Nitration of Phenol Discussion and Conclusion - The goal of...

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The goal of this experiment was to conduct a nitration using phenol and dilute nitric acid,which in turn yields a crude product mixture of o-nitrophenol, p-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol (picric acid). The goal was then to separate, analyze, and characterize these products using semi-microscale column chromatography to separate the products based on their polarity, TLC to analyze the separated products based on their polarity, IR Spectroscopy to characterize the separated products based on functional groups, and H C NMR to characterize the products based on their magnetic environments to determine structure.When an aromatic compound such as phenol undergoes nitration, it does so through an Electrophilic Aromatic Substitution (EAS). Undergoing this reaction requires two steps. The firststep is the addition of the electrophile, which in this lab was the Nitronium ion formed by the dilute nitric acid solution. This is the rate determining step for this reaction, as during this step aromaticity is lost and the arenium ion is formed. The position of the electrophile to be added is determined by how well the arenium ion can be stabilized once the initial addition occurs. In the case of phenol the greatest stabilization occurs when the electrophile is added to the ortho or parapositions due to the ability of the alcohol to donate electrons into the ring system. Meta addition does not occur as it does not lead to an as stable arenium ion intermediate. This is seen and discussed using resonance forms for both the para and meta substitutions in the “Pre-Lab” portion of this lab write up. The second step is the elimination of a hydrogen atom to reform the double bond lost and regain aromaticity. This reaction was able to happen during designated lab time due to the fact that a phenol was used. Phenols or more reactive than unsubstitued benzene rings due to the presence of the alcohol on the benzene ring. The alcohol is considered an activating group due to the oxygen’s ability to donate its lone pairs into the benzene ring thus giving it more electrons and thus making it more nucleophilic and more likely to react with the
introduced electrophilic species. As aforementioned, there are various products formed in this reaction the two major products formed though are the ortho and para products. It is debatable which product is more prominent due to steric reasons and the capability of each product to conduct in hydrogen bonding. The ortho products allows for extra stabilization due to hydrogen bonding between the alcohol and nitro group adjacent to one another. The issue here is that these are both fairly large molecules therefore there may be a bit of steric hindrance making this a bit less stable. As for the para product the alcohol and nitro group are on opposite sides of the molecule therefore steric hindrance is no longer an issue but it no longer has the capability to conduct in hydrogen bonding due to the distance between the nitro and alcohol groups and therefore some stability is loss in that perspective. The di- and tri- substituted products may also

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