When an alcohol (substrate) is placed in an excess equimolar amounts of chloride

Question

When an alcohol (substrate) is placed in an excess equimolar amounts of chloride ions and bromide ions in acid media the corresponding alkyl halides are obtained. The ratio of the alkyl halides depends on several factors: the nature of the substrate, the reaction conditions, the solvent and the mechanism of the reaction. When tert-butanol is placed in the presence of excess equimolar amounts of chloride and bromide ions in aqueous acid experimental data indicates 45% tert-butyl chloride and 55% tert-butyl bromide is obtained. This outcome can be explained by the fact that the rate determining step of the reaction does not involve the nucleophile, and once the carbocation is formed it is equally available to each nucleophile. On the other hand, when n-butanol is placed in the presence of excess equimolar amounts of chloride and bromide ions in aqueous acid experimental data indicates 16% of n-butyl chloride and 84% of nbutyl bromide is obtained. The difference in the outcome can be explained by the fact that the rate determining step of this reaction requires the presence of the nucleophile, and in aqueous media the small size chloride ion gets “trapped” in the aqueous solvent cage and the bromide ion therefore gets a better chance to attack the substrate. This point can be demonstrated by changing the solvent. When n-butanol is placed in the presence of excess equimolar amounts of chloride ions and bromide ions in acid dimethyl sulfoxide the experimental data provides an 84% n-butyl chloride and 16% n-butyl bromide as the outcome.Calculate the theoretical yield for chlorides and bromides if each reaction was started with 2.5 mL of the corresponding alcohol, using an aqueous acid media. Place your results in a data table format. Calculate the expected percentage yield of each halide if you obtained a total of 0.025 moles of tertbutyl halides and 0.022 moles of n-butyl halides, and your % composition corresponds to the values of refractive index indicated and using an aqueous acid media. (hint: complete the data table)

% Cl (RI)

%Br (RI)

Mass Cl

Mass Br %

Yield

% RCl % RBr

1Tert-butyl 1.4042 refractive index 25.7 temperature

1n-butyl 1.4311 refractive index 26.1temperature

What would be the expected refractive index for the reaction of n-butanol with an excess of equimolar amounts of chloride and bromide ions if the reaction took place in dimethyl sulfoxide? No need to do calculations for the third reaction, just the first two.

Explanation / Answer

Substrate : tert-butyl alcohol

moles = 2.5 ml x 0.781 g/ml/74.12 = 0.0263 mol

45% tert-butyl chloride = 0.45 x 0.0263 = 0.0118 g

55% tert-butyl bromide = 0.55 x 0.0263 = 0.0145 g

Actual yield of halides = 0.025 mol

total yield = 0.025 x 100/0.0263 = 95.06%

tert-butyl chloride yield = 0.45 x 95.05 = 42.77%

tert-butyl bromide yield = 0.55 x 95.05 = 52.28%

Substrate : n-butyl alcohol

moles = 2.5 ml x 0.81 g/ml/74.12 = 0.0273 mol

45% tert-butyl chloride = 0.45 x 0.0273 = 0.0123 g

55% tert-butyl bromide = 0.55 x 0.0273 = 0.0150 g

Actual yield of halides = 0.022 mol

total yield = 0.022 x 100/0.0273 = 80.59%

tert-butyl chloride yield = 0.45 x 80.59 = 36.26%

tert-butyl bromide yield = 0.55 x 80.59 = 44.32%

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