MCAT Biology : Organic Chemistry, Biochemistry, and Metabolism
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Example Question #25 : Reaction Types
Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:
A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.
In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride 
When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more 
Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.
When a patient is not ventilating efficiently, the kidneys try to compensate by producing more bicarbonate to buffer the decreasing pH of the blood due to the buildup of 
I. Increase
II. Increase 
III. Increase metabolism
I only
I and III
III only
II only
I, II, and III
I and III
To answer this question, we must understand Le Chatelier’s principle. The reaction favorites the product when there is more of the reactants. The opposite is also true in that when there is a buildup of the products, the reaction will reverse and favorite the reactants. Increasing the metabolism will produce more
Example Question #26 : Reaction Types
Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:
A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.
In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride 
When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more
Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.
Based on the passage, which of the following statements, if true, will contradict the effectiveness of carbonic anhydrase inhibitors as a treatment?
Acidic molecules have a better ability to cross the membrane than basic molecules
Basic molecules will release its proton in the basic environment of the lumen of the collecting tubule
Basic Molecules will not release its proton in the basic environment of the lumen of the collecting tubule.
Acidic molecules will release its proton in the basic environment of the lumen of the collecting tubule
Acidic molecules will not release its proton in the basic environment of the lumen of the collecting tubule
Acidic molecules will not release its proton in the basic environment of the lumen of the collecting tubule
Carbonic anhydrase inhibitors are used to alkalinize the urine. When the urine is alkalinized, acidic molecules will lose its proton and go into a charged state. Charged molecules are unable to cross the membrane of the lumen of the collecting tubule. Without the ability to cross the membrane, the molecule is therefore unable to be reabsorbed. Therefore, if the statement "Acidic molecules will not release its proton in the basic environment of the lumen of the collecting tubule" was true, then alkalinizing the urine will have no effect.
Example Question #1 : Substitution And Elimination Mechanisms
Which of the following reactions is the nucleophile potassium tert-butoxide often used for?
E1
SN1
SN2
E2
E2
Tert-butoxide is a large, sterically hindered, strong nucleophile that is often used in E2 reactions. Strong nucleophiles usually undergo the SN2 or E2 pathway, but tert-butoxide is much too large to undergo a substitution reaction.
Example Question #25 : Organic Chemistry
Which of the following factors do NOT favor an SN2 reaction of an alkyl halide?
A tertiary carbocation
A primary halide
A polar aprotic solvent
A good nucleophile
A tertiary carbocation
The way the question is phrased, three answer choices must favor an SN2 reaction, while the "correct" answer is a factor that does not favor, or disfavors an SN2 reaction.
SN2 reactions are bimolecular, and thus their rate of reaction depends on both the substrate and the nucleophile, forming a high energy transition state in which the nucleophile will displace the substate's leaving group at an angle of 180o. The more sterically hindered the compound is, the higher in energy the transition state will be, and the slower the rate of reaction will be. Consequently, SN2 reactions are favored when the leaving group (a halogen in this case) is on a primary carbon center. Additionally, because the reaction is bimolecular, step two of the reaction will NOT occur without a good nucleophile to displace the leaving group. Finally, all SN2 reactions are favored by polar aprotic solvents.
Because SN2 reactions proceed via a transition state, no carbocation intermediate is formed (that happens in SN1 reactions) and therefore the formation of any carbocation favors an SN1 reaction, not an SN2 reaction.
Example Question #3 : Substitution And Elimination Mechanisms
Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.
Reaction 1:
Reaction 2:
The reaction depicted in reaction 1 takes place in solution with a solvent. What type of solvent is most likely to be preferred for the reaction to occur as written?
This reaction requires water as a solvent
Polar, protic solvent
Nonpolar, aprotic solvent
Polar, aprotic solvent
Nonpolar, protic solvent
Polar, aprotic solvent
Reaction 1 is an SN2 reaction. This type of substitution reaction prefers a polar, aprotic solvent. The polarity helps to solvate the nucleophile. Aprotic solvents help mediate the transition state and increase reaction rate.
Example Question #1 : Substitution And Elimination Mechanisms
Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.
Reaction 1:
Reaction 2:
In reaction 1, a scientist is trying to modify the reaction by using a weaker nucleophile. Which of the following is a weaker nucleophile than what is used above (hydroxide ions)?
Nucleophilicity increases to the left on the periodic table. Nucleophilicity will also generally increase with charge. The only equally charged ion in the answers that is present to the right of oxygen on the periodic table is the fluoride ion.
Example Question #1 : Substitution And Elimination Mechanisms
Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.
Reaction 1:
Reaction 2:
Using the product of reaction 2, a scientist adds bromine gas to the reaction chamber. After the bromine and the alkene react, he finds that his product consists entirely of single bonds, with two bromine atoms on the carbon chain. What kind of reaction most likely took place?
Halogenation reaction
Substitution reaction
Addition reaction
Elimination reaction
Oxymercuration/demercuration reaction
Addition reaction
The addition of bromine gas (
Halogenation reactions refer to reactions between a halogen and an alkane, while addition reactions occur between a halogen and an alkene (such as the product in reaction 2).
Example Question #6 : Substitution And Elimination Mechanisms
Students in an organic chemistry lab perform two E2 elimination reactions, using compounds 1 and 2. The students observe that while compound 2 undergoes elimination with reagent X (not shown), compound 1 is unreactive. What is the best explanation for this discrepancy?
Compound 1 has no hydrogen molecules anti-periplanar to the bromine leaving group
Bromine is not a good enough leaving group
Reagent X is more selective for compound 2 than for compound 1
Compound 1 is too sterically hindered
Compound 1 has no hydrogen molecules anti-periplanar to the bromine leaving group
This question tests your understanding on E2 elimination reactions. Because E2 reactions have no carbocation intermediates, the reaction proceeds through a transition state. The bromine atom (Br) must be displaced by the bond between an adjacent carbon and hydrogen, and that hydrogen must be anti-periplanar to the bromine. Compound 2, unlike compound 1 has one hydrogen that is anti-periplanar to the bromine, and thus undergoes an E2 elimination.
If compound 2 reacts, it is unlikely the reason that compound 1 would not react is because of steric hindrance. Additionally, aside from stereochemistry, there is nothing different enough about the two compounds that would suggest any reagent would be more favorable for one over the other.
Example Question #1 : Substitution And Elimination Mechanisms
Which of the following compounds could NEVER undergo an E2 reaction when treated with potassium tert-butoxide?
3-methyl-3-iodopentane
Cyclopentylbromide
Bromoethane
Cis-2-bromo-1-methylcyclohexane
Benzylbromide
Benzylbromide
For an E2 reaction to occur, there must be a hydrogen on the carbon adjacent to the carbon with the leaving group. Benzyl bromide contains no hydrogens on the carbon next to the carbon with the bromide, and would therefore undergo only a substitution reaction.
Example Question #1 : Substitution And Elimination Mechanisms
Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.
Reaction 1:
Reaction 2:
In reaction 2, which of the following describe the rate limiting step?
I. It involves the formation of carbocation
II. It is favored by the presence of substituents on the central carbon
III. It involves a transition state, but no intermediate
I, only
III, only
II, only
II and III
I and II
I and II
Reaction 2 represents an E1 reaction. The rate limiting step of reaction 2 involves the formation of a carbocation, whose stability is favored by the presence of substituents on the carbon involved. Carbocations are considered intermediates due to their relative stability compared to transition states.
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