pre and post lab essay

Experiment 5 – Title of Experiment
Rafael Nadal
University of South Carolina
Chem 332L-334L
TA Luis Ramos
June 22, 2020
Purpose
To learn how nucleophilic substitution can be used to allow the interconversion of functional groups and how choosing certain conditions influence what mechanism will take place during that particular reaction.
Introduction
Nucleophilic substitution is one of the most thoroughly studied class of reactions in organic chemistry (Atim, 2019). If the reaction is displaying second-order kinetics, it is occurring by a SN2 mechanism, as a concerted process and the rate of the reaction is proportional to the concentrations of both the nucleophile and the alkyl halide (Atim, 2019). If the reaction is displaying first order kinetics, it is occurring by a SN1 mechanism in which the reaction first begins with a carbocation intermediate being generated, which then captures a nucleophile, with the concentration of the alkyl halide being solely responsible for the rate of reaction (Atim, 2019). The use of certain conditions can be used to influence how nucleophilic substitution reactions take place (Atim, 2019). Hyperconjugation as well as the inductive effect can be used to stabilize the carbocations (Atim, 2019).
Objectives
1. Utilize a SN1 reaction to synthesize triphenylmethyl bromide from triphenylmethanol and record the mass, melting point and percent yield of the product. Analyze the reactants and products via IR, NMR.
Reaction Schemes / Structures

Reagents
Name, structure MW g/mol MP BP Density g/mL Properties Safety
Acetic Acid (CH3COOH)

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60.0524 16.6 117.9 1.05 Corrosive material; colorless liquid or solid with a strong vinegar-like odor Corrosive to tissue and irritating to the skin, mucous membranes, upper respiratory tract, and eyes
Hydrobromic Acid
HBR 80.9119 -87 -66.38 1.49 CORROSIVE MATERIAL; colorless gas Corrosive to tissue and irritating to the skin, mucous membranes, upper respiratory tract, and eyes
Triphenylmethanol (C19H16O)

260.3348 163 360 – Crystalline white powder Irritating to the eyes, skin, and respiratory tract
Hexane (C6H14)

86.1766 -95 69 .659 Colorless flammable liquid Irritating to the eyes, skin, and respiratory tract
Triphenylmethyl bromide (C19H15Br)

323.2315 152-154 230 at 15 mmHg – White solid –

Procedure
1. Set up a hot water bath to 60-65 °C using a hot plate and beaker.
2. Obtained a small test tube, put 108 mg (4.15 x 10-4 mol) of triphenylmethanol into it, add 2 ml (0.0350 mol) of acetic acid, and placed into the hot water bath, allowing the solution to heat until the solid dissolved. Stirred with a glass rod.
3. Added 0.2 ml (0.004 mol) of concentrated HBr into the test tube, observed and documented the color change and the formation of precipitate.
4. Continued to heat the solution for an additional 5 – 10 minutes in the hot water bath and then allowed the test tube to cool to room temperature slowly, then took the test tube and placed into an ice bath allowed it to cool further.
5. In a separate test tube, placed 3 or 4 drops of hexane and put it into the ice bath as well.
6. Used vacuum filtration and collected the cooled crystalline product, rinsed the product first with ice cold deionized water and then with the ice cold hexane.
7. Drained the product well and allowed it to air dry. Covered it with a paper towel, sealed it and save it in a drawer until lab class the following week.
8. At the beginning of the following lab, weighed the product and determined the percent yield, then crushed the crystals and determined the melting point of the product. All of the results were documented.
Waste Reagent Disposal
Excess aqueous solutions can be washed down the drains as where the organic solvents are to be disposed of in the organic liquid waste container. Any soiled gloves or paper towels should be disposed of in their appropriate containers along with any broken glassware.
Observations
The triphenylmethanol a white crystalline powder substance. When the acetic acid was initially added, the triphenylmethanol did not dissolve completely. After it was placed into the hot water bath, it dissolved fully. When the hydrobromic acid was added, as it entered the solution it turned bright yellow. Once it was completely added the solution was yellow and precipitate began to form. Once removed from the ice bath, the triphenylmethyl bromide was collected via vacuum filtration and the crystalline product consisted of large white crystals.
Data / Results
Mass = density x volume
Mass of Acetic Acid = 1.05 g/ml x 2 ml = 2.10 g
Mass of Hydrobromic Acid = 1.49 g/ml x 0.2 ml = 0.298 g
Moles = mass / molecular weight
Moles of Acetic Acid = 2.10 g / 60.0524 g/mol = 0.035 mol
Moles of Hydrobromic Acid = 0.298 g / 80.9119 g/mol = 0.004 mol
Moles of Triphenylmethanol = 0.108 g / 260.3348 g/mol = 4.15 x 10-4 mol
Mass = moles of reactant x (1 mol product/1 mol reactant) x (molecular weight of product / 1 mol product)
Mass of Triphenylmethyl bromide = 0.000415 (1 mol Triphenylmethyl bromide / 1 mol Triphenymethanol) x (323.2315g / 1 mol Triphenylmethyl bromide) = 0.134g
Percent yield = (actual mass of product / theoretical yield of product) x 100%
Percent yield of Triphenylmethyl bromide = (0.098g / 0.134g) x 100% = 73%
Moles of Triphenylmethyl bromide = 0.098 g / 323.2315 g/mol = 3.03 x 10-4 mol
Melting point of product = 152.3℃

Conclusions
Using the correct solvent, in this case acetic acid, alcohols can be converted to another functional group. Triphenylmethanol was able to be converted to triphenylmethyl bromide, an alkyl halide, through an SN1 mechanism by first converting to a trityloxonium ion. Starting with 0.108 g (4.15 x 10-4 mol) of triphenylmethanol, after completing the experiment, 0.098 g (3.03 x 10-4 mol) of triphenylmethyl bromide was recovered, resulting in a 73% recovery. The percent recovery could have been affected by the humidity in the room preventing all of the product from being collected as well as a high level of impurities in the reactant. Ways to prevent these possibilities include utilizing a better controlled environment that would regulate the humidity at optimal levels and evaluating the supplier for the reactants and the accepted impurity level in their products. The melting point of the recovered product was 152.3℃ which is consistent with pure triphenylmethyl bromine. Looking at the IR spectra you can see that all three compounds share similar frequencies such as the extremely large spike at 700 cm-1 which is consistent with all three being benzene derivatives. Another commonality is found at approximately 750 cm-1 where all have peaks that are consistent the C-H bending. Some differences are found in areas such as the 3200-3550 cm-1 range where the triphenylmethanol has a broader peak due to the O-H stretching and the other two compounds have sharper peaks at approximately 3100 cm-1 due to C-H stretching. As well as the spike of triphenylmethanol at 1000 cm-1 which is due to the C-O bond.
Resources
Atim, S., Chemistry 331L/333L Laboratory Manual Essentials of Organic Chemistry Laboratory. QDE Press Inc. Montgomery, Al, 2019. p.105-118,
NIST Chemistry Webbook, https://webbook.nist.gov/chemistry/
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