Preparation of 1-Bromobutane and Reactivity of Alkyl Halides

Topics: Nuclear magnetic resonance, Proton NMR, Chemical shift Pages: 7 (1544 words) Published: February 18, 2014
Experiment 1: Preparation of 1-Bromobutane and Reactivity of Alkyl Halides

Objective: The purpose of this lab is to prepare 1-bromobutane from 1-butanol in an acid-catalyzed substitution reaction. While the reaction would be expected to occur as SN2 due to the primary nature of the substrate, because H2SO4 is used as a solvent, the conditions are very polar and the reaction can proceed via an SN1 reaction. The main objective is to obtain test results to determine the mechanism of the reaction and purity of the product.

Introduction: The techniques used in this laboratory included reflux, distillation, and extraction as well as obtaining IR and NMR spectroscopy. The reaction of 1-Butanol to 1-Bromobutane occurs via a substitution reaction. This reaction type can proceed via either the SN1 or SN2 mechanisms. SN2 reactions have a second order rate and proceed in a single step (ie. The leaving group leaves and nucleophile attacks all in one step) and leads to inversion of configuration. In contrast, an SN1 reaction occurs at a first order rate and proceeds in two steps between which an intermediate carbocation is formed. In this reaction type there is both inversion and retention of configuration. Analysis will be done to determine by which mechanism the reaction proceeds. To catalyze the substitution reaction, Reflux is employed. Reflux is a technique that involves the evaporation of liquid, condensation of vapors, and the return of these condensed vapors to the reaction vessel by condensation on the cool glass. Reflux is used in this experiment to catalyze the chemical reaction from 1-Butanol to 1-Bromobutane because it allows the reaction to be heated for a long period of time without losing the volatile reactants or products. Similar instruments are used for the distillation method, which allows the separation of two liquids based on their boiling points. The more volatile components (which have a higher vapor pressure and lower boiling point) will be vaporized first and then cooled down the condenser, condensing the gas back into liquid and collecting this liquid in a receiving flask. Thus, the less volatile material remains in the original container while the more volatile material travels to the receiving flask, separating the two liquids. A number of tests were also applied (including the Sodium Iodide, Lucas, and Silver Nitrate Test) to determine the reactivity and mechanism (SN1 or SN2) of a number of organic compounds including the prepared compound.

Reaction Equation:

+

Reagents used:

Table 1: Data for Reactants and Products

1-Butanol
NaBr
H2O
H2SO4
1-Bromobutane
Mass (g)
12.15 g
20.0 g
15.0 g
27.45 g
22.47 g(theoretical)
Molecular weight (g/mol)
74.12
58.44
18.02
98.08
137.03
Moles(mol)
0.164
0.342
0.83
0.28
0.164 (theoretical)
Density (g/cm^3)
0.81
3.21
1.00
1.83
1.276
Equivalence(g)
12.15
9.58
2.96
16.08
22.47

Procedure:
The procedure was followed as outlined in Experiment 1: Preparation and Reactivity of Alkyl Halides part A and B. Minor changes were however made in Part A and part B. In part A, a 150 mL round bottom flask was used instead of a 250 mL round bottom flask. The mixture was only distilled once and simply until approximately 25 mL were obtained, rather than reaching the head temperature of 120° C. Thus, the crude sample was the solution obtained after the distillation, and the purified sample was the solution obtained after gravity filtration. While IR and NMR spectra were obtained, the prepared compound was not analyzed by gas chromatography but one was given in the lab manual. In Part B of the Experiment 1 lab manual, the only changes were the absence of testing of the prepared products as they were not yet complete when the test were being done, as well as in the Silver Nitrate Test, Benzyl Chloride was tested in place of Benzyl Bromide.

Results:
a.) Yield
Compound
Mass (g)
% yield

Theoretical
22.47...
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