Electrophilic Substitution of Benzene

Benzene electrophilic substitution reactions

You might already be familiar with the organic molecule benzene (C₆H₆). It is an example of an aromatic compound: a molecule with a ring of delocalised pi electrons. In particular, benzene has six delocalised electrons, but other aromatic compounds may have more or fewer. Aromatic molecules are also known as arenes.

Benzene frequently takes part in electrophilic substitution reactions, which we defined above. In these reactions, benzene is attacked by an electrophile. Consequently, one of benzene's hydrogen atoms is replaced by a different functional group such as a halogen atom, hydroxyl group, or nitrate group.

Typical electrophilic substitution reactions of benzene include:

• Nitration.
• Halogenation (such as chlorination or bromination).
• Friedel-Crafts acylation.
• Friedel-Crafts alkylation.

Each of these reactions has three steps:

1. The electrophile is generated.

2. The electrophile reacts with benzene using the general benzene electrophilic substitution mechanism.

3. The catalyst is regenerated.

But before we delve any further into the world of electrophilic substitution reactions of benzene, let's think about why benzene takes part in this type of reaction.

Why does benzene undergo substitution reactions?

As we mentioned earlier, benzene is an aromatic molecule. All aromatic molecules are characterised by their ring of delocalised pi electrons. The ring of delocalisation is an area of high electron density - after all, it contains six free-moving delocalised electrons! This ring is particularly attractive to electrophiles, which are electron-pair acceptors, and so benzene commonly finds itself under electrophilic attack.

However, the aromatic ring is relatively strong and stable because it spreads the charge of the six delocalised electrons over a greater area. It takes a lot of energy to disrupt the delocalisation. Therefore, benzene electrophilic attack doesn't usually result in reactions that involve breaking the ring by forming additional bonds with the delocalised electrons - which is exactly the case in addition reactions. Instead, attack from electrophiles often results in substitution. Substitution keeps the delocalised electron ring intact and so it is a lot more favourably energetic for benzene than addition. This is why benzene prefers to take part in electrophilic substitution reactions.

Electrophilic substitution of benzene equation

We now know what an electrophilic substitution reaction is. Let's learn the general equation for an electrophilic substitution reaction of benzene.

$$C_{6}H_{6} + El^{+}\rightarrow C_{6}H_{5}El + H^{+}$$

In electrophilic substitution reactions, benzene (C₆H₆) reacts with an electrophile (El⁺) to produce a benzene derivative (C₆H₅El) and a hydrogen ion (H⁺). The benzene derivative is much like benzene except one of its hydrogen atoms is replaced by the electrophile.

Electrophilic substitution of benzene mechanism

It's now time to look at the general mechanism of an electrophilic substitution reaction of benzene.

Here, the electrophile is represented by El⁺.

Fig. 1: The general mechanism of benzene electrophilic substitution.StudySmarter Originals

1. The electrophile is attracted to benzene’s aromatic ring of delocalisation. The electrophile forms a bond with one of benzene’s carbon atoms, using two electrons from the aromatic ring. This partially destroys the ring of delocalisation and leaves benzene with a positive charge.

2. Benzene would be a lot more stable if it repaired its disrupted electron ring by replacing the two lost delocalised electrons. To do this, one of its C-H bonds breaks heterolytically. Both electrons go to the ring of delocalisation, restoring it back to six electrons, and the hydrogen atom is lost as a hydrogen ion (H⁺).

3. The final organic product is a benzene derivative with the electrophile in place of one of its hydrogen atoms. We also end up with a hydrogen ion.

We'll now practice applying this mechanism to the four benzene electrophilic substitution reactions we mentioned earlier: nitration, halogenation, Friedel-Crafts acylation, and Friedel-Crafts alkylation.

Electrophilic substitution of benzene examples

We covered examples of electrophilic substitution reactions of benzene in the article Reactions of Benzene. In that article, you learned about the reactants and conditions for various different benzene electrophilic substitution reactions. We'll visit these examples again, but this time we'll also focus on the reaction mechanism. First up: nitration.

Nitration of benzene

We nitrate benzene by heating it under reflux at 50 °C with a mixture of concentrated sulphuric and nitric acids (H₂SO₄ and HNO₃). The sulphuric acid acts as a catalyst. It is used to generate our electrophile, which is the nitronium ion (NO₂⁺), but is reformed at the end of the reaction. Here's the equation for the formation of NO₂⁺:

$$H_{2}SO_{2} + HNO_{3}\rightarrow NO_{2}^{+} + HSO_{4}^{+} + H_{2}O $$

The nitronium ion electrophile NO₂⁺ then reacts with benzene in an electrophilic substitution reaction using the general mechanism that we learned above. The reaction swaps one of benzene's hydrogen atoms for the nitrate group (-NO₂) and releases a hydrogen ion (H⁺). The hydrogen ion then reacts with the HSO₄^- produced when we generated our electrophile to reform the sulphuric acid catalyst.

$$H^{+} + HSO_{4}^{-} \rightarrow H_{2}SO_{4}$$

Overall, we produce nitrobenzene (C₆H₅NO₂) and water. The reaction has the following equation:

$$C_{6}H_{6}+ HNO_{3} \rightarrow C_{6}H_{5}NO_{2} + H_{2}O$$

Now it's your turn - practice applying the general electrophilic substitution mechanism to the nitration of benzene. Our worked example below will help you if you get stuck.

