Passage
Students perform a synthesis of a carboxylic acid derivative.They mix a 10.00-mL sample of hexanoic acid, 2.33 mL of ethanol, and 0.64 mL of concentrated sulfuric acid in a flask with a water condenser fitted to its neck. The mixture is boiled for one hour and then poured into water when the reaction is complete. The aqueous and organic layers are separated, and the organic layer is washed once with NaHCO 3 and twice with water. Solid Na 2 SO 4 is added to the organic layer to remove any water present. The organic layer and Na 2 SO 4 are separated by filtration. The filtrate is purified by fractional distillation to yield the carboxylic acid derivative, a colorless liquid.
-A majority of excess hexanoic acid is removed from the product in the extraction but a small amount remains, making the product impure. Which statement accurately describes the subsequent distillation of the product?

Question

Passage
Students perform a synthesis of a carboxylic acid derivative.They mix a 10.00-mL sample of hexanoic acid, 2.33 mL of ethanol, and 0.64 mL of concentrated sulfuric acid in a flask with a water condenser fitted to its neck.  The mixture is boiled for one hour and then poured into water when the reaction is complete.  The aqueous and organic layers are separated, and the organic layer is washed once with NaHCO 3 and twice with water.  Solid Na 2 SO 4 is added to the organic layer to remove any water present.  The organic layer and Na 2 SO 4 are separated by filtration.  The filtrate is purified by fractional distillation to yield the carboxylic acid derivative, a colorless liquid.
-A majority of excess hexanoic acid is removed from the product in the extraction but a small amount remains, making the product impure.  Which statement accurately describes the subsequent distillation of the product?

Quizplus · Accepted Answer

11ecba2d_12b1_a7fa_a3bf_e962658a4a3c_MD0008_00 Fractional distillation is a purification technique that separates molecules based on their boiling point, which is dependent on the intermolecular forces holding the molecule together. Stronger intermolecular forces require more energy to break, causing molecules with these forces to have a higher boiling point than molecules of similar molecular weight with weaker intermolecular forces. Hydrogen bonds are strong intermolecular forces that consist of a donor and acceptor. Donors are hydrogen atoms bonded to an electronegative atom from the second row of the periodic table, and acceptors are electronegative atoms with a lone pair of electrons (eg, oxygen, nitrogen, fluorine). Hexanoic acid can hydrogen bond with itself because it has a hydrogen bond donor (acidic hydrogen) and two acceptors (oxygen atoms). The reaction product ethyl hexanoate (an ester) is only a hydrogen bond acceptor, and therefore cannot hydrogen bond with itself. Esters experience dipole-dipole interactions, which are weaker than hydrogen bonds. Therefore, ethyl hexanoate has a lower boiling point and will distill before hexanoic acid. (Choice A) Hexanoic acid can hydrogen bond to itself whereas ethyl hexanoate only experiences dipole-dipole interactions and cannot hydrogen bond to itself. Because hexanoic acid experiences stronger intermolecular forces than ethyl hexanoate, it will have a higher boiling point and distill after ethyl hexanoate. (Choice B) Decreasing the pressure by doing the distillation under vacuum would decrease the boiling point of both compounds and would not make the boiling points farther apart. (Choice D) Increasing the length of the column in a fractional distillation will improve the separation of the compounds. Educational objective: Boiling points are dependent on a molecule's intermolecular forces, and strong intermolecular forces lead to higher boiling points. Carboxylic acids contain a hydrogen bond donor (hydroxyl hydrogen) and hydrogen bond acceptors (carbonyl oxygen and hydroxyl oxygen) and therefore can hydrogen bond to themselves. Because hydrogen bonds are strong intermolecular forces, carboxylic acids have higher boiling points than molecules of a similar molecular weight that cannot hydrogen bond to themselves and have weaker intermolecular forces.

Passage Students Perform a Synthesis of a Carboxylic Acid Derivative.They Mix