Benzene and Aromatic Substitution: Quick Revision

A brief note on Benzene and Aromatic Substitution. Read it before a test. Try each question at the end before you open its answer.

Structure

• Benzene, $\text{C}_{6}\text{H}_{6}$: flat, six $\text{sp}^2$ carbons, six equal C-C bonds, a delocalised cloud of six pi electrons.
• Bond lengths: single $154\ \text{pm}$, double $134\ \text{pm}$, benzene $139\ \text{pm}$. Resonance energy about $150\ \text{kJ/mol}$.
• Aromatic: cyclic, planar, fully conjugated, $4n + 2$ pi electrons. Cyclobutadiene ($4n$) is antiaromatic.
• Disubstituted benzenes: 3 isomers (ortho, meta, para). The formula $\text{C}_{8}\text{H}_{10}$ has 4 aromatic isomers.
• Made from ethyne (red-hot iron tube), from sodium benzoate with soda lime, and from phenol with zinc.

Electrophilic substitution

• Two steps: the electrophile attacks (slow), giving an arenium ion. A proton leaves (fast), and the ring is aromatic again.
• Nitration: conc. $\text{HNO}_{3}$ and conc. $\text{H}_{2}\text{SO}_{4}$, electrophile $\text{NO}_{2}^{+}$.
• Halogenation: $\text{X}_{2}$ with $\text{FeX}_{3}$ or $\text{AlCl}_{3}$. Sulphonation: fuming $\text{H}_{2}\text{SO}_{4}$, $\text{SO}_{3}$, reversible.
• Friedel-Crafts: $\text{RCl}$ or $\text{RCOCl}$ with anhydrous $\text{AlCl}_{3}$. Alkylation can rearrange and can go on to more than one group. Acylation cannot.

Directing effects

• Activating, ortho-para: $\text{OH}$, $\text{NH}_{2}$, $\text{OR}$, alkyl groups. Deactivating, ortho-para: the halogens.
• Deactivating, meta: $\text{NO}_{2}$, $\text{CN}$, $\text{CHO}$, $\text{COR}$, $\text{COOH}$, $\text{SO}_{3}\text{H}$.
• The order of the steps in a synthesis decides the isomer.

Other reactions

• Side chain: halogen with light. Ring: halogen with a Lewis acid in the dark. Hot permanganate turns an alkyl group into $\text{COOH}$.
• Addition: $\text{H}_{2}$ with nickel gives cyclohexane. $\text{Cl}_{2}$ in sunlight gives $\text{C}_{6}\text{H}_{6}\text{Cl}_{6}$.
• Benzene and benzo[a]pyrene are carcinogens.

A worked example

Make m-bromonitrobenzene from benzene.

1. Choose the first group. The nitro group directs meta, so it goes on first: $\text{C}_{6}\text{H}_{6}\ +\ \text{HNO}_{3}\ \longrightarrow \ \text{C}_{6}\text{H}_{5}\text{NO}_{2}\ +\ \text{H}_{2}\text{O}$.
2. Add the second group. Bromine with iron(III) bromide goes meta to the nitro group.
3. Check the other order. Bromine first would direct the nitro group ortho and para. The meta isomer would be a small part of the mixture.

Practice with new numbers

A program to try

The program lists groups with the way they activate the ring and direct the next group.

Mistakes to avoid

• Benzene as an alkene. It substitutes. It keeps its ring.
• No Lewis acid for halogenation. Bromine alone does not react.
• Rearranged alkylation. 1-Chloropropane gives isopropylbenzene. Use acylation, then reduction.
• Halogens as meta directors. They direct ortho and para.
• Side chain with a Lewis acid. The side chain needs light.

Check yourself

1. How many pi electrons does benzene have?

2. What is the electrophile in nitration?

3. Which group directs the next group to the meta position: $\text{OH}$ or $\text{NO}_{2}$?

4. What does toluene give with chlorine in ultraviolet light?

5. Why does benzene add hydrogen only under severe conditions?

Sources

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons".
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons".