Carboxylic Acids

Question 1

Define:

carboxylic acid

Answer 1

A carboxylic acid is an organic acid that contains a carboxyl (-COOH) group.

Carboxylic acids have IUPAC names that end in “-oic acid” (propanoic acid, for example).

Question 2

Which of the following acids (top or bottom) is a carboxylic acid?

Answer 2

The top molecule is a carboxylic acid due to the presence of a carboxyl (-COOH) group.

To be classified as a carboxylic acid, an acid must contain at least one carboxyl group. Here, the molecule on the bottom contains a sulfonyl instead.

Question 3

Which of the following acids is a carboxylic acid?

• phosphoric acid
• benzenesulfonic acid
• pentanoic acid

Answer 3

Pentanoic acid is a carboxylic acid.

Carboxylic acids are named by adding the suffix “-oic acid” to the IUPAC name for the base chain. Pentanoic acid consists of a five-carbon chain (“pentane”) with a carboxyl group (“-oic acid”).

Question 4

What is the IUPAC name of this molecule?

Answer 4

This molecule is ethanoic acid.

Carboxylic acids are named by adding the suffix “-oic acid” to the IUPAC name for the base chain. Ethanoic acid is commonly known as acetic acid. For the MCAT, you should know both the IUPAC and common names for this molecule.

Question 5

What is the IUPAC name of this molecule?

Answer 5

This molecule is ethanedioic acid.

Dicarboxylic acids are named by adding the suffix “-dioic acid” to the IUPAC name, indicating that two carboxyl groups are attached. Ethanedioic acid is commonly known as oxalic acid (H₂C₂O₄)

Question 6

What is the IUPAC name of this molecule?

Answer 6

This molecule is benzoic acid.

Carboxylic acids are named by adding the suffix “-oic acid” to the IUPAC name for the base chain. In this case, the benzene ring is the base chain.

Question 7

Define:

hydrogen bonding

Answer 7

Hydrogen bonding is the strongest of the dipole-dipole interactions. It occurs between molecules that contain hydrogen bound to highly electronegative atoms (F, O, or N).

Carboxylic acids contain O-H bonds and can participate in hydrogen bonding, including the formation of dimers.

Question 8

How many hydrogen bonds can a single carboxylic acid molecule form with another?

Answer 8

Carboxylic acids can form two hydrogen bonds, one from the polar C=O group to a new hydrogen and one from the O-H group to an oxygen.

This results in the formation of cyclic dimers and very high boiling points.

Question 9

How do the boiling points of carboxylic acids compare to those of other types of molecules?

Assume all compounds have similar molecular weight, chain length, and branching.

Answer 9

Carboxylic acids tend to have higher boiling points due to their strong intermolecular forces.

Carboxylic acids contain O-H groups that allow them to form hydrogen bonds. Hydrogen bonding is a stronger force than dipole-dipole or dispersion forces, and it significantly increases boiling point.

Question 10

Rank the following molecules from highest to lowest boiling point:

• Butanone
• Butanol
• Butanoic acid
• Butane

Answer 10

Butanoic acid > butanol > butanone > butane

Since butanoic acid is able to form hydrogen bond dimers, it has the highest boiling point. Though less significant, the hydrogen bonding in butanol is stronger than the dipole-dipole forces in butanone. Finally, as a nonpolar molecule, butane has only weak dispersion forces.

Question 11

Define:

dimer

Answer 11

A dimer is a pair of two structurally similar molecules linked together by chemical bonds.

Here, dimers refer to carboxylic acids linked by hydrogen bonds. In the biology section of the MCAT, however, dimers can relate to a variety of proteins and other molecules.

Question 12

How many hydrogen bonds are involved when two molecules of acetic acid form a dimer?

Answer 12

The homodimer formed would be linked by two hydrogen bonds.

A homodimer implies that both molecules are the same. Though it is possible to form heterodimers between different carboxylic acids, this is rarely tested on the MCAT.

Question 13

How many hydrogen bonds are involved when two molecules of benzoic acid form a dimer?

Answer 13

The homodimer formed would be linked by two hydrogen bonds.

A homodimer implies that both molecules are the same. Though it is possible to form heterodimers between different carboxylic acids, this is rarely tested on the MCAT.

Question 14

Which type of solvent is best for dissolving carboxylic acids?

Answer 14

Carboxylic acids are most soluble in polar protic solvents.

Remember that “like dissolves like.” Carboxylic acids are highly polar due to the presence of the carboxylate group, and would thus dissolve best in a polar solvent. They also have the ability to form hydrogen bonds, which is a characteristic of polar protic solvents in particular.

