(1) Recall the different types of intermolecular interactions. (2) Look at the structure and determine the correct answer.

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1 MCAT rganic Chemistry - Problem Drill 11: Carboxylic Acids Question No. 1 of 10 Question 1. What kinds of interactions are holding together the carboxylic acid dimer shown? Question #01 3 C C 3 (A) Van der Waals interactions (B) London dispersion forces (C) Dipole-dipole interactions (D) ydrogen bonds Van der Waals interactions are another name for London dispersion forces. They are relatively weak forces found in nonpolar molecules. Look at the atoms involved in the interactions above and review the different types of intermolecular interactions. London dispersion forces are another name for Van der Waals interactions. They are relatively weak forces found in nonpolar molecules. Look at the atoms involved in the interactions above and review the different types of intermolecular interactions. Dipole-dipole interactions are found in polar molecules like acids. owever, there are specific types of interactions holding the molecules of this dimer together. Look at the atoms involved in the interactions above and review the different types of intermolecular interactions. D. Correct! Carboxylic acids can act both as hydrogen bond donors and acceptors. This is the case in this dimer. Each carbonyl oxygen is acting as a donor while each hydroxyl group acts as an acceptor. (1) Recall the different types of intermolecular interactions. We learned in an earlier tutorial there were three main types of intermolecular interactions: London dispersion forces, dipole-dipole interactions, and hydrogen bonds. London dispersion forces (AKA Van der Waals forces) are commonly found in nonpolar molecules and are relatively weak. Dipole-dipole interactions are found between polar molecules and are stronger than London dispersion forces. ydrogen bonds, a special type of dipole-dipole interaction, occur between molecules with, -N or F. These are fairly strong interactions. (2) Look at the structure and determine the correct answer. Looking at the structure, we can see the molecules contain hydroxyl groups. It appears that the hydrogen of the hydroxyl group is being shared between the oxygens of the carbonyl and hydroxyl groups. This is indicative of a hydrogen bond. Therefore, the correct answer is (D).

2 Question No. 2 of 10 Question 2. Which functional group below is most acidic? Question #02 (A) Carboxylic acids (B) Alcohols (C) Anhydrides (D) Acid chlorides A. Correct! f the functional groups given, carboxylic acids are the most acidic. While alcohols are weakly acidic, there is a functional group listed that is more acidic than an alcohol. Review the acidity table in the tutorial or consult one that is the appendix of an organic chemistry textbook. While it is possible for anhydrides to have acidic α-hydrogens, they themselves do not contain an acidic hydrogen. Review the acidity table in the tutorial or consult one that is the appendix of an organic chemistry textbook. While it is possible for acid halides to have acidic α-hydrogens, they themselves do not contain an acidic hydrogen. Review the acidity table in the tutorial or consult one that is the appendix of an organic chemistry textbook. (1) Consider the structure of each functional group listed. Do any of them contain an acidic hydrogen? The best way to approach this problem is to first draw out the functional groups listed: R R R R R Look for acidic hydrogens in each structure. Both carboxylic acids and alcohols contain acidic hydrogens (indicated with arrows). While anhydrides and acid chlorides may have acidic α-hydrogens, these functional groups do not contain an acidic hydrogen. We now have the list of possible answers narrowed down to two: carboxylic acids or alcohols. It is a good idea to memorize the relative pkas of acidic hydrogens in various functional groups. If you recall, the carboxylic acids are more acidic (have a lower pka) than alcohols. In general, carboxylic acids have a pka around 5 whereas alcohols are generally around 15. But what if you didn t have the relative pkas memorized? Could you still have arrived at the correct answer? Yes, by looking at the anions formed when an acidic hydrogen is removed from a structure. When you deprotonate an alcohol, you form an alkoxide ion: R Notice the negative charge is localized on the oxygen. When you deprotonate a carboxylic acid, you form a carboxylate anion: R R δ R δ Notice the negative charge is delocalized through resonance. This means the carboxylate anion is resonance stabilized. Since its anion is more stable than the alkoxide ion, carboxylic acids should be more likely to give up a proton. Therefore, the correct answer is (A).

