Organic Chemistry Lab Report #6 PDF

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Melina Rosado Rosales

Chemistry 2612 Organic Chemistry Lab

Lab Report #6

Synthesis of 1-Bromobutane from 1-butanol (SN2)

04-03-17

Introduction The purpose of this experiment is to form 1-bromobutane from 1-butanol and sulfuric acid via SN2 reaction mechanism. SN2 is a substitution reaction mechanism that exhibits 2nd order kinetics. The rate for SN2 is rate = k [Nu][R-Lg]. This means that the rate determining step is an interaction between the nucleophile and the organic substrate including the leaving group. SN2 is a concerted process which occurs in a single step with one transition state and no intermediates. Within this one transition state there is a simultaneous formation of the carbonnucleophile bond and breaking of the carbon-leaving group bond. The nucleophile must be strong, which is usually a charged halogen such as Br- and Cl-. The leaving group also must be a weak base when displaced by the strong nucleophile. A main characteristic of SN2 is the backside attack of the nucleophile, meaning it is forming a bond on the opposite side of the leaving group resulting in an inversion of configuration. Polar aprotic solvents are utilized to enhance the reactivity of the nucleophile and promote an SN2 reaction. SN2 is most efficient when the carbon that the nucleophile is attacking is not sterically hindered. A primary carbon & 1° carbons will undergo SN2 reaction faster than a 2° carbon, but 3° carbons do not undergo SN2 reaction mechanism.

Reaction Mechanism Step 1: Production of H-Br

Step 2: Protonation of alcohol by H-Br, formation of good leaving group

Step 3: Nucleophilic attack by Br-

Properties & Safety of Chemicals Compound

Boiling Point (°C)

D (g/mL)

1-Bromobutane

117.4

0.81

Hazards

-

1-Butanol

Sulfuric Acid

101.4 – 102.9

337

1.27

1.84

-

Irritant of skin, eye, ingestion & inhalation Flammable liquid

-

Irritant of skin, eye, ingestion & inhalation Toxic for aquatic life

-

Oxidizer corrosive

Experimental Section 11.1 g of NaBr, 10 mL of 1-Buanol, and 20 mL of 50% H2SO4 were mixed into a 100 mL RB flask with a stir bar, which was hooked up to a condenser for reflux. Mixture was heated until solid completely dissolved. A 45-minute reflux was conducted and 2 distinct layers were formed. Reflux was removed and 15 mL H2O was added and a simple distillation was set up. 5 – 10 mL of product was collected while temperature is below 80 °C. Product was transferred to a separatory funnel and 10 mL of 5% NaOH was added. Then the separatory funnel was shaken and vented and the bottom layer was transferred into a beaker which is the butyl bromide. The top aqueous layer was poured out through the top. A pH test was conducted on the base extract to make sure it’s basic. Anhydrous sodium sulfate which is a drying agent was added to the “now product”. The liquid was transferred to a tarred vial and weighed to determine percent yield. An IR of the sample was conducted and compared to 1-butanol. A silver nitrate and sodium iodide confirmation tests were also taken.

Data & Observations 1-Butanol: 10 mL NaBr: 11.1 g 1-Bromobutane(product): 5.4 mL

Calculations % Yield = (

actual ) *100 theoretical

Molar Mass 1-Butanol: 10 g * (1mol/74g) = 0.135 mol 1-Butanol Molar Mass NaBr: 11.1 g * (1mol/103.9g)= 0.107 mol NaBr 3.303 g 1-Bromobutane * (1mol/103.9g)= 0.032 mol 1-Bromobutane Theoretical yield = (10 ml 1-Butanol/ 74g/mol)(103.9 g/mol) = 7.12 g % yield = 3.303 / 7.12 g = 46.39%

Conclusion/Discussion After conclusion of the experiment, the first step was mixing the NaBr, 1-butanol, and H2SO4 which initiated the production of H-BR. Then protonation of alcohol by H-Br occurs which leads to the formation of a good leaving group, when the hydrogen on the hydrobromic acid protonized the alcohol group on 1-butanol forming H2O which is the good leaving group and forms Br- which is the strong nucleophile. Next a nucleophilic attack occurs from Br- while the leaving group H2O leaves. During this transition state the backside nucleophilic attack and the leaving group leaving occurs simultaneously which is a characteristic of an SN2 reaction. The backside attack results in an inversion of stereochemistry meaning that the Br- will be on the opposite side where the leaving group was. Refluxing the mixture in the flask for 45 minutes helps speeds up the mechanism. After the experiment was completed my percent yield was 29.95%. A silver nitrate and a sodium iodide test was conducted to confirm whether the experiment was an SN1 or SN2 reaction mechanism. The silver nitrate test yielded a precipitate which indicates that the reaction

mechanism was in fact an SN2. An IR was taken as well which indicated that there was starting material in my product by a “thumbprint” at around 3500 cm-1 which is the OH peak. This is due to rookie mistakes made in the organic chemistry lab such as excess drainage of product yielding a low percent yield and inefficient decanting techniques. The peak at 643.84 cm-1 indicates a CBr bond meaning that I in fact did have 1-Bromobutane in my product....