocalised π electron cloud is spread throughout the benzene ring, minimising repulsion and stabilising the structure

Three sp² hybrid orbitals
Each C atom has 3 sp² hybrid orbitals that are arranged in a trigonal planar manner 2 sp² hybrid orbital are used to overlap head-on with the sp² hybrid orbitals of 2 adjacent C atoms to form 2 C-C σ bonds 1 sp² hybrid orbital is used to overlap head-on with the 1s orbital of the H atom to form the C-H σ bond The benzene molecule is entirely planar and all bond angles in the molecule is 120° (trigonal planar shape)
One unhybridized p orbital
Each carbon atom also has an unhybridized p orbital occupied by a single electron Each singly-filled p orbital overalls sideways ( π bonding) with the adjacent p orbital on either side This continuous overlap of the p-orbitals results in a cloud of cyclic delocalised π electrons above and below the plane of the ring (hence resonance is present in benzene) The delocalised π electron cloud is spread throughout the benzene ring, minimising repulsion and stabilising the structure

Note for students: Due to this resonance stabilisation from the delocalisation of the π electrons, benzene rings prefer to undergo substitution rather than addition, where the electron cloud is less nucleophilic compared to that in a C=C. Hence, benzene ring requires a stronger electrophile to react.

Arenes are generally unreactive due to the resonance stabilisation as a result of delocalisation of π electrons. Hence, the most common chemical reaction benzene undergoes is electrophilic substitution. This is because the delocalised π electrons in the benzene ring makes it “electron-rich” and attracts electrophiles easily. In electrophilic substitution, the H atom of the benzene ring is substituted by electrophiles.

Now, let’s apply the above concept to a commonly-tested exam question.

[2019 HCI Prelim H2 Chemistry Paper 2 Q4(a), (b)]

Aromatic compounds like benzene tend to undergo electrophilic substitution reactions.

(a) Explain why aromatic compounds are reactive towards electrophiles but not nucleophiles.

Solution:
The π cloud has high electron density/ is electron-rich that will attract electron-deficient electrophiles, but will repel electron-rich nucleophiles.

(b) Explain why aromatic compounds tend to undergo substitution reactions instead of addition reactions.

Solution:
Electrophilic addition destroys the delocalisation in the π electron cloud/disrupts
aromaticity which requires a significant amount of energy, whereas electrophilic
substitution retains aromaticity.

The various different types of electrophilic substitution reactions are as shown:

Reaction and conditions	Products
X₂ , anhydrous FeX₃/AlX₃ at room temperature (where X = Cl, Br, I)	Halogeno-benzene Eg. Chlorobenzene
Concentrated HNO₃, concentrated H₂SO₄, 55°C	Nitrobenzene
RX, anhydrous FeX₃/AlX₃ at room temperature (where X = Cl, Br, I and where R= any alkyl group)	Alkyl benzene Eg. Methyl benzene

Reaction and conditions
X₂ , anhydrous FeX₃/AlX₃ at room temperature (where X = Cl, Br, I)
Concentrated HNO₃, concentrated H₂SO₄, 55°C
RX, anhydrous FeX₃/AlX₃ at room temperature (where X = Cl, Br, I and where R= any alkyl group)

Products
Halogeno-benzene Eg. Chlorobenzene
Nitrobenzene
Alkyl benzene Eg. Methyl benzene

The electrophilic substitution mechanism is as shown:

Checklist for drawing electrophilic substitution mechanism

❏     Name of the mechanism (electrophilic substitution)

❏     Steps in sequential order (generation of electrophile, formation of carbocation intermediate, regeneration of catalyst)

❏     Curved arrows (from benzene ring to electrophile – point to the atom in electrophile to be directly bonded to benzene ring; from C-H bond of tetrahedral C on intermediate to restore aromatic ring)

❏     Labels (slow step, fast step)

❏     Identify correctly the carbocation intermediate and major product formed

Substituent Effects in Electrophilic Substitution

When a substituent is already present in the benzene ring, electrophilic substitution will be affected in two ways:

1. Position of the next substituent
2. Reactivity of the substitution

A benzene ring with a substituent attached can react during electrophilic substitution to give three products:

Isomers with substitution at the 6th site are the same as those substituted at the 2ⁿᵈ site.The three isomers are usually not formed in equal amounts. The ratio of the products formed is usually influenced by the substituent already present in the ring.