3 Question No. 3 of 10 Question 3. Which compound shown will have the lowest pka? Question #03 Br F Consider the inductive electron withdrawing effects of halogens. Which halogens would have the strongest electron withdrawing effects? Also, does the number of halogens present affect the acidity of the compound? Remember that pkas have an inverse relationship with acidity. The lower the pka, the stronger the acid. Also consider the inductive electron withdrawing effects of halogens. Which halogens would have the strongest electron withdrawing effects? C. Correct! The combined inductive electron withdrawing effects of the chlorine and fluorine, two very electronegative atoms, make this acid more acidic than the others. Consider the inductive electron withdrawing effects of halogens. Which halogens would have the strongest electron withdrawing effects? This is governed by the electronegativity of the atoms. (1) Consider the trend the question is implicitly asking about. The question asked which of the four given compounds is most acidic. When you survey the compounds, you notice that all are carboxylic acids that are substituted with halogens. This question is testing you on your knowledge of how halogen substituents affect the acidity of carboxylic acids. (2) Recall the inductive effects of halogens on the acidity of compounds. alogens are electronegative and can exert electron withdrawing effects in molecules. In this case, the electron withdrawing effect can help stabilize the carboxylate anion that forms when the acid loses its acidic hydrogen. Anything that stabilizes the anion that forms from deprotonation will increase the acidity of the proton. Remember that the more electronegative the atom, the stronger the electron withdrawing effect. Also, this effect falls off with distance so the further the electronegative atom is from the acid group, the less it can stabilize the anion that results from deprotonation. (3) Based on this information, choose the correct answer. II should be the least acidic since it contains a bromine atom that is three carbons away from the acid group. I should be more acidic than II as it contains a chlorine atom (which is more electronegative than bromine) that is one carbon closer to the acid group. IV will be more acidic than I since there are two chlorines alpha to the acid group instead of one. III should be the most acidic of all four compounds since it has a chlorine and a fluorine (the most electronegative atom) alpha to the acid group. Remember, the question asked for the compound with the lowest pka. The pka values indicate acidity with an inverse relationship: the lower the pka, the stronger the acid. So compound III is the most acidic and should possess the lowest pka of all four compounds. Therefore, the correct answer is (C).

4 Question No. 4 of 10 Question 4. Which compound shown corresponds to 4-ethyl-3-methoxyhexanoic acid? 3 C 3 C 3 C C 3 Question #04 While this compound contains a 6 carbon carboxylic acid and a methoxy group at the appropriate carbon, it does not have an ethyl group at carbon 4. Go back and review the IUPAC nomenclature rules. B. Correct! This compound is a carboxylic acid with a chain of 6 carbons. There is an ethyl group at carbon 4 and a methoxy substituent at carbon 3. While this compound contains a 6 carbon carboxylic acid, it does not contain a methoxy group at carbon 3 and also does not have an ethyl group. Go back and review the IUPAC nomenclature rules. This compound is an acid halide. The suffix anoic acid indicates a carboxylic acid will be the main functional group in the correct answer. Go back and review carboxylic acid IUPAC nomenclature. (1) Examine the IUPAC name given in the problem. Identify the functional group in the parent chain from the given name. ere, the parent ends in anoic acid. This is the suffix for carboxylic acids. Based on this fact, we can already eliminate one choice. IV is an acid chloride and not a carboxylic acid so it cannot be the correct answer. The other three choices are acids so we can move forward with them. (2) Identify the number of carbons in the parent chain. In this problem the parent has a prefix of hex- which means there are 6 carbons in the longest chain. If you number the longest chain in each of the remaining answers, you will find we cannot eliminate any other choices as they all have a chain of 6 carbons. (3) Identify the type of substituents present from the given name. Two substituents are given in the IUPAC name. We can expect the structure of the molecule to include both an ethyl group and a methoxy group. I, II, and III all have a methoxy group. owever, I and III each have a methyl group present rather than an ethyl group. So they can be eliminated leaving II. (4) Identify the location of the substituents/functional groups from the given name. From the given name, we can determine the correct molecule will have a carboxylic acid at carbon 1, a methoxy group at carbon 3, and an ethyl group at carbon 4. This leaves II as the only possible answer. Therefore, the correct answer is (B).

