Chapter 17: Reactions of Aromatic Compounds

Transcription

1 1 Chapter 17: Reactions of Aromatic Compounds I. Introduction to Electrophilic Aromatic Substitution (EAS) A. General Mechanism II. Reactions of Electrophilic Aromatic Substitution A. Halogenation (E = Cl or Br ) Overall Reaction: 1. Bromination: a. Comparison with Alkenes

2 2 An analogous addition reaction between benzene and bromine: General Problem: Solution: o Bromine itself is not electrophilic enough to react with benzene. o The addition of a strong Lewis acid (electron pair acceptor), such as FeBr 3, catalyses the reaction, and leads to the substitution product. b. Mechanism of Electrophile Formation:

3 3 c. Energy Profile: 2. Chlorination of Benzene The chlorination proceeds analogously to the bromination except this time the Lewis acid catalyst used is AlCl 3 (can also use FeCl 3 ). Observed Reaction: Mechanism:

4 4 3. Iodination of Benzene The iodination procedure requires an acidic oxidizing agent, such as nitric acid. The nitric acid is a strong oxidizer (i.e. removes electrons, converts iodine into I ), this makes the iodine a much stronger electrophile. The nitric acid is consumed in the reaction, it is therefore a reagent, not a catalyst. B. Nitration (E = NO 2 ) Observed Reaction: Mechanism:

5 5 C. Sulfonation (E = HSO 3 ) Benzene will react with sulfur trioxide in the presence of an acid (7% SO 3 in H 2 SO 4 = fuming sulfurinc acid), arylsulfonic acids are produced. Observed reaction: Mechanism: Reverse reaction is also an observed reaction (Desulfonation) Reaction is an equilibrium that can be controlled by using Le Chatelier s principle:

6 6 D. Nitration of Toluene Toluene reacts 25 times faster than benzene. The methyl group is an activating group.

7 7 III. Substituent Effects on Electrophilic Aromatic Substitution A. Classification of Substituents on Aromatic Rings There are two basic types of substituents on aromatic rings: 1. Electron donating groups (EDG) increase the electron density of neighboring atoms. 2. Electron-withdrawing groups (EWG, or acceptors) decrease the electron density of neighboring atoms.

8 8 B. Substituents Can Have Different Effects on the Reactivity: 1) Inductive Effects: donation or withdrawing of electrons through bonds. inductive electron-withdrawing groups inductive electron-donating groups 2) Resonance Effects: donation or withdrawing of electrons through resonance and the formation of bonds. Examples:

9 9 IV. Activating Ortho/Para Directing Substituents A. Alkyl Substituents The alkyl group is an activating group, and is ortho and para directing. B. Substituents with Nonbonding electrons (lone pairs). 1. The methoxy group: Methoxybenzene (anisole) undergoes nitration around 10,000 times faster than benzene, and about 400 times faster than toluene. The lone pair on the substituent can be used to stabilize adjacent positive charges through resonance.

10 10 Example: Let s look at the sigma complexes ortho attack OCH 3 H NO 2 OCH 3 OCH 3 H H NO 2 NO 2 OCH 3 H NO 2 meta attack OCH 3 OCH 3 OCH 3 H NO 2 H NO 2 H NO2 para attack OCH 3 OCH 3 OCH 3 OCH 3 H NO 2 H NO 2 H NO 2 H NO 2 The methoxyl group is so activating that anisole will react with bromine itself, and if excess bromine is used, the tribromide is readily generated.

11 11 2. The Amino Group In a similar fashion (to methyoxy groups), the lone pair of electrons on the nitrogen in an amino group causes the -NH 2 substituent to be a powerful activating group with strong ortho and para directing effects. Aniline and anisole will react with bromine without a catalyst to generate tribromoaniline. Again it is the non bonding electrons that provide resonance stabilization of the sigma complex when the attack is ortho and para. Summary:

12 12 V. Deactivating, Meta Directing Substituents Typically, the atom attached to the aromatic ring will have a partial positive charge or a full positive charge. Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. Nitrobenzene is about 100,000 times less reactive than benzene towards EAS.

13 13 Other Deactivators Aldehydes and Ketones Summary of Deactivators:

14 14 VI. Exceptions to the Rules, Halogens Features: 1) The halogens are very electronegative. They can powerfully withdraw electron density from the ring inductively through the sigma bond (therefore deactivating). 2) The halogens have lone pairs of electrons that can donate electron density (resonance donation) through bonding (therefore ortho and para directors). Resonance of the sigma complex: Reaction at the meta position does not allow for the positive charge to be placed adjacent to the halogen, and therefore does not result in any stabilization Halogens are deactivating because of the inductive withdrawal of electron density from the ring, yet are ortho para directors since they can use resonance donation to stabilize adjacent carbocations.

15 15 Summary of Activators and Directors VII. Effects of More than One Substituent Two or more substituents produce a combined effect on the reactivity of an aromatic ring. These effects either compliment each other or conflict with each other. 1. Compliments A. Meta-xylene.

16 16 B. p-nitrotoluene. 2. Conflicts When there is a conflict between an activating group and a deactivating group, usually the stronger activating group dominates the orientation of substitution. Substituents can be divided into three groups, differing in the strength of their directing abilities. (From strongest to weakest). 1) Powerful o/p directing groups with lone pairs (resonance stabilizers) 2) Moderate o/p directors such as alkyl groups and halogens 3) Meta directors

17 17 A. It is more complicated if the directing effects conflict with each other. Often in these cases, mixtures of products are produced. Example. o-xylene.