Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 501:

  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 1

  Keto-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 2 Another 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 3

  The two products formed by the racemization reaction in Figure 3 could best be described as:

  A. enantiomers

  B. diastereomers

  C. conjugate acid and base

  D. same compound

  Correct Answer: 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.

• Question 502:

  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 1

  Keto-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 2

  Another 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 3

  The 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?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: 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.

  

• Question 503:

  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 1

  Keto-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 2 Another 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 3

  Which of the following ketones will have the most acidic -hydrogen:

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Alpha hydrogens are relatively acidic because of resonance stabilization as shown below. The acidity of the alpha hydrogens will be increased by proximity to electron withdrawing groups.

  

  The chloride groups in choice B are electron withdrawing and will help stabilize the negative charge. Choice A is incorrect because dimethylketone has no additional electron withdrawing groups. Choice C is incorrect because an amine is an

  electron donating group and would tend to decrease the acidity of the alpha hydrogen.

  Choice D is incorrect because there is no alpha hydrogen to be removed.

• Question 504:

  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 1

  Keto-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 2

  Another 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 3

  A scientist attempts to follow the progress of the -deuteration shown in Figure 2 using proton NMR. Which of the following would be the best indicator that the reaction has proceeded to completion?

  A. Singlet at 9.8 ppm

  B. Quadruplet between 1.0 and 2.0 ppm

  C. Doublet between 3.0 and 4.0

  D. No signal

  Correct Answer: D

  Proton NMR is used to identify the number and environments of 1H nuclei. Therefore, deuterium, 2H, will not appear on the NMR and as the alpha hydrogens are replaced with deuterium, the signals will disappear. Choice A is incorrect because a peak at 9.8 ppm, which, by the way, is rarely a singlet, indicates an aldehyde proton. No aldehydes are present here. Choice B is incorrect because a quadruplet in the alkane range (1.0?.0) indicates hydrogens with 3 neighboring hydrogens, leading to splitting. Choice C is incorrect because a peak between 3.0?.0 may indicate an alcohol. A doublet indicates splitting by a single neighboring hydrogen.

• Question 505:

  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 1

  Keto-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 2

  Another 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 3

  The IUPAC name for the reactant in Figure 3 is:

  A. (R)-3-methyl-2-pentanone

  B. (R)-3-ethyl-2-butanone

  C. (R)-3-ethyl-3-methyl-propanal

  D. (R)-2,2-diethyl-propanone

  Correct Answer: A

  The structure can be drawn as shown below:

  

  The longest chain has 5 carbons and this compound will be named with the pent- prefix. This is enough to select Choice A. Numbering proceeds to give the lowest numbered ketone (the highest priority group in this compound).

  Choice B is incorrect because it does not number the longest chain correctly, naming the molecule as an ethyl substituted butanone.

  Choice C is incorrect because it too does not number the longest chain correctly.

  Choice D describes an impossible molecule.

• Question 506:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  Researchers have discovered that the stereochemistry of the alkaloids is critical to their physiological effects. For example, (?-methadone produces powerful analgesic effects while (+)-methadone produces no effect. Which of the following is the most likely explanation for this difference in activity?

  A. (+)-Methadone does not follow the "morphine rules."

  B. Alkaloids bind to stereospecific receptors.

  C. Analgesic effects require the rotation of plane polarized light.

  D. (?-Methadone is the naturally occurring enantiomer

  Correct Answer: B

  Enantiomers like (?-Methadone and (+)-Methadone do not differ in their physical or chemical properties. They can only have different physiological effects by interacting with a stereospecific receptor ?a receptor that is itself chiral.

  Choice A is incorrect because (+)-Methadone would follow all of the morphine rules because it has the same connectivity of atoms as (?-Methadone.

  Choice C is incorrect because analgesic effects are physiological and would not depend on the rotation of light. The rotation of plane polarized light is used as a test for chirality. The light would rotate an equal degree measure, but in the

  opposite direction.

