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Introduction to Organic Chemistry Essay

Amines are compounds composed of nitrogen atoms bearing alkyl or aromatic compounds. Amines undergo interesting reactions, one of which is with the reaction with nitrous acid producing an azo dye. In this study, the experiment focused on synthesizing an observing the physical properties of Sudan-1. Sudan-1 is of the most common dyes found in waxes, oils and in some food ingredients specifically curry and chilli powder.

Furthermore, this study aimed to understand the mechanism behind the synthesis of 1-phenylazo-2-naphtol. To be able to synthesize Sudan-1, preparation of phenyldiazonium chloride solution and β- naphthol solution were done. Ingrain dyeing was also done in this experiment. The synthesis of Sudan-1 has a two-step reaction – diazotization and coupling reactions. Diazotization is the formation of diazonium salt, meanwhile, the coupling reaction took place when an activated aromatic compound, β-naphtol was reacted with the diazonium salt, benzene diazonium chloride, to form the azo compound known as the 1-phenylazo-2-naphthol. As a result, an orange-red precipitate was formed after series of reaction. Hence, all the said objectives in this experiment were achieved.

Amines are compounds that are composed of a nitrogen atom bearing alkyl or aromatic groups. They are basic and nucleophilic because of their lone pair. They occur both in plants and animals. Amines produces some of the most interesting effects and of the common reaction of aminewith nitrous acid producing a dye[4]. Alizarin, for example is a red dye extracted from madder root used by Egyptians and Persians. However, in this experiment, it aimed to produce a dye commonly known as Sudan-1. Sudan-1 is a lysochrome with the chemical formula 1-phenylazo-2-naphthol. It is a powdered substance with an orange-red color.

This azo dye is most commonly found in waxes, oils, and also in some food coloring ingredients – curry powder and chili powder. However, the presence of Sudan-1 in most foods now is currently being banned because it has been classified to be carcinogenic. This experiment focused on synthesizing of 1-phenylazo-2-naphthol which is a two-step reaction. The first reaction is the reaction of aniline with nitrous acid, which is called diazotization and second, the reaction of diazonium salt and beta-naphtol to form azo dye which is the coupling reaction.

Figure 1 Diazotization Reaction of Aniline to Produce a Diazonium Salt Figure 1 Diazotization Reaction of Aniline to Produce a Diazonium Salt In diazotization reaction, there is a formation of diazonium salts. This reaction is made possible when a primary aromatic amine is treated with nitrous acid. Then in coupling reaction, the electrophilic substitution reaction of a diazonium salt with an activated aromatic ring formed a azo compound specifically an azo dye.[3] The main objective of this study was to be able to synthesize Sudan-1. Also, it aimed to characterize the azo dye with its most distinguishing physical properties. Furthermore, this experiment also aimed to understand the mechanism behind the synthesis of Sudan-1.

Figure 2 Coupling Reaction of Benzene Diazonium Chloride with β -Naphthol Figure 2 Coupling Reaction of Benzene Diazonium Chloride with β -Naphthol Aniline was reacted NaNO2 crystals under acidic condition using HCl in a cold temperature. The solution was done in a very cold temperature because the phenyldiazonium intermediate easily decomposes back to its aniline counterpart at a slightly high temperature; hence the temperature of the solution was maintained in an ice bath below 5°C.

Rock salt may also be added to the ice bath to maintain the temperature. However, in this experiment, no rock salts were added instead constant monitoring of the temperature was done. β-naphthol solution was used as a coupling reagent in synthesizing Sudan-1. In preparing β-naphthol solution, β-naphthol was dissolved in 5% of aqueous NaOH and was also cooled in an ice bath below 5°C, this was to avoid the decomposition of the compounds. The main reaction that occurred in the preparation of phenyldiazonium chloride solution was diazotization reaction. Diazotization is the reaction between a primary aromatic amine and nitrous acid at cold temperatures to diazonium salt compound.[2] Figure 1 below is the reaction exhibited by the phenyldiazonium chloride solution.

As this experiment aimed to synthesize Sudan-1, two steps are done. The first step would be the reaction of a primary aromatic amine to produce a diazonium salt as seen in Figure 1. The second step, then, is the reaction of the diazonium salt with a strongly activated aromatic syste,l known as coupling reactions. Azo coupling is the reaction between a diazonium compound and aniline, phenol or other aromatic compound which produces an azo compound.[5] In this experiment β-naphthol couples with the diazonium salt. Figure 2 below shows the coupling reaction of the benzene diazonium chloride with β-naphthol and having the product of Sudan-1. Furthermore, figure 3 below is the summary of reactions of the synthesis of Sudan-1 in this experiment. Figure 3 Summary of Reactions in Synthesizing Sudan-1

Figure 3 Summary of Reactions in Synthesizing Sudan-1

In this experiment, a filter paper was used to undergo ingrain dyeing. Ingrain dyeing is an irreversible chemical reaction of the diazonium salt solution and the activating aromatic solution. An orange-red filter paper was produced after such procedure. The presence of orange-red color in filter indicates the presence of the azo dye (see appendix for the orange-red filter paper produced). The Sudan dye is synthesized right in the spaces between the filter paper such that they are permanently trapped inside the fiber spaces of the filter paper.[2]

After mixing the phenyldiazonium chloride solution with the β-naphthol solution, an orange-red paste-like solution was formed. Furthermore, the mixed solution was also reacted at a temperature not exceeding 4˚C for 1-5 minutes. Afterwards, the mixture was also filtered was washed with several portions of water to filter the product, Sudan-1.

Recrystallization was also done when the filtrate was steamed bath after dissolving it with 95% hot ethanol. AS a result, orange-red crystals were formed which is the Sudan-1 product. The crystal appeared to be orange-red in color due to the N=N bond present in Sudan-1. The N=N is responsible for the absorption of

light thus reflecting a color which is orange-red. The structure of Sudan-1 is shown in Figure 4 below showing the N=N bond of the compound. The N=N is known as the chromophores which are responsible for the color. The –OH group attached in the structure is also responsible for enhancing the orange-red color. The –OH functional group is known as the auxochrome, which modifies the ability of the chromophore to absorb the light.[1]

Figure 4 Structure of Sudan-1

Azo-compounds, compounds with general formula Ar-N+=N-Ar-, are coupling products from the reaction of diazonium salts with amines. The general reaction pattern for Sudan synthesis first undergoes diazotization reaction and then coupling reaction with highly activated aromatic compounds. In this experiment, the diazotization reaction of aniline with NaNO2 and HCl yielded a diazonium salt, benzene diazonium chloride. Furthermore, the diazonium salt then underwent coupling reactions with an activated aromatic ring which is β-naphthol.

The coupling reaction yielded an azo compound which is most commonly known as the Sudan-1 with an IUPAC name of 1-phenylazo-2-naphthol. As a result of the reactions in this experiment, an orange-red color of solution was produced. This experiment aimed to understand the reactions that underwent to synthesize Sudan-1; as a result, figure 3 was the summary of reactions. As a physical result, orange-red colored crystals were produced representing the azo compound, Sudan-1. However, some minor errors will not be ignored in this experiment.

Some errors like human errors might have affected the results in yielding a pure azo compound. One human error, would be the measuring of the reagents used to yield the said product. Also, the misreading of some measurements may have also affected the results of this experiment. Also, some impurities in the chemicals used will also not be ignore, since this impurities may have led to a not so visible side reactions in the said experiment. After being said and done, all the said objectives in this experiment were met.


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