Understanding Electrophilic Substitution Reactions of Benzene

1. Thursday, 23 November 2017 Electrophilic substitution reactions of benzene

3. Benzene reactions Benzene and it’s derivatives undergo substitution reactions where a hydrogen on the ring is substituted out for an electrophile group. Define electrophile Define substitution

4. Nitration: Benzene reacts with a mixture of the reagent, concentrated nitric acid, and the catalyst, concentrated sulphuric acid – this is called a nitrating mixture. The product is nitrobenzene, providing the temperature is kept below 55°C.

5. The reaction is heated to 50°C and kept in a water bath to maintain a steady temperature If the temperature goes above 55°C multiple substitution occurs and 1,3-dinitrobenzene forms. Nitration is an importantstarting material in the preparation of dyes, pesticides and pharmaceuticals, including paracetamol.

6. Add a description for each step

7. Halogenation of benzene • Benzene does not react with bromine unless there is a catalyst of iron (III) bromide or iron filings (this catalyst is called a halogen carrier) • The halogen carrier can be generated in the reaction vessel by the reaction between the metal and the halogen

8. Halogenation of benzene • Benzene is too stable to react with the non-polar bromine molecule so a bromonium ion must be generated using the halogen carrier catalyst.

9. Halogenation of benzene Write the three steps required (including mechanism) for the electrophilic substitution of chlorine on to a benzene ring using aluminium trichloride as the halogen carrier

10. Alkylation: substituting an alkyl group in a benzene ring. • Reflux benzene with a halogenoalkanes in the presence of an anhydrous halogen carrier catalyst. (e.g. anhydrous aluminium chloride) under dry conditions. This is sometimes known as a Friedel-Crafts reaction

11. Could you predict the three steps for this reaction?

12. Acylation: • Similar to alkylation but benzene is refluxed with an acyl chloride – the product is a ketone. • The reaction forms carbon-carbon bonds and is, again, like alkylation, useful in organic synthesis

13. Alkenes vs. Arenes Alkenes can decolourise bromine by an electrophilic reaction. Draw out the reaction between cyclohexene and bromine. Draw the mechanism for this reaction

14. In this reaction bromine adds across the double bond. • The π-bond in the alkene contains localised electrons above and below the plane of two carbon atoms producing an area of high electron density • The localised electrons produce a dipole in the non-polar bromine molecule • The slightly positive bromine atom allows the bromine molecule to act like an electrophile

15. Why don’t electrophilic addition reactions happen with benzene? • The delocalised electrons means there is insufficient π-electron density around any two carbon atoms. • What is the effect of this on the bromine molecule? • What is the effect of this on the overall reaction?