Halogenation of benzene

Benzene is halogenated using a halogen (X₂) and its corresponding aluminium halide (AlX₃). For example, chlorination uses chlorine gas (Cl₂) and aluminium chloride (AlCl₃), whilst bromination uses bromine water (Br₂) and aluminium bromide (AlBr₃). The aluminium halides act as catalysts. Alternatively, you can catalyse the reactions using iron(III) halides (FeCl₃ or FeBr₃ respectively). Benzene halogenation is another type of electrophilic substitution reaction and uses the same general mechanism as nitration. All the benzene halogenation reactions take place at room temperature.

Our electrophile is a positive halogen cation. It is generated by reacting the aluminium halide with the halogen. This also forms a negative aluminium tetrahalide ion (AlX₄^-). Here's the equation:

$$X_{2} + AlX_{3}\rightarrow X^{+} + AlX_{4}^{-}$$

The halogen cation then reacts with benzene using the general mechanism we learned earlier. This forms a halogenoarene (C₆H₅X) and a hydrogen ion (H⁺). The hydrogen ion reacts with the negative aluminium tetrahalide ion we produced when generating the electrophile to produce a hydrogen halide (HX) and reform the catalyst (AlX₃):

$$H^{+} + AlX_{4}^{-} \rightarrow AlX_{3} + HX$$

Overall, we form a halogenoarene (C₆H₅X) and a hydrogen halide (HX) using the following equation:

$$C_{6}H_{6} + X_{2} \rightarrow C_{6}H_{5}X + HX$$

Friedel-Crafts acylation of benzene

One method of acylating benzene involves reacting benzene with a mixture of an acyl chloride (RCOCl) and aluminium chloride (AlCl₃) in a further example of a benzene electrophilic substitution reaction. The reaction takes place in anhydrous conditions under reflux. Here, aluminium chloride acts as a catalyst and is used to generate the electrophile, which in this reaction is the positive RCO⁺ cation. Notice that the other product of the electrophile generation is once again a negative aluminium tetrachloride ion (AlCl₄^-).

$$RCOCl + AlCl_{3} \rightarrow RCO^{+} + AlCl_{4} ^{-}$$

The RCO⁺ electrophile reacts with benzene using the same mechanism as all of our other examples. Again, it releases a hydrogen ion. The hydrogen ion reacts with the negative aluminium tetrachloride ion (AlCl₄^-) to produce hydrogen chloride (HCl) and regenerate the catalyst (AlCl₃):

$$H^{+} + AlCl_{4}^{-} \rightarrow AlCl_{3} + HCl$$

Overall, the reaction produces a ketone with a benzene ring attached (C₆H₅COR - this is also known as an aromatic ketone) and hydrogen chloride (HCl). We use the prefix phenyl- to name the organic ketone.

$$C_{6}H_{6} + RCOCl \rightarrow C_{6}H_{5}COR + HCl $$

Friedel-Crafts alkylation of benzene

The last example of electrophilic substitution of benzene that we'll look at today is Friedel-Crafts alkylation. In this reaction, we swap one of benzene's hydrogen atoms for an alkyl group using a mixture of a chloroalkane (RCl) and aluminium chloride (AlCl₃). Aluminium chloride is once again a catalyst and helps generate the electrophile, which is a positive R⁺ carbocation:

RCl + AlCl₃ → R⁺ + AlCl₄^-

The R⁺ carbocation then reacts with benzene using the same general electrophilic substitution mechanism as before to produce an alkylarene (C₆H₅R) and a hydrogen ion (H⁺). The hydrogen ion reacts with the aluminum tetrachloride (AlCl₄^-) produced when generating the electrophile to form hydrogen chloride (HCl) and revive our catalyst (AlCl₃). Here's the equation for the regeneration of the catalyst:

$$H^{+} + AlCl_{4}^{-} \rightarrow AlCl_{3} + HCl $$

Overall, we produce an alkylarene (C₆H₅R) and hydrogen chloride (HCl).

$$C_{6}H_{6} + RCl \rightarrow C_{6}H_{5}R + HCl$$

Have a go at drawing the mechanism for this next example.

Directing effects of the electrophilic substitution of benzene

In all of the examples we've investigated so far, we've only made one substitution into the aromatic benzene ring. However, if you provide an excess of the electrophile, you can actually repeat the reaction over and over again, substituting many of benzene's hydrogen atoms for other functional groups.

The first substitution of benzene is always random. After all, benzene's carbon and hydrogen atoms are identical - the chances of the electrophile attacking any one of its carbon atoms are equal. We call the carbon atom with the newly-substituted functional group carbon 1.

But subsequent substitutions aren't random. This is because some functional groups attached to the benzene ring have specific directing effects. They encourage the next electrophile to attack a certain carbon atom. Here are their effects.

• Electron-donating groups encourage the next electrophile to attack carbon atoms 2, 4, and 6in the benzene ring. These groups include:
  • Alkyl groups (-R).
  • The hydroxyl group (-OH).
  • The amine group (-NH₂).
  • The ketone carbonyl group (-COR).
• Electron-withdrawing groups encourage the next electrophile to attack carbon atoms 3 and 5in the benzene ring. These groups include:
  • The nitrate group (-NO₂).
  • The carboxyl group (-COOH).

Now have a go at applying your knowledge about the directing effect to some practice questions.

Now try this second example. It draws on the information we learned earlier about substitution involving halogen atoms.