The experiment focuses on finding out what kind of activating effects that four different substituents will have on an aromatic benzene ring. The substituents being tested are aniline, anisole, acetamide (acetanilide), and phenol. All four of these groups are either para or ortho activating. Bromination is the reaction that will be carried out. The melting point ranges of the final products will be taken in order to determine their identities and reactivity. It is predicted that substitution order from most to least reactive should be aniline, phenol, anisole, and acetamide. Theory:
Regioselectivity and the rate of electrophilic aromatic substitution are affected by the substituents attached to the original benzene. In electrophilic aromatic substitution, (EAS for short), the rate determining step is the first step of the reaction. This experiment deals with activating substituents that increase the rate of reaction. In the first rate determining step, the electron density rich pi bonds of benzene react with the electrophile (Bromine) to form a resonance stabilized carbocation. This step is the most important factor that decides which substituents make benzene react faster. This is because the rate of the reaction can be determined through the stability of the carbocation transition state hybrid, which is also known as resonance effect and the Hammond Postulate. In other words, if a carbocation intermediate is more stable, there is less energy needed in the transition state to form the carbocation. Less energy needed translates to a faster reaction.
All in all, substituents that increase the electron density on the ring contribute by making the benzene ring more nucleophilic through increased electron density. The increased electron density around the ring would help to stabilize the positively charged carbocation intermediate. This means that this intermediate is more likely to form. Electron donating substituents would activate the ring towards EAS, which means the overall rate would be faster compared to benzene. The aniline group forms the most stable carbocation because it has the same resonance effects as the other groups but because nitrogen is less electronegative than oxygen, its inductive effects are weaker. Therefore aniline should react the fastest.
Acetamide would be last in terms of reactivity because it has the weakest resonance effects compared to inductive effects, in other words it is the weakest electron donating group. The order of reactivity from strongest activator to least should be aniline, phenol, anisole, and acetamide. It is also predicted that all of the groups should react towards a polyhalogenated product except for acetamide. Specifically, they should all be tri-substituted except for acetamide. Steric hindrance also plays a factor in preventing Br from being added to the ortho positions, so it should be expected that the acetamide product should only have a substituent added to the para position.
Melting Point Range (⁰C)
40-43 and 55.3-73.7
Anisole product did not recrystallize so data is taken from another lab’s.
Crude mass product of acetamide: 0.235g
Recrystallized product mass of acetamide: 0.087g
Theoretical yield: 0.0625g
Percent yield: 139%
Aniline and phenol both formed a tri-substituted product in two ortho and one para positions according to their respective melting point ranges. This confirms expectations that these two substituents are the strongest activators. This is because the NH2 and OH groups are very electronegative and reactive which allows their carbocation resonance structures to be stabilized. Even though both aniline and phenol had the same amount of polybromination, we know that since nitrogen is less electronegative than oxygen, aniline has less inductive effects. In turn, it means that aniline would have more electron density which make makes it more reactive to electrophiles. Although the anisole did not form a product, retrieved data suggests that the product is is actually disubstituted this contradicts the prediction that it would be trisubstituted also it is not surprising because anisole has an extra carbon attached to the oxygen which could weaken resonance effects.
Lastly, the melting point ranges for the acetamide product suggest that it formed 4-bromoacetamide. Based on the data gathered, the ranking in order of increasing activity would be aniline, phenol, anisole, and acetamide. This matches up with initial predictions. These results make sense because anisole and acetamide have resonance structures where the electron density is moved outside of the ring so it cannot activate the benzene ring as well as the other two. With regards to the efficiency of the reaction, three of the reactions appeared to be efficient. Aniline, phenol, and acetamide all reacted efficiently. This is observed through their products which displayed conclusive melting point ranges that confirmed their predictions. Although 10% bromine solution was used these reactions carried to completion and their yields were decent. Conclusion:
Aniline and Phenol both yield trisubstituted products of 2,4,6-bromoaniline and 2,4,6-bromophenol according to the melting point ranges obtained. This supports predictions that these amine and hydroxyl groups would be the strongest activating groups of benzene. Anisole yielded a disubstituted product which suggests that it is a more moderate activator when compared to aniline and phenol. Acetamide, which has the bulkiest substituent, yield a monosubstituted product which suggests that it is the weakest activator of the four. These results match up with ranking predictions but differ with substituent predictions. It was predicted that anisole’s activating strength would be on par with that of phenol and aniline when results indicate that it is actually considerably weaker. Sources of Error:
After recrystallzation and weighing out the final product identified as 4-bromoacetamide. It was observed that the actual yield was higher than the theoretical yield. This could have been due to impurities in our final product.