Question 15

In which of the following solvents will a carboxylic acid dissolve most readily?

• water
• ethane
• cyclohexane

Answer 15

A simple carboxylic acid will dissolve most readily in water, a polar solvent that forms hydrogen bonds.

Remember that “like dissolves like.” Both ethane and cyclohexane are nonpolar, making them very different from a carboxylic acid.

Question 16

How is the acidity of a carboxylic acid affected by the inductive effects of its substituents?

Answer 16

Acidity increases if substituent groups are electron-withdrawing. Such groups can delocalize the negative charge on the carboxyl group, stabilizing the conjugate base.

In contrast, electron-donating groups decrease the acidity, since they contribute extra electron density and destabilize the carboxyl group.

Question 17

Which of the following (top or bottom) is a stronger acid?

Answer 17

The bottom acid (acetic acid) is stronger.

The top acid is propanoic acid. Its extra alkyl group (circled) is electron-donating, which destabilizes the negative charge of the carboxylate ion. The less stable the conjugate base, the weaker the original acid, and vice versa.

Question 18

How does resonance affect the acidity of a carboxylic acid?

Answer 18

Resonance increases the acidity. It allows the carboxylate ion’s negative charge to be delocalized between the oxygen atoms of the carboxyl group.

Each oxygen atom receives a partial negative charge, as well as a bond strength of 1.5 between it and the central carbon.

Question 19

Which of the following two molecules (top or bottom) is a stronger acid?

Answer 19

The top (carboxylic acid) is much more acidic than the bottom (alcohol) molecule. This increased acidity is due to the resonance stabilization of the carboxylate ion.

Resonance delocalizes the negative charge, which stabilizes the conjugate base and increases acidity. In general, alcohols have relatively low acidity.

Question 20

How does the number of electron-withdrawing groups affect the acidity of a carboxylic acid?

Answer 20

The more electron-withdrawing groups, the stronger the acid. These groups stabilize the conjugate base by delocalizing the electron density of the negative charge.

Additional electron-donating groups, in contrast, decrease the acidity.

Question 21

Which of the following molecules is a stronger acid, and why?

Answer 21

The bottom molecule (the carboxylic acid with two chlorine substituents) is a stronger acid.

The presence of an additional electron-withdrawing group increases the stability of the carboxylate ion.

Question 22

Which of the following molecules is a stronger acid, and why?

Answer 22

The top molecule (the carboxylic acid with fewer alkyl substituents, which is isobutyric acid) is a stronger acid.

This molecule has fewer electron-donating groups. The more such groups present on the molecule, the more electron density they contribute to the carboxylate ion, and the more the conjugate base is destabilized (which decreases acidity).

Question 23

How is the acidity of a carboxylic acid affected by the electronegativity of its substituents?

Answer 23

More electronegativity near a carboxyl group increases the acidity.

The higher the electronegativity of a substituent, the more it delocalizes the negative charge on the carboxyl group, resulting in a more stable conjugate base and a stronger acid.

Question 24

Which of the following molecules is a stronger acid, and why?

Answer 24

The top molecule (chloroacetic acid) is a stronger acid.

Chlorine is more electronegative than bromine. It is thus better able than bromine to delocalize the carboxylate ion’s negative charge. More delocalization of charge means greater acidity.

Question 25

How is the acidity of a carboxylic acid affected by the distance of an electron-withdrawing group from the carboxyl carbon?

Answer 25

A carboxylic acid with an electron-withdrawing group farther from the carboxyl carbon will be less acidic than one with a closer electron-withdrawing group.

The inductive effects of both electron-donating and electron-withdrawing substituents decrease with distance.

Question 26

Which of the following carboxylic acids is a stronger acid, and why?

Answer 26

The top acid is a stronger acid because its electron-withdrawing group is closer to the carboxyl.

The inductive effects of electron-withdrawing groups decrease with distance. For this reason, a group on the alpha carbon will exert a greater effect than one on the beta carbon.

Question 27

In a carboxyl group, what charges do each of the atoms carry?

Answer 27

The oxygens carry a partial negative charge, while the carbon carries a partial positive charge.

Question 28

In a reaction, what role would likely be played by the carbon atom in a carboxyl group?

Answer 28

The carboxyl carbon usually acts as an electrophile.

Electrophiles are usually positively charged, or electron-deficient. They receive electrons in a reaction and are subject to nucleophilic attack. With a partial positive charge, the