Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 551:

  Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols, such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and carboxylic acids.

  

  Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol, another naturally-occurring organic compound; this conversion is shown in Reaction B.

  Reaction A

  

  Reaction B

  

  What simple chemical test could be used to distinguish between the following two compounds?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Compound I is para-cresol or para-methylphenol, and Compound II is benzyl alcohol, which behaves more like an aliphatic alcohol than a phenol. We can take advantage of this fact in order to distinguish between these two compounds. One of the easiest ways to do this is by their solubility behavior, which is also an easy way of separating them out of a mixture. Phenols are appreciably acidic, so they are quite soluble in aqueous sodium hydroxide. Alcohols, including benzyl alcohol, are NOT acidic and won't be soluble in aqueous sodium hydroxide. Now there is one exception; very small alcohols, with fewer than five carbons, are water-soluble, so they would dissolve in ANY aqueous solution. However, benzyl alcohol is too big a molecule to be water-soluble, so it won't be soluble in sodium hydroxide solution. So, the solubility of paracresol, Compound I, in aqueous sodium hydroxide would provide an effective test to distinguish between the two compounds, and thus choice B is the correct answer. Notice that this means you could separate a mixture of these two compounds by dissolving it in an organic solvent, and then extracting the solution in a separatory funnel with aqueous layer. Choice C is wrong, because compound I won't react with a bromine solution and decolorize it. Both compounds will react under stronger conditions, namely, the presence of a Lewis acid, but bromine in solution is too mild a reagent to react with the stable aromatic ring. Therefore, compound I, and compound II for that matter, won't decolorize bromine solution and choice C is incorrect. Choice D is incorrect because neither of the two compounds will be soluble in sodium bicarbonate. Sodium bicarbonate is a weak base, but since phenols are fairly weak acids, it takes a fairly STRONG base to make a phenol give up its acidic proton. Thus, para-cresol won't dissolve in sodium bicarbonate. As we've said, benzyl alcohol is an even weaker acid than para-cresol, so it certainly won't dissolve in sodium bicarbonate. And since neither compound will be soluble in sodium bicarbonate, sodium bicarbonate can't be used to distinguish between them.

• Question 552:

  Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols, such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and carboxylic acids.

  

  Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol, another naturally-occurring organic compound; this conversion is shown in Reaction B.

  Reaction A

  

  Reaction B

  

  The reaction of phenol with dilute nitric acid produces which of the following compounds?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  To answer this question, you need to know that hydroxyl groups are electron-donating and so they activate aromatic rings towards electrophilic aromatic substitution. Like all electrondonating groups, hydroxyl groups are ortho-para directors. So in this case, the nitro group will add to phenol to form two products, ortho-nitrophenol and para-nitrophenol. This means choice A is the correct answer. Choice B, which has only para-nitrophenol, is incomplete and therefore wrong. Choices C and D can be eliminated because they both include meta-nitrophenol, which will NOT be formed. The only way to get meta-nitrophenol would be to start with nitrobenzene and turn it into a phenol, which would work because the nitro group is an electron-withdrawing, deactivating meta director. The reaction will produce only negligible amounts of meta-nitrophenol, if any.

• Question 553:

  Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols, such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and carboxylic acids.

  

  Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol, another naturally-occurring organic compound; this conversion is shown in Reaction B.

  Reaction A

  

  Reaction B

  

  Which of the following shows the order of decreasing acidity among the four compounds below?

  

  A. I, III, IV, II

  B. IV, I, II, III

  C. IV, III, II, I

  D. IV, II, I, III

  Correct Answer: D

  In order to answer this question, you need to know what characteristics of a substituted phenol will tend to increase its acidity. The more electron-withdrawing groups a phenol has, the more any negative charge can be dispersed and stabilized by resonance. Since resonance stabilization stabilizes the phenoxide ion more than the phenol, increased resonance stabilization will make a phenol more acidic. Looking at the four phenols, three of them have varying numbers of nitro groups, which you should know are strongly electron-withdrawing, and the fourth has a methyl group, which is electron-donating and should have the opposite effect from the nitro groups. So trinitrophenol, roman numeral four, which has the most nitro groups, is the most acidic, followed by dinitrophenol, which is roman numeral two, then para-nitrophenol, which is roman numeral one and, finally, para-cresol, roman numeral three, with the methyl group.

• Question 554:

  Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols, such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and carboxylic acids.

  

  Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol, another naturally-occurring organic compound; this conversion is shown in Reaction B.

  Reaction A

  

  Reaction B that phenol is more acidic than cyclohexanol. Which of the following explain(s) the acidity of phenol?

  

  I. The exceptionally strong hydrogen bonding possible with phenol facilitates the loss of a proton, making it more acidic than cyclohexanol.

  II. Phenol's conjugate base, phenoxide, is stabilized by resonance to a greater extent than phenol itself.

  III.

  The negative charge of the oxygen atom on the phenoxide ion is delocalized over the benzene ring.

  A.

  I only

  B.

  II only

  C.

  II and III only

  D.

  I, II, and III

  Correct Answer: C

  To answer this, you need to understand the resonance stabilization effect that makes phenols more acidic than aliphatic alcohols, such as cyclohexanol. In the phenoxide ion, the negative charge on the oxygen is dispersed throughout the benzene ring which, in effect, acts like an "electron sink," withdrawing electron density. This delocalizing charge effect, as it's called, stabilizes the phenoxide ion, and is described by Statement III, which therefore must be included in the correct answer. So you can eliminate the choices that don't include III, which are choices A and B. Now, the phenoxide ion has more possible resonance structures than the undissociated phenol, which still has its proton; thus Statement II is also correct. As for Statement I, it's factually true -- that is, phenols CAN hydrogen-bond more strongly than aliphatic alcohols, as indicated by the higher boiling points mentioned in the passage -- but this is really a consequence, not a cause, of phenol's greater stability. So even though Statement I is true, it doesn't explain phenol's acidity, and so choice D, which includes Statement I, is incorrect.

• Question 555:

  Alleles are created when a single gene undergoes several distinct mutations. These alleles may have different dominance relationships with one another; for example, there are three alleles coding for the human blood groups, the IA, IB, and i alleles. Both the IA and IB alleles are dominant to the i allele, but IA and IB are codominant to each other.

  A multiple-allele system has recently been discovered in the determination of hair coloring in a species of wild rat. The rats are found to have one of three colors: brown, red, or white. Let B = the gene for brown hair; b = the gene for red hair; and w = the gene for white hair. The results from nine experimental crosses are shown below. The males and females in Crosses 1, 2, and 3 are all homozygous for hair color.

  

  If it were discovered that the alleles for red and white hair were actually incompletely dominant and produced a pink hair color in rats with one copy of each allele, what would be the expected phenotypic ratio in a cross between a Bb male and a pink female?

  A. 2 brown : 1 red : 1 white

  B. 2 brown : 1 red : 1 pink

  C. 1 brown : 2 white : 1 pink

  D. 1 brown : 1 white

  Correct Answer: B

  What we need to do is figure out the genotype of the pink female. That's not so hard, since the question stem tells us that rats with pink hair have one copy of the red allele -- little b, and one copy of the white allele -- w. So, the genotype of the pink female would be little b, w. So if this pink female is crossed with a brown male, the cross is big B, little b ?little b, w. If we work out the Punnett square, 25% of the progeny would be big B, little b, which is phenotypically brown; 25% would be big B, w, which is also brown; 25% would be little b, little b, which is red; and finally 25% would be little b, w, which is pink if these two alleles are incompletely dominant. Thus, the phenotypic ratio of the offspring is 2 Brown: 1 Red : 1 Pink, which is choice B.

• Question 556:

  Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols, such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and carboxylic acids.

  

  Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol, another naturally-occurring organic compound; this conversion is shown in Reaction B.

  Reaction A

  

  Reaction B

  

  Reaction A is an example of:

  A. a free radical substitution.

  B. an electrophilic aromatic substitution.

  C. an electrophilic addition.

  D. a nucleophilic aromatic substitution.

  Correct Answer: B

  Reaction A is an electrophilic aromatic substitution reaction, in which thymol is formed from meta-cresol. Both the methyl and hydroxyl substituents in meta-cresol are ortho-para directing activators. However, hydroxyl is the more powerful of the two and if you look at reaction A you can see that substitution occurs ortho to the hydroxyl group. However, we are not too concerned about substituent effects but rather the mechanism of the reaction, so let's briefly review what happens. Initially, phosphoric acid abstracts an electron from propene creating a secondary carbocation. This carbocation then acts as an electrophile and adds to the electron rich benzene ring, ortho to the hydroxyl group. This results in the formation of an arenium ion and aromaticity is regained by loss of a proton, generating thymol. Therefore, this mechanism is electrophilic aromatic substitution -- making choice B the correct answer. From this mechanism, it should be pretty easy to eliminate the other answer choices. Choice C is wrong, because although the carbocation adds to the ring, a proton is lost in order to regain aromaticity. Therefore, this is an example of substitution NOT addition. Choice D is wrong because the carbocation which adds to the ring is acting as an electrophile, not a nucleophile. Remember that benzene is electron rich, and the substituents on meta- cresol enhance this, so there is no way that meta-cresol could be susceptible to nucleophilic attack. Finally, choice A -- free radical substitution -- is also wrong. There is nothing in reaction A to suggest that free radicals are formed and phosphoric acid will not induce radical formation.

• Question 557:

  Alleles are created when a single gene undergoes several distinct mutations. These alleles may have different dominance relationships with one another; for example, there are three alleles coding for the human blood groups, the IA, IB, and i alleles. Both the IA and IB alleles are dominant to the i allele, but IA and IB are codominant to each other.

  A multiple-allele system has recently been discovered in the determination of hair coloring in a species of wild rat. The rats are found to have one of three colors: brown, red, or white. Let B = the gene for brown hair; b = the gene for red hair; and w = the gene for white hair. The results from nine experimental crosses are shown below. The males and females in Crosses 1, 2, and 3 are all homozygous for hair color.

  

  A white male is crossed with the heterozygous red female from Cross 9. What is the expected ratio of red to white offspring?

  A. 3:1

  B. 1:3

  C. 1:1

  D. 2:1

  Correct Answer: C

  This question is really quite simple because we're told that the red female has the heterozygous genotype, which we know is little b, w. We don't have to look at the progeny of Cross 9 and try to work backwards from there. Since white fur is recessive, the white male in the cross must have the genotype ww. So the cross is little b, w ?w, w. 50% of the offspring have the genotype b, w, which phenotypically, is read fur; the other 50% of the offspring have the genotype w, w, which corresponds to white fur. Thus, the ratio of red offspring to white offspring is 50% to 50%, which is the same as 1:1, choice C.

• Question 558:

  Alleles are created when a single gene undergoes several distinct mutations. These alleles may have different dominance relationships with one another; for example, there are three alleles coding for the human blood groups, the IA, IB, and i alleles. Both the IA and IB alleles are dominant to the i allele, but IA and IB are codominant to each other.

  A multiple-allele system has recently been discovered in the determination of hair coloring in a species of wild rat. The rats are found to have one of three colors: brown, red, or white. Let B = the gene for brown hair; b = the gene for red hair; and w = the gene for white hair. The results from nine experimental crosses are shown below. The males and females in Crosses 1, 2, and 3 are all homozygous for hair color.

  

  Based on the experimental results, what is the genotype of the female in Cross 5?

  A. Bb

  B. BB or Bb

  C. BB or Bw

  D. BB, Bb, or Bw

  Correct Answer: D

  In Cross 5, a brown male is mated with a brown female, and we're told that 100% of their offspring are brown. Since we know that the big B allele is dominant to the little b and w alleles, there are three types of genotypically distinct crosses between two brown parents that result in all brown progeny; again, you might want to write these down to avoid confusion: (1) big B, big B ?big B, big B; (2) big B, big B ?big B, w; and (3) big B, big B ?big B, little b. From this we see that at least one of the parents must be big B, big B. If the female is big B, big B, then the male can be big B, big B; big B, little b; or big B, w. Likewise, if the male is b B, big B, then the female can be big B, big B; big B, little b; or big B, w.

