The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction. Another method for forming esters is:RCOO- + RX RCOOR + X-Why does this reaction occur?
Answer(s): D
This question asks you to explain why a reaction occurs between carboxylic acids and alkyl halides. This reaction is an example of a nucleophilic substitution reaction with the carboxylate ion serving as the nucleophile and the leaving group. On the basis of this information, choice D is the correct response. Choice A is wrong because, in this case, the halide is a very good leaving group. Choice B is wrong because the halide is not the nucleophile but the leaving group. And finally, choice C is incorrect because the leaving group is an electron withdrawing group by virtue of its electronegativity.
The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction. Which of the following alkyl halides would be most likely to react with sodium butanoate to form an ester?
Answer(s): B
In this question, you are asked to decide which alkyl halide would be most likely to react with sodium butanoate and form an ester. The reaction between a carboxylate anion and an alkyl halide is an SN2 reaction. Because this reaction involves partial bonding between the attacking nucleophile (carboxylate) and substrate (alkyl halide), it will not occur easily with bulky substrates. Bulky substrates sterically hinder the reaction, by shielding the susceptible carbon atom from the attacking nucleophile. Choice B, methyl chloride, has only hydrogen and chlorine substituents, whereas all the other choices have bulkier alkyl substituents. This makes B the least sterically hindered of all the choices, and it will therefore react most easily with sodium butanoate.
Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen.While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.In a paternity case, the mother has type A+ blood and her son has type O- blood. If the husband has type B+ blood, which of the following is true?
Answer(s): A
Let's determine as much as we can about the blood types of the parents and the son. We know that the mother is blood type A positive, and therefore one of her alleles codes for the A antigen. We don't know whether the other allele for that locus is an O or A. The mother is also Rh positive, and again, similar reasoning applies here. We know that she has at least one Rh allele that codes for the Rh antigen, which makes her Rh positive, since the Rh allele is dominant. We don't know about the second allele at that locus; it might be either Rh positive of Rh negative. Let's take a look at the son. The son is O negative, so here we know his genotype precisely. His genotype for the ABO blood groups is OO. Likewise, with respect to the Rh locus, he's doubly negative. If he had just one Rh allele he would be Rh positive, since the allele is dominant. So the son's genotype must be OO Rh negative Rh negative. Working backwards, we can deduce that his mother's genotype must be AO Rh positive Rh negative, since the son inherited one allele per locus from his mother.Let's take a look at the husband. The husband is type B positive, so we know that he has to have at least one B allele and one Rh allele. In the paternity case the question to be addressed is, is it possible for the husband to be the father of this boy, knowing what we know about the mother? The answer is yes, it's possible. We know that the mother's genotype is AO Rh positive, Rh negative; now suppose that the husband's blood type is BO Rh positive Rh negative. If this were the case, then the son could have inherited one O allele from each parent and one Rh negative allele from each parent. Therefore, it is possible for these two people to conceive an O negative child. thus, the correct answer is choice A; the husband could have been the father. We can't answer D, that the husband was definitely the father, because we just don't know; maybe the mother had an affair with another man whose blood type contained both the O allele and the Rh negative allele. From our discussion, choices B and C are obviously incorrect. And besides, the gender of the child has no influence whatsoever on blood type.
Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen.While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.A couple decide to have a child. If the father's genotype is AO and the mother has type B blood of unknown genotype, which of the following are possible blood types for their child?I). AII). BIII). A, BIV. O
This is one of those questions requiring an understanding of simple genetics and the ABO system. If you didn't already know it, you're told in the passage that the A and B alleles are codominant to each other, and that the O allele is recessive. Codominance means that both the alleles are phenotypically expressed. So, when a person has both the A and the B alleles, a person is said to have type AB blood and expresses the properties ascribed to BOTH alleles that is, their red blood cells have both the A and B antigens on their surface. During sexual reproduction, the mother and father each donate one allele to their offspring. In this case we know the father's genotype is AO. This means that he can donate either an A allele or an O allele to his child. We don't, HOWEVER, know the mother's genotype; we only know that her phenotype is type B blood. Well, this means that her genotype is either BO or BB we simply don't know which one it actually is. Let's first assume the mother to have the genotype BB and must therefore donate a B allele to the child. In this situation, if the father donates an A, the child's genotype and phenotype will b ABk. If the father donates an O allele, the child's genotype will be BO and the phenotype will be type B. Therefore, statements II and III are correct. Well, since III is correct, you can rule out choices A and C because they don't contain it. Now let's assume the mother's genotype to be BO. This means that she can donate an O allele to the child. In this case, if the father donates an A allele, the child's genotype will be AO and the phenotype will be A. This means that statement I is also correct; but this doesn't help you decide between choices B and D because they both contain statement I). In fact, you should have known that I was correct because it appears in both of these remaining choices. So what it comes down to is whether or not this child could have type O blood. Well, if the father donates an O allele, the child's genotype will be OO and the phenotype could have type O. This means that statement IV is also correct; all four blood types are possibilities.
Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen.While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk. The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby's blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother's serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby's red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby's red blood cells must have been attacked by its mother's anti-Rh antibodies.A new virus has been discovered that evades detection by the immune system of only those individuals with type A or type AB blood. Which of the following best accounts for this observation?
From this question stem you know that this virus avoids detection by the immune system of only those individuals who have type A or AB blood. From this fact you can conclude that the virus must NOT evade the immune system of people with B or O blood types. So the correct answer will differentiate between these two groups. Well, from this piece of information you can eliminate choice D. If the viral antigens are too small to elicit an immune response, the virus would evade all immune systems, not just those of individuals with A or AB blood types. Choice C can also be eliminated since the presence of the Rh factor is independent of ABO blood type. So now we need to decide between choices A and B. How do type A and type AB blood differ from type B and type O blood. Well, individuals with type A and AB blood express the A antigen on the surface of their red blood cells. These individuals will therefore NOT produce antibodies to the A antigen. On the other hand, individuals with type B and O blood do NOT express the A antigen, thus they will produce antibodies to the A antigen. Well if the virus' antigen mimicked the A antigen, then people who normally express the A antigen that is, people with type A or type AB blood would not recognize the virus as foreign. Thus, no immune response would be elicited, and the virus would be able to persist in these people. Thus, choice A is the correct answer. If the viral antigens mimicked the B antigen, as in choice B, then the virus would evade detection by the immune system in those individuals who normally expressed the B antigen type B and type AB people, but would be detected by the immune system of those individuals who do not normally express the B antigen, namely, type A and type O people.
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