5 Question No. 5 of 10 Question 5. What is the product of the reaction shown? MgBr 1. C Question #05 Br ose! Go back and review the reactants in this reaction. What is added to the starting material in this reaction? An alcohol is not the final product of this reaction. Go back and review the reactions covered in this tutorial. C. Correct! The final product is a carboxylic acid. This one has 5 carbons in its longest chain. A carbon was added to the chain when the Grignard reagent attacked carbon dioxide. An acid halide is not the final product of this reaction. Go back and review the reactions covered in this tutorial. (1) Recall the reactions covered in this tutorial. The tutorial covered reactions to synthesize carboxylic acids and the reactions carboxylic acids undergo. (2) Determine what kind of reaction is taking place. To determine what kind of reaction is taking place, you will first need to identify the reactants being used. The starting material is an organomagnesium compound (AKA Grignard reagent). The other reactants are carbon dioxide and aqueous acid. You may have already recognized this reaction as the carboxylation of a Grignard reagent to form a carboxylic acid. If you did not, go back and review the reactions in this tutorial. It is not enough to just recognize the type of reaction taking place. You must also understand the mechanism in order to accurately predict the product. Recall that the reaction begins when carbon of the Grignard reagent attacks carbon dioxide to form a carboxylate anion. This step will add one carbon to the alkyl chain of the Grignard reagent. In the last step the carboxylate anion is protonate by the aqueous acid to give a carboxylic acid as the final product. (3) Predict the product(s). Based on the known mechanism of this reaction, we can predict the final product should be a carboxylic acid. We can eliminate II and IV since they are not carboxylic acids. Look closely at I and III. ow do they differ? I has 4 carbons in its longest chain while III has 5 carbons. ow many carbons were in the alkyl group of the Grignard reagent? Four carbons were present. We know from the mechanism of the reaction that one more carbon is added to the alkyl portion of the Grignard so we are looking for a product that will have 5 carbons in the longest chain. Therefore, the correct answer is (C).

6 Question No. 6 of 10 Question 6. What is the product of the reaction shown? N 3 + Question #06 A. Correct! This reaction is the hydrolysis of a nitrile. The final product is a carboxylic acid with 4 carbons in its longest chain. An alcohol would not be obtained under these conditions. Go back and review the reactions in this tutorial. Careful! Count the number of carbons in this compound and then count the number of carbons in the starting material. This compound has one more carbon than the starting material. Since there is no other source of carbon in the reaction, you would not obtain this product under these conditions. An aldehyde would not be obtained under these conditions. Go back and review the reactions in this tutorial. (1) Recall the reactions covered in this tutorial. The tutorial covered reactions to synthesize carboxylic acids and the reactions carboxylic acids undergo. (2) Determine what kind of reaction is taking place. To determine what kind of reaction is taking place, you will first need to identify the reactants being used in the reaction. The starting material is a nitrile. The other reactant is aqueous acid. You may have already recognized this reaction as the hydrolysis of a nitrile to form a carboxylic acid. If you did not, go back and review the reactions in this tutorial. (3) Predict the product(s). Since we know the final product is a carboxylic acid, we can eliminate II and IV since they are not acids. Looking carefully at I and III, find out how they differ. They differ only in the number of carbons in their longest chain. I has 4 and III has 5. ydrolysis of a functional group involves the addition of water not carbon. Since the starting nitrile has 4 carbons, we would expect our product to have the same number. Therefore, the correct answer is (A).

7 Question No. 7 of 10 Question 7. Which of the following reagents will not oxidize a primary alcohol to a carboxylic acid? Question #07 (A) Cr 3 (B) KMn 4 (C) N 3 (D) 2 S 4 Chromium trioxide is a strong oxidant and can transform an alcohol to a carboxylic acid. Go back and review the different oxidants covered in the tutorial. Potassium permanganate is a strong oxidant and can transform an alcohol to a carboxylic acid. Go back and review the different oxidants covered in the tutorial. Nitric acid is a strong oxidant and can transform an alcohol to a carboxylic acid. Go back and review the different oxidants covered in the tutorial. D. Correct! Sulfuric acid will not oxidize an alcohol by itself and is therefore incapable of transforming an alcohol to a carboxylic acid. (1) Recall the oxidants that can be used to oxidize a primary alcohol up to a carboxylic acid. Primary alcohols can be oxidized with Na 2 Cr 2 7, K 2 Cr 2 7, KMn 4, Na, N 3, and Cr 3 to carboxylic acids. (2) Read the possible answers carefully and pick the one that will not oxidize a primary alcohol. Sulfuric acid is not known to oxidize alcohols by itself. All the other reagents are listed as oxidants that can oxidize alcohols to carboxylic acids. Therefore, the correct answer is (D).

