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?
Answer(s): 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.
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?
Answer(s): 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.
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 AReaction BReaction A is an example of:
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.
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 AReaction B Comparing the Ka values for cyclohexanol (Ka = 10-13) and phenol (Ka = 1.3 × 10-10) reveals 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.
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.
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 AReaction BWhich of the following shows the order of decreasing acidity among the four compounds below?
Answer(s): 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.
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