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Experiment 9: Aromatic Substitution Reactions of Veratrole

Introduction

Worth 20%

Experimental Method

15%

Discussion

20%

Conclusion

5%

Questions

30%

Layout

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Preeti Dave TA: Naeem Lodhi 212902128 Date of Experiment: July 25th, 2017 Submission: August 1st, 2017

INTRODUCTION Electrophilic aromatic substitution (EAS) is the substitution of a proton for an electrophile on an aromatic ring. EAS is a mechanism by which substituted aromatic compounds can be synthesized, which is especially significant. In the case of nitration, sulfuric acid reacts with nitric acid to form the electrophile (the nitronium ion). The attack on the electrophile forms the sigma complex, and finally the loss of a proton gives nitrobenzene (Figure 1). Substituents have effects on further substitutions; their position is non-random. When considering the substitution pattern, the structure of the intermediate sigma complex should be considered because the formation of the sigma complex is the rate limiting step in EAS. For instance, when benzene reacts with the nitronium ion, the charge is distributed over three 2° carbocations. In a substituted aromatic compound such as toluene, the charge is spread over one 3° carbocation in the case of an ortho, para attack. This is more favourable because the positive charge is delocalized onto the tertiary carbon and this further stabilizes the sigma complex. Therefore, toluene is an ortho, para director because it destabilizes the ortho, para sigma complex less than the meta position. Some substituents such as methoxide are electron donating; they donate electron density to a pi system by inductive effects. They primarily activate the ortho and para positions. Electron withdrawing groups such as NO2 remove electron density from a pi system, making it electrophilic. They are usually meta directing with the exception of halogens because they can donate a lone pair of electrons in resonance. Where a substituent is placed on a substituted aromatic compound depends on the kind of substituent; resultantly, this affects the reaction rate. In this experiment, veratrole was reacted with nitric acid and a structure was proposed. The structure was deduced from the H1 NMR and overall knowledge of the effects of substituents on aromatic substitution. The structure was proposed to be 4-nitroveratrole (Figure 2). EXPERIMENTAL METHOD When the veratrole solution was added to the diluted nitric acid, a brown-orange slurry had formed. The veratrole was added dropwise and the temperature was periodically checked so that the solution did not exceed 20°C. As veratrole was being added, the temperature of the

solution continued to increase and it had to be placed in an ice bucket regularly to maintain temperature under 20°C. The crude product was filtered using vacuum filtration and weighed to be 1.38 g. The melting point was 85-90°C. For recrystallization, methanol was added to the crude product and heated until all the product was dissolved. The product was scratched with a glass rod for crystallization. The pure product was filtered using vacuum filtration and weighed to be 0.83 g. The melting point was 85-87°C. The theoretical yield was calculated to be 1.41 g with calculations shown below. % yield was calculated to be 44.4 %. Theoretical yield mols of veratrole:

Theoretical mass of veratrole

n = CV

Mass = mol x molar mass

= 0.003 L x 3.4 mol/L

= 0.102 mol x 138.16 g/mol

= 0.102 mol veratrole

= 1.41 g veratrole

1. Stoichiometry to determine mass of 4-nitroveratrole (1.41 g veratrole) x ( 1 mol veratrole / 138. 16 g mol-1) x ( 1 mol 4-nitroveratrole / 1 mol veratrole) x ( 186 g mol-1) = 1. 869 grams 4-nitroveratrole Percent yield 4-nitroveratrole % yield = actual / theoretical = 0.83 g / 1.869 g = 44.4% DISCUSSION There was a large deshielding effect of 3 different types of aromatic protons, as indicated by three different peaks found in the region from around 7-8 ppm. This region is characteristic of aromatic hydrogens. The singlet at 7.75 ppm would be the isolated hydrogen found between the nitronium group and methoxide group. The doublet around 8 ppm would be the hydrogen in between another hydrogen and the OCH3 group. The other doublet around 7 ppm would be the hydrogen in between the H and nitronium group. This doublet is found more upfield because the proton is strongly shielded from the nitro group. There are also two singlets found

around 4 ppm; these would be the methoxide groups. They are singlets because they are isolated from other protons. Another isomer could occur with the nitronium group placed at the ortho position of either methoxide group. However, this was not as likely as veratrole is a bulky compound and substitution occurring at the ortho position would cause steric hindrance. Therefore, the proposed structure would be 1,2-dimethoxy-4-nitrobenzene or 4-nitroveratrole (Figure 2). CONCLUSION In this experiment, an electrophilic aromatic substitution was performed by reacting a poly substituted aromatic compound (veratrole) with nitric acid. Electrophilic nitration was done on veratrole to produce 4-nitroveratrole (Figure 2). Understanding the directing effects of substituents and looking at the H1 NMR led to prediction of this likely product. Since methoxide is an activating group, it was ortho, para directing. More than likely, substitution occurred at the para position from the first carbon to avoid steric hindrance. The singlet at 7.75 ppm was indication of a hydrogen placed in between a methoxide and nitronium group, further evidence supporting the nitronium group being placed in the para position. In conclusion, the aromatic substitution reaction of veratrole produced 4-nitroveratrole.

Figure 1. Full mechanism of nitration of benzene

Figure 2. Structure of 4-nitroveratrole

QUESTIONS

NO2

1.

NO2

NO2 NO2 4,5-dinitroveratrole

5,6-dinitroveratrole

Because there are two nitro groups being added to the compound, 2 substitutions reactions are taking place. The first substitution would be faster because there are two activating groups. The second substitution would take longer because there is a deactivating group (-NO2 group) on it. The intermediate structure would also take longer for substitution due to the overall bulkiness of the group. The first isomer has the NO2 group in the para positions of either methoxide group, named 4,5-dinitroveratrole. This isomer is favoured because methoxide is an ortho, para directing group. Since the group is bulky with the -OCH3 substituents, the ortho position is less likely to be occupied. The second isomer is 5,6-dinitroveratrole. The nitronium group is added to the ortho position of the methoxide at the first carbon and added to the para position of the methoxide at the second carbon. The veratrole compound is bulky, so both ortho positions would less likely be occupied. 2. 4-(3-methylbutane)-veratrole is produced when veratrole reacts with 3-methyl-2butanol. This is the likely structure because when 3-methyl-2-butanol loses the hydroxide to form a carbocation, it undergoes a carbocation rearrangement to form a tertiary carbocation. The rearrangement occurs through a hydride shift to form a more

stable, tertiary carbocation from a secondary carbocation. Afterwards, monoalkylation will proceed to form the likely product.

3. a)

Compound A b) DMBA undergoes the reaction at the indicated position because it is para to the methoxide (OCH3) on the third carbon. It is also ortho to the –CH2OH group. In addition, the benzylic carbocation is very bulky and another alternative such as the ortho position would less likely occur due to steric hindrance. Addition at the ortho position would create an unstable compound. c)

Compound C

d)

CTV Structure...