The hydrogens of alkanes have pKa values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have pKa values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored.Figure 1Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of D2O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2.Figure 2Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3.Figure 3The two products formed by the racemization reaction in Figure 3 could best be described as:
Answer(s): A
The passage states that a racemic mixture is produced, indicating the presence of enantiomers in equal concentrations. Figure 3 shows the structure of the two compounds. The compounds differ in configuration at their only chiral center, indicating that they are enantiomers.Choice B is incorrect because diastereomers are not mirror images. Diastereomers may have optical rotations of different magnitudes. Thus, a solution with equal concentrations of two diastereomers may still rotate plane polarized light. In contrast, a racemic mixture does not rotate plane polarized light.Choice C is incorrect because the compounds are not conjugate acids and bases.Choice D is incorrect because meso compounds are compounds that have chiral centers but lack optical activity due to an internal plane of symmetry.
The hydrogens of alkanes have pKa values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have pKa values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored.Figure 1Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of D2O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2.Figure 2Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3. Figure 3The aldol condensation proceeds through formation of an enolate anion. This is followed by nucleophilic attack by the enolate ion on the carbonyl carbon of the original aldehyde. Which of the following reactants would lead to the product indicated below?
Answer(s): C
The reaction proceeds through the following steps: Choices A, B, and D are incorrect because they would not lead to the indicated aldol condensation product.
The hydrogens of alkanes have pKa values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have pKa values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored.Figure 1Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of D2O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2.Figure 2Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3. Figure 3Which of the following best represents the enolate ion intermediate of the racemization reaction shown in Figure 3?
The enolate anion is stabilized by resonance which delocalizes the negative charge to the electronegative oxygen.Choice A is incorrect because it shows delocalization of a positive charge. The enolate ion is a negative ion (anion) created by loss of an alpha proton as described in the passage.Choice B is incorrect because, although this enolate anion may form through abstraction of an alpha proton from the methyl group, it will not lead to racemization of the chiral center. Note that the structure in Choice B maintains a chiral center which has not been turned into an achiral, planar configuration which would lead to racemization. Choice D is incorrect because this resonance structure would not occur.
Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials.If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.Table 1Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis, the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.In order to prevent the net movement of water between the blood and the dialyzing fluid, the dialyzing fluid:
Answer(s): B
If the dialyzing fluid is isoosmotic, it has the same concentration of particles and thus, the same osmotic pressure exists on either side of the membrane. There will be no net flow of water by osmosis between the blood and the dialyzing fluid.Choice A is incorrect because a hypoosmotic dialyzing fluid would lead to flow of water into the circulation from the dialyzing fluid.Choice C is incorrect because a solution with a higher concentration of solutes is hyperosmotic. A hyperosmotic dialyzing fluid would lead to flow of water out of the circulation from the dialyzing fluid.Choice D is incorrect because hydrophilicity has nothing to do with the net flow of water. A hydrophilic protein is "water-loving" because it contains polar amino acids.
Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials.If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.Table 1Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis, the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.Which of the following provides the best explanation for the urea plasma concentration in individuals with renal failure?
Reading from Table 1, it is clear that the urea concentration in a patient with renal failure is much higher than that in a normal individual. Filtration of urea decreases, leading to decreased excretion and a higher urea concentration in the plasma.Choice B is incorrect because a decrease of absorption from the nephron tubule would tend to increase excretion and decrease the plasma urea concentration.Choice C is incorrect because increased urea filtration would lead to increased excretion.Choice D is incorrect because increased secretion of urea into the nephron tubule would increase excretion.
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