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Bryanna Tanase John Torres Geoffery Gray Multistep Synthesis of p-bromoanaline
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Introduction Bromination is a type of electrophilic aromatic substitution in which a hydrogen atom on an aromatic ring is replaced by a bromine atom 2 . The first step in bromination is the formation of the electrophile 2 . The electrophile is created by reacting bromine with iron tribromide (Lewis acid catalyst) to form a complex that makes bromine more reactive 2 . A pair of pi electrons on the aromatic ring then attack the electrophile, displacing iron tetrabromide 2 . This step causes aromaticity to be destroyed and yields a cyclohexadienyl cation intermediate 2 . Finally, Iron tetrabromide left in solution takes off the hydrogen on the sp 3 carbon which holds the bromine, restoring aromaticity to the ring and regenerates hydrobromic acid and iron tribromide 2 . An example of the mechanism for this reaction is shown below in Figure 1, in which the aromatic compound used is benzene. Figure 1: Bromination of benzene 2
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Sometimes if the ring being used in electrophilic aromatic substitution contains a strong activating group, such as an amino group and hydroxyl group, a protecting or blocking group must be used 3 . This is because strong activating groups cause an aromatic ring to become extremely reactive, therefore increasing the probability for side products to be formed 3 . The protecting group helps make the group less reactive and thus prevents formation of unwanted products, allowing for greater control of the reaction 3 . An example of an instance in which a protecting group is needed would be when an amide group is converted into an amine 3 . Amine groups have a high reactivity and could form side products at several points in a reaction , such as in the preparation of sulfanilamide 3 . In the reaction sequence, acetanilide is used as the staring material rather than aniline because aniline is too reactive 3 . In the formation of the electrophile for the reaction, an amine such as the one in aniline could react with ammonia in the same way as a sulfonyl chloride group because the two are both nucleophiles and have a similar structure 3 . At the reaction’s start, the amine group would become protonated by the strong acid and become a meta directing group, rather than para directing group as desired, forming a different product 3 . Amino groups can be protected by acylation with either acetic anhydride or acid chloride and protected by hydrolysis in hydrochloric acid and addition of sodium hydroxide 1 . The objective of this experiment is to perform a multistep synthesis of aniline to form p- bromoanaline 1 . The mechanism of the reaction is shown below in Figure 2.
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Figure 2: Multistep synthesis of p-bromoanaline
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In the first step of the synthesis, aniline is protonated by stealing hydrogen from hydrochloric acid to from a protonated aniline. The pronated aniline then reacts intramolecularly with sodium acetate as a solvent to form generated aniline, which then reacts with acetic anhydride (protection) to form N-phenylacetamide. Once aniline is protected, the electrophile is formed for
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