Advanced Projects in Chemistry - Electrophilic Substitution Reactions

Study of Bromination Reactions

The theoretical study of electrophilic substitution reactions is best understood with the actual execution of the reactions in the laboratory. A comparative study of the product yield under the available conditions gives a better understanding of the spontaneity as well as feasibility of the reaction. At the K-11-12 level, the instruction of electrophilic substitution reactions is superficial, until it is impressed upon by lab work. A student studies that an electrophile is an electron loving substituent, which is attracted to electrons and participates in a chemical reaction by accepting an electron pair. In aromatic substitution reactions, the benzene ring acts as a source of electrons. For an electrophilic substitution reaction the hydrogen atoms on the benzene ring are replaced by the attacking reagent which is deficient in electrons. Bromination is an electrophilic substitution reaction with Br+ as the electrophile. Benzene, C6H6, the substrate is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached. There are delocalised electrons above and below the plane of the ring. The presence of the delocalised electrons makes benzene particularly stable. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.

Two effects work in electrophilic substitution reactions. One is induction, which is an effect that occurs through the sigma- bond system. Electron withdrawing groups will "pull" electron density away from the ring making the electrons of the ring less available for attack by an electrophile. Electron donating groups do the opposite. Induction also plays a role in some of the directional effects. The other effect is resonance, which occurs through the pi system of bonds in the molecule. Resonance plays an important role in the stability of intermediates of the reactions.

Sometimes these two effects are opposite to one another. One effect may be stronger and exert greater influence. The ease with which a compound suddenly becomes receptive to bromination due to presence of electron donating groups like -OH in phenol and -NH2 in aniline is studied. Phenols are potentially very reactive towards electrophilic aromatic substitution. In aqueous solutions, phenol undergoes ionization to give the phenoxide ion. The negative charge on the oxygen of the phenoxide ion donates electrons into the benzene ring to a large extent. The strong activation often means that milder reaction conditions than those used for benzene and also a trisubstituted reaction product. The reaction is carried out in the laboratory without a strong Lewis acid like Friedel Crafts catalyst. If bromine water is added to a solution of phenol in water, the bromine water is decolorized and a white precipitate is formed which smells of antiseptic. The precipitate is 2,4,6-tribromophenol.
Ar-OH+3Br2---> Ar-OH Br3 +3HBr
The NH2 group in aniline strongly activates the aromatic ring through the delocalization of the lone pair of electrons of the N-atom over the entire aromatic ring. Aromatic amines undergo the reaction readily and it is impossible to stop the reaction at the monosubstitution stage. Aniline too on bromination gives a tribromosubstituted derivative.
Ar –NH2+ 3Br2--> 3HBr+ Ar-NH2-Br3

Cinnamic acid is an alkenoic acid. It undergoes an electrophilic addition reaction. The yield of the product is fairly high as the reaction mechanism does not involve large change in bond energies. The double bond present in cinnamic acid consists of one sigma and one pi-bond. Pi- electrons form an electron cloud which lies above and below the plane of the sigma bonded carbon atoms. The pi electron cloud is thus more exposed and less tightly held by the two carbon atoms. The pi electrons attract electrophiles and undergoes electrophilic addition reactions. Bromine molecule is non-polar but when it comes in the vicinity of a double bond, the pi electrons of the double bond begin to repel the bromine molecule. The bromine molecule gets polarized and the positive end of the bromine molecule is attracted towards the pi-electrons forming a carbocation. This is the slow and rate determining step of the reaction.

The carbocation is higly reactive and undergoes a nucleophilic attack by the bromide ion giving a dibromo addition product across the double bond. One gm of cinnamic acid was placed in a conical flask and dissolved in10mL of hot water. The hot solution was added to 10 mL of a strong solution of 20% bromine. The derivative was filtered and purified in hot alcohol. The melting point of the di- bromo derivative of cinnamic acid was determined to be 195oC
Ar –CH=CHCOOH+3Br2 Ar-CHBr-CHBr-COOH

Acknowledgments - The bromo derivatives were patiently prepared by Manasvi Lalvani, Shreya Krishnan and Sanjana Siddhra, Grade 12, of R.N. Podar Sr. Secondary CBSE High School, Santacruz (W).