  Choice D is incorrect because the passage states that methadone is a synthetic alkaloid. Even if it was naturally occurring, Choice D might be true but would still not explain the difference in physiological activity.

• Question 507:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  Hofmann elimination involves methylation of the amine nitrogen followed by elimination (E2). Which of the following represents a possible product of one sequence of Hofmann elimination on Meperidine (demerol)?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Hofmann elimination proceeds through exhaustive methylation of the amine to the quaternary ammonium compound followed by elimination.

  

  Choice B shows a structure that has the double bond and the tertiary amine produced by Hofmann elimination. Note that the alkene bond could have formed on the other side of the nitrogen by an equally likely elimination giving the product shown below:

  

  Choice A is incorrect because the structure indicated is the intermediate of the Hofmann elimination. Choice A shows the structure following methylation but prior to elimination. Choice C is incorrect because the amine is not fully methylated (to the tertiary amine). Note the similarity between choice C and the valid elimination product shown above. Choice D is incorrect because it does not show the cleavage of the N-C bond that occurs in a Hofmann elimination (as described in the question stem).

• Question 508:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  Which of the following compounds would be most likely to have morphine-like biological effects?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  The passage states that there are certain requirements, the "morphine rules," that give a compound morphine-like biological activity. These requirements are an aromatic ring attached to a quaternary carbon and a tertiary amine situated two

  carbons away from that quaternary carbon. A quaternary carbon is a carbon bonded to four alkyl groups. A tertiary amine is bonded to three alkyl groups. The only structure that meets these requirements is structure D. The other answer

  choices look like the structures in the passage but do not meet all of the "morphine rules". Note that if the written description of the "morphine rules" is not clear enough, you can look at the figures in the passage as well. Choice A is incorrect

  because the aromatic ring is not attached to a quaternary carbon. It is attached to two secondary carbons.

  Choice B is incorrect because it lacks a tertiary amine. It has a secondary amine bonded to only 2 alkyl groups.

  Choice C is incorrect because the tertiary amine is not located two carbons away from the quaternary carbon. It is only one carbon away from the quaternary carbon.

• Question 509:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  Morphine can be reacted with 2 equivalents of ethanoyl chloride (acetyl chloride) to form heroin. In this reaction, the hydroxy groups of morphine function as:

  A. nucleophiles.

  B. electrophiles.

  C. leaving groups.

  D. Lewis acids.

  Correct Answer: A

  This reaction proceeds through nucleophilic attack by the hydroxy oxygen (of morphine) on the electrophilic carbonyl carbon of ethanoyl chloride. A nucleophile is an electron rich (Lewis Base) species that can attack an electrophile (electron poor species).

  

  Thus, the hydroxy groups on morphine act as nucleophiles.

  Choice B is incorrect because the carbonyl carbon is the electrophile in this reaction. The carbonyl group is polarized. Electrons are drawn towards the electronegative oxygen leaving the carbon with a slightly positive charge. Thus, the

  carbon is electron deficient and "wants" electrons. It is an electrophile. Choice C is incorrect because chloride acts as the leaving group in this reaction. Choice D is incorrect because Lewis acids are electron pair acceptors. The hydroxy

  group acts as an electron pair donor, a Lewis base.

• Question 510:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  How many chiral carbons are there in morphine?

  A. 4

  B. 5

  C. 6

  D. 7

  Correct Answer: B

  Any carbon bonded to four different substituents will be chiral. Morphine has five such carbons as shown below: Note that the nitrogen atom is also a chiral center. If nitrogen's lone pair of electrons is considered a fourth substituent bonded to nitrogen, it can be seen that nitrogen can also act as a chiral center. Some amines can invert configuration (and thus are not chiral). The tertiary amine of morphine cannot invert ?it is locked into its configuration by the ring structure. However, the question asks for the number of chiral carbons, not the number of chiral centers.