Preparation of 4-Bromoaniline
Preparation of 4-bromoaniline
Introduction
Aromatic compounds tend to undergo electrophilic aromatic substitutions rather than addition reactions. Substitution of a new group for a hydrogen atom takes place via a resonance-stabilized carbocation. As the benzene ring is quite electron-rich, it almost always behaves as a nucleophile in a reaction which means the substitution on benzene occurs by the addition of an electrophile. Substituted benzenes tend to react at predictable positions. Alkyl groups and other electron-donating substituents enhance substitution and direct it toward the ortho and para positions. Electron-withdrawing substituents slow the substitution and direct it toward the meta positions.
Aromatic compounds also undergo reactions in their side-chains, often at the benzylic position next to the aromatic ring. Reactions at benzylic positions are often promoted by resonance stabilization of the intermediate and/or transition state with the aromatic ring.
Aromatic amides are formed via electrophilic substitution. The NH2 group is electron-donating and therefore the substituted ring is considered “activated” and will often react even without a catalyst. The nitrogen based activating group increases reactivity by a resonance effect: The resonance stabilisation from an amide group in an aromatic compound is mainly due to the delocalisation of the nitrogen lone pair onto the amide oxygen (which makes the lone pair less available to the aromatic ring) and the increased steric bulk of the group, favouring substitution at the 4-position over the 2-positions which it shields.
Aniline is the simplest aromatic amine and is synthesised by first nitrating benzene using a concentrated mixture of nitric acid and sulphuric acid to give nitrobenzene which is then hydrogenated in the presence of a nickel catalyst to give the final product. Aniline undergoes very readily electrophilic substitution reactions as the aromatic ring of aniline is very electron
Introduction
Aromatic compounds tend to undergo electrophilic aromatic substitutions rather than addition reactions. Substitution of a new group for a hydrogen atom takes place via a resonance-stabilized carbocation. As the benzene ring is quite electron-rich, it almost always behaves as a nucleophile in a reaction which means the substitution on benzene occurs by the addition of an electrophile. Substituted benzenes tend to react at predictable positions. Alkyl groups and other electron-donating substituents enhance substitution and direct it toward the ortho and para positions. Electron-withdrawing substituents slow the substitution and direct it toward the meta positions.
Aromatic compounds also undergo reactions in their side-chains, often at the benzylic position next to the aromatic ring. Reactions at benzylic positions are often promoted by resonance stabilization of the intermediate and/or transition state with the aromatic ring.
Aromatic amides are formed via electrophilic substitution. The NH2 group is electron-donating and therefore the substituted ring is considered “activated” and will often react even without a catalyst. The nitrogen based activating group increases reactivity by a resonance effect: The resonance stabilisation from an amide group in an aromatic compound is mainly due to the delocalisation of the nitrogen lone pair onto the amide oxygen (which makes the lone pair less available to the aromatic ring) and the increased steric bulk of the group, favouring substitution at the 4-position over the 2-positions which it shields.
Aniline is the simplest aromatic amine and is synthesised by first nitrating benzene using a concentrated mixture of nitric acid and sulphuric acid to give nitrobenzene which is then hydrogenated in the presence of a nickel catalyst to give the final product. Aniline undergoes very readily electrophilic substitution reactions as the aromatic ring of aniline is very electron
References: 1. Harwood, L.M.; Moody, C.J.; Percy, J.M.; “Experimental organic chemistry: standard and microscale”, Blackwell Science 1999, Edition 2; p439-443. 2. http://www.chemicalbook.com/ChemicalProductProperty_EN_CB9760655.htm 3. http://www.chemexper.com/chemicals/supplier/cas/103-88-8.html 4. http://www.uni-r.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/chembox_brom_arom-e.htm 5. http://edivini.pbworks.com/f/bromination_acetanilide_JCE1996p0267.pdf