• Question 559:

  Alleles are created when a single gene undergoes several distinct mutations. These alleles may have different dominance relationships with one another; for example, there are three alleles coding for the human blood groups, the IA, IB, and i alleles. Both the IA and IB alleles are dominant to the i allele, but IA and IB are codominant to each other.

  A multiple-allele system has recently been discovered in the determination of hair coloring in a species of wild rat. The rats are found to have one of three colors: brown, red, or white. Let B = the gene for brown hair; b = the gene for red hair; and w = the gene for white hair. The results from nine experimental crosses are shown below. The males and females in Crosses 1, 2, and 3 are all homozygous for hair color.

  

  If a large number of brown offspring from Cross 8 are mated with each other, what is the expected percentage of white offspring?

  A. 6.25%

  B. 8.33%

  C. 12.5%

  D. 25%

  Correct Answer: A

  Cross 8 is between a brown male and a red female. The fact that 25% of their offspring are white indicates that both parents are heterozygotes, since white fur is a recessive trait. This means that the genotype of the brown male must be big B, w, and the genotype of the red female must be little b, w. Now we need to figure out the genotypes of the brown offspring. A cross between this brown male and red female results in 25% ww offspring, which are white; 25% little b, w, which are red; 25% big B, w, which are brown; and 25% big B, little b, which are brown. So there are two different brown genotypes in the offspring -- big B, little b and big B, w. If a large number of these brown offspring are mated with each other there are four different crosses possible, and you might want to write them down to avoid confusion; (1) big B, little b ?big B, little b; (2) big B, w ?big B, w; (3) big B, little b ?big B, w; and don't forget (4) big B, w ?big B, little b -- because this is the second way that the third cross can occur. Now we need to figure out what percentage of the offspring produced in these crosses will have white fur. Well, if we work out the Punnett squares, we find that neither the first, the third, nor the fourth crosses yield any white rats. On the other hand, the second cross, big B, w ?big B, w, yields 25% white offspring. But you're not through yet; 25% is not your answer. You have to take into account that only one fourth of the total number of possible crosses between two brown rats yields 25% white offspring. Well, one fourth of 25% is 6.25%, which is the correct answer, choice A.

• Question 560:

  A student was given a sample of an unknown liquid and asked to determine as much as possible about its structure. He was told that the compound contained only carbon, hydrogen, and oxygen, and had only one type of functional group. The student found its boiling point to be 206. Using mass spectroscopy, he determined its molecular weight to be 138 g/mol. Finally, he took the infrared spectrum of the compound, which is shown below.

  

  From this spectrum, the student quickly reached a conclusion about the functional group. He then turned his attention to the fingerprint region of the compound, which generally has a complicated pattern of peaks that are determined by the structure of the hydrocarbon portion of a molecule. The student decided that the large peak at 750 cm-1 must indicate that this was a disubstituted aromatic compound.

  The student decides to carry out some simple tests on the compound in order to confirm his identification. Which of the following statements is NOT true?

  A. He could distinguish between a phenol and a benzoic acid by seeing if the unknown can be extracted with a weak base.

  B. He could distinguish between a benzyl aldehyde and a benzyl ketone by seeing if the unknown will react with cold KMn.

  C. He could distinguish between a benzyl alcohol and a phenol by attempting to dissolve the unknown in an aqueous solution of HCl.

  D. He could distinguish between a benzyl alcohol and a benzoic ester by attempting to dissolve the unknown in an aqueous solution of NaOH.

  Correct Answer: C

  This answer choice is false because neither one will dissolve in aqueous hydrochloric acid. You could use a strong base to distinguish between these two and then the slightly acidic phenol would deprotonate and dissolve as an ion in the aqueous solution. The alcohol, on the other hand, remains uncharged and would not dissolve in the aqueous solution because of its large organic phenyl group. The other three choices are all true statements. For choice A, the benzoic acid will become deprotonated, while the phenol won't. For choice B, the aldehyde will be oxidized, yielding a carboxylic acid, while the ketone won't. Finally, for choice D, sodium hydroxide will hydrolyze the ester-so it will dissolve, while benzyl alcohol will remain insoluble.