In essence, 2,4-directing substituent groups will direct the next substituent to the 2ⁿᵈ and 4ᵗʰ sites. Majority of the products will hence be 2 and 4-substituted products. The same can be said for 3-directing substituent groups.

Substituents which cause the benzene ring to be more reactive are known as activating groups while substituents which cause the benzene ring to be less reactive are known as deactivating groups.
Electron donating groups are usually activating groups, as they increase the electron density of the π benzene ring, thus enhancing the availability of electrons about the ring and vice versa for electron-withdrawing groups.

Generally, activating groups are 2,4-directing while deactivating are 3-directing. However, it is best to reference this table provided in the data booklet, as there are exceptions such as halogens. Halogens are deactivating but 2,4-directing.

The position of the incoming group, E, is determined by the nature of the group, G, already bonded to the ring , and not by the nature of the incoming group E.

Tip: When drawing the mechanism for electrophilic substitution of an already subsitutent ring, the right major products must be drawn otherwise you might be penalised.

Let’s look at a worked example below:

Ethylbenzene can react with bromine in two ways, depending on the conditions of the reactions.

In the boxes provided, draw the structure of the major product, assuming monobromination, and state the mechanism that ethylbenzene undergoes for each reaction.

For Reaction A, the reagents and conditions suggest that ethylbenzene will undergo Electrophilic Substitution (Friedel-Craft Alkylation), and since there is the presence of -CH₂CH₃ electron-donating alkyl group attached to the benzene ring, it is 2,4 directing. Thus, the -Br will attach to the benzene at either position 2 or 4 from the alkyl position.

As for Reaction B, the reagents and conditions suggest that ethylbenzene will undergo Free Radical Substitution (FRS), where the Br does not attach directly to the benzene ring, but replaces a H atom on the alkyl group -CH₂CH₃.

Sometimes reactions can take place at a lower temperature as well, due to the substituent in the ring being activating. An example learnt in this chapter is the electrophilic substitution of ethyl benzene.

Equation
Reagents and conditions	Concentrated HNO₃, concentrated H₂SO₄, 30℃
Observations	A pale yellow liquid with the smell of almonds is produced

Equation
Reagents and conditions
Concentrated HNO₃, concentrated H₂SO₄, 30℃
Observations
A pale yellow liquid with the smell of almonds is produced

Nitration of benzene would involve the same reagents, but at a higher temperature of typically 55℃. However, nitration of methylbenzene is more reactive due to the electron donating methyl group already present in the ring. It increases the electron density of the ring, thus making methylbenzene more susceptible to electrophilic attack.

Methylbenzene (as well as other alkyl benzenes) is able to undergo 2 other reactions, mainly free radical substitution of the side chain as well as side chain oxidation.

The -CH₃ substituent in methylbenzene (or any corresponding alkyl substituent) shows reaction expected of any alkyl group. Applying the concepts of what you have previously learned in the chapter of alkane, the mechanisms of free radical substitution will similarly apply to the side chain. Let’s look at Worked Example 3 to best illustrate this.

Describe the mechanism for the reaction between methylbenzene and chlorine in the presence of UV light

Solution:
Mechanism: Free Radical Substitution

Step 1 – Initiation

Step 2 – Propagation

The (a) and (b) are repeated continuously

Step 3 – Termination

The second reaction that alkyl benzenes can undergo is a side chain oxidation reaction. It is important to note that any benzene derivative with an alkyl side chain (irregardless of how long the alkyl side chain is) will produce the same product, benzoic acid.
– REAGENTS AND CONDITIONS: KMnO₄ (aq), H₂SO₄ (aq), heat
– Side chain is destroyed, forming carbon dioxide and water

Reminder to students: Alkyl benzenes WITHOUT a benzylic H (no H atom bonded to the C atom that is directly bonded to the benzene ring) do not undergo oxidation when heated with acidified KMnO₄. This can be illustrated with the following molecule.