8 Question No. 8 of 10 Question 8. What is the product of the reaction shown? S 2 Question #08 An aldehyde would not be obtained under these conditions. An aldehyde would represent a reduction of the carboxylic acid. The thionyl chloride is not a reducing agent so an aldehyde can be ruled out as a possible choice. Go back and review what thionyl chloride is used for in this tutorial. An alkyl halide would not be obtained under these conditions. Be cautious. Thionyl chloride can be used to accomplish different transformations. We have seen it in an earlier tutorial being used to convert an alcohol to an alkyl halide. Go back and review how thionyl chloride reacts with a carboxylic acid. An alcohol would not be obtained under these conditions. An alcohol would represent a reduction of the carboxylic acid. The thionyl chloride is not a reducing agent so an alcohol can be ruled out as a possible choice. Go back and review what thionyl chloride is used for in this tutorial. D. Correct! Thionyl chloride can transform a carboxylic acid to an acid chloride. (1) Recall the reactions covered in this tutorial. The tutorial covered reactions to synthesize carboxylic acids and the reactions carboxylic acids undergo. (2) Determine what kind of reaction is taking place. To determine what kind of reaction is taking place, you will first need to identify the reactants being used in the reaction. The starting material is a carboxylic acid. The other reactant is thionyl chloride. You may have already recognized this reaction as the conversion of a carboxylic acid to an acid chloride. If you did not, go back and review the reactions in this tutorial. (3) Predict the product(s). Be careful. ften times reagents have many uses in organic chemistry and the reagents may not always react in the same manner in every situation. This is the case with thionyl chloride. We saw it earlier being used to convert an alcohol to an alkyl halide. ere, it transforms a carboxylic acid to an acid chloride. Since we know the final product is an acid chloride, we can eliminate all but one choice: IV Therefore, the correct answer is (D).

9 Question No. 9 of 10 Question 9. What is the product(s) of the reaction shown? + Question #09 + A. Correct! In this reaction, the hydroxyl of the carboxylic acid attacks the carbonyl of the acid halide, displacing the halide, to form an anhydride. An acid chloride and a carboxylic acid would not be the products obtained under these conditions. Go back and review the reactions of carboxylic acids covered in this tutorial. This compound would not be the product obtained under these conditions. Go back and review the reactions of carboxylic acids covered in this tutorial. This hydroxyester would not be the product obtained under these conditions. Go back and review the reactions of carboxylic acids covered in this tutorial. (1) Recall the reactions covered in this tutorial. The tutorial covered reactions to synthesize carboxylic acids and the reactions carboxylic acids undergo. (2) Determine what kind of reaction is taking place. To determine what kind of reaction is taking place, you will first need to identify the reactants being used in the reaction. The starting material is a carboxylic acid. The other reactant is an acid chloride. You may have already recognized this reaction as the conversion of a carboxylic acid to an anhydride. If you did not, go back and review the reactions in this tutorial. (3) Predict the product(s). Since we know the final product is an anhydride, we can eliminate all but one choice: I Therefore, the correct answer is (A).

10 Question No. 10 of 10 Question 10. What is the product(s) of the reaction shown? heat Question #10 A β-hydroxyketone is not obtained under these conditions. Go back and review the decarboxylation reactions in this tutorial. B. Correct! After the acid decarboxylates, an enol is formed initially. This enol quickly converts to its keto form to give a ketone as the final product. III is simply the deprotonated form of the starting material. Go back and review the decarboxylation reactions in this tutorial. A β ketoaldehyde will not be formed under these conditions. Go back and review the decarboxylation reactions in this tutorial. (1) Recall the reactions covered in this tutorial. The tutorial covered reactions to synthesize carboxylic acids and the reactions carboxylic acids undergo. (2) Determine what kind of reaction is taking place. To determine what kind of reaction is taking place, you will first need to identify the reactants being used in the reaction. The starting material is a β-ketoacid. There are no other reagents. The reaction is heated to obtain a product. You may have already recognized this reaction as a decarboxylation reaction. If you did not, go back and review the reactions in this tutorial. In a decarboxylation reaction, the starting material typically loses one carbon the one that was part of the carbonyl of the acid. A proton replaces it. In the case of a β-ketoacid, the initial product is an enol but it usually quickly reverts to its keto form. (3) Predict the product(s). Since we are familiar with what occurs during a decarboxylation reaction, we can eliminate some of the possible choices. III is simply the deprotonated starting material and is not the final product. In IV, the carboxylic acid apparently was reduced to an aldehyde. A reduction of the acid will not occur under these conditions. The same is true for I. An alcohol (an apparent reduction of the acid) will also not be formed under these conditions. Compound II, however, is a ketone. If you count the number of carbons in II you will find II has one less carbon than the starting acid. This is the product that would be expected in a decarboxylation reaction of a β-ketoacid. Therefore, the correct answer is (B).