CHEM 254 Report 3 PDF

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lab report 3...

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Experiment 3: Electrophilic Aromatic Substitution Reactions Expt. 3: Electrophilic Aromatic Substitution Reactions Date of Experiment: 11 Feb 2021 Purpose To synthesize p-bromonitrobenzene via an electrophilic aromatic substitution (EAS) reaction wherein a nitro group (NO2) is substituted onto the benzene ring of bromobenzene, and to observe the relative rates of bromination of substituted benzenes. Theory Electrophilic aromatic substitution reactions (EAS) are useful for synthesizing aromatics. The presence of a substituent on the starting benzene ring influences the reactivity of the ring and the orientation on the ring in reactions. An activating group substituent makes the ring more reactive than benzene alone in substitution reactions, thus increasing the rate of reaction. Activating groups are electron-donating and stabilize the carbocation intermediate. Conversely, deactivating groups are electron-withdrawing, destabilize the intermediate, decrease the reactivity of the benzene ring, and reduce the rate of reaction. Substituents on a benzene ring may either be meta directors (resulting in addition of incoming substituents at the meta position) or para/ortho directors (resulting in incoming substituents adding to the ortho or para positions). The bromine substituent in bromobenzene is an ortho/para director. The para position is favored due to steric effects. In the reaction that takes place in this lab, a nitro group is substituted onto bromobenzene at the para position to form 1-bromo-4-nitrobenzene. To determine if a substituent on a benzene ring is an ortho/para director or a meta director, one can measure its rate of reaction. Reaction Reaction: bromobenzene + HNO3 + H2SO4  1-bromo-4-nitrobenzene Amounts: 2.1 mL 4 mL 4 mL

Bromobenzene

Nitric acid

Sulfuric acid

p-bromonitrobenzene

Synthesis Table

Chemical Name

Scale (mol/mmol/umol ) MW

Densit y

Mass/ Volum e

1

Bromobenzene 1-bromo-4nitrobenzene

mol

157.0 2 202.0 1

Nitric acid

mol

Sulfuric acid Dichloromethan e

mol

1.495

2.1 mL

--

0.68 g

63.01

1.513

4 mL

mol

98.08

1.840

4 mL

mol

84.93

1.330

20 mL

Yield Calculations: Bromobenzene: Br∗1.495 g Br ∗1 mol Br 1 mL Br ∗1 mol 1−Br 157.02 g Br ∗202.01 g 1−Br 1mol Br =4.039 g 1−bromo −4−nitrobenzene 2.1 mL 1 mol 1−Br HNO3: 3∗1.513 g HNO 3 ∗1 mol HNO 3 1 mL HNO 3 ∗1 mol 1 −Br 63.01 g HNO 3 ∗202.01 g 1−Br 1mol HNO 3 =19.403 g 1−bromo −4−nitrobe 4 mL HNO 1 mol 1−Br

The theoretical yield from bromobenzene (4.039 g) is less than the theoretical yield from HNO3 (19.403 g); thus, bromobenzene is the limiting reagent and the theoretical yield is 4.039 g 1bromo-4-nitrobenzene. Theoretical Yield: 4.039 g Actual Yield: 0.68 g 0.68 ∗100 %=16.836 % Percent Yield: 4.039 Methods/Procedures The experiment was followed verbatim from the textbook as found on pages 16-24, with no exceptions. Bromobenzene, 1-bromo-4-nitrobenzene, nitric acid, sulfuric acid, and dichloromethane are skin and eye irritants. Sulfuric acid is corrosive to metals, may be fatal if inhaled, and experiences an exothermic reaction with water. Dichloromethane is a carcinogen. Dichloromethane and bromobenzene (liquid and vapor) are flammable and must be kept away from heat and flame. Nitric acid is toxic if inhaled. When adding sulfuric and nitric acids, sulfuric acid must be added to nitric acid, and not the other way around. The experiment was conducted using proper safety precautions and wearing goggles and gloves to avoid skin and eye

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contact. Phenol, used in part B, is very caustic, so if exposed, the experimenter must wash the exposed area well with water. Observations/Results Whilst adding the bromobenzene to the sulfuric/nitric acid mixture, the solution bubbled somewhat aggressively and released a somewhat pleasant scent. After adding bromobenzene to the solution, solids formed and clumped together. After washing the dichloromethane layer with saturated aqueous NaHCO3 it turned yellow. The 0.05 M Br2 in 90% acetic acid solution used in Part B was very pungent. The experiment yielded 0.68 grams of product, resulting in a percent yield of 16.836%. The melting point range of the product was 124-130. This is fairly close to the expected melting point pure 1-bromo-4-nitrobenzene, indicating the product is fairly pure, though somewhat crude. The experiment was successful and produced 1-bromo-4-nitrobenzene. Below is a table of the results for Part B: Compound (Room Temp) Anisole Acetanilide Phenol Salicylic acid Toluene p-xylene Ethyl benzoate

Time to react 1 min; rank 3 4 min; rank 4 30 sec; rank 2 Almost instant; rank 1 Rank 6 Rank 5 Rank 7 (darkest)

Compound (Hot Water Bath) Anisole Acetanilide Phenol Salicylic acid Toluene p-xylene Ethyl benzoate

Time to react Instant (rank 1/2/3) Instant (rank 1/2/3) 1 min (rank 4) Instant (rank 1/2/3) Equal hue (rank 5/6) Equal hue (rank 5/6) Darkest hue (rank 7)

Based on the tables above, we can determine that salicylic acid is the compound which reacts the fastest, indicating it is an activating group, characterized by preference for ortho/para substitution and an increased reaction rate. Contrastingly, ethyl benzoate resembles a deactivating group, as it has a slow rate of reaction. Discussion/Conclusion The experiment went as planned, resulting in the formation of 0.68 grams of pbromonitrobenzene via electrophilic aromatic substitution; a percent yield of 16.836%. The product was determined to be fairly pure, as indicated by its melting range’s proximity to the melting range of the pure compound. The rates of bromination of the substituted benzenes in Part B shed light on the regioselectivity of these compounds. The compounds with faster rates (acetanilide, anisole, phenol, salicylic acid, and potentially p-xylene) likely contain a para/ortho

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director, while those with slower rates (ethyl benzoate and toluene, and potentially p-xylene) likely display a meta director. Exercises 1) Write the equation for each of the reactions that you conducted in Part B of this experiment. For compounds with o,p directors, you will have two products to show.

5) Write a mechanism for the nitration of bromobenzene (Part A) according to the procedures followed in this experiment.

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Raw Data

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