3. In liquid-liquid extractions, the organic solution is shaken with aqueous sol

Question

3. In liquid-liquid extractions, the organic solution is shaken with aqueous solutions to remove byproducts or excess reactants. Identify one compound that is being removed from the organic solution when it is shaken with each aqueous solution below. 4pts a. Extraction of Grignard reaction mixture with water b. Extraction of Grignard reaction mixture with saturated sodium chloride c. Extraction of bleach oxidation mixture with sodium bicarbonate d. Extraction of either oxidation mixture with sodium thiosulfate 4. The Grignard reaction in week one (Figure 6) makes a new stereocenter in 2-methyl-4- heptanol. a. Draw the structure of the alcohol and label the stereocenter with an asterisk. b. Do you expect this reaction to produce a single enantiomer (either only R or only S) or a mixture of enantiomers (both R and S)? Explain. 3pts 5. Show a calculation for overall percent yield if you had performed a 3 step reaction with a yield of 90% each step. 2 pts

Explanation / Answer

3.

a) Grignard reagents are extremely mositure sensitive organomagnesium compounds. Extracting a Grignard reaction with water results in quenching the reaction thus preventing excess formation of side products or even avoid their formation. Moreover, extraction of the organic layer with water results in the hydrolysis of the bond between the metal and the product thus giving the actualy product. Thus, extraction of Grignard reaction with water results in the removal of the excess Grignard regent and the magnesium halide salt.

b) Extraction of organic layers with saturated sodium chloride helps remove completely all the salts that have formed in the reaction thus making the organic layer free of any water-soluble substance. Here, using saturated NaCl solution helps completely remove magnesium halides from the Grignard reaction.

c) Oxidation with NaOCl proceeds through the formation of HOCl and using sodium bicarbonate neutralizes the weak acid formed thus preventing over-oxidation of the reactant. Sodium bicarbonate being an aqueous soluton also helps remove NaOCl.

d) Sodium thiosulphate is a good reducing agent and so using it to quench oxidation reaction results in the reduction of the oxidants used in the reactions thus rendering the oxidants inactive. This again is done to stop further oxidation of the reactant(s) by making the oxidant inactive and removing it from the organic layer, away from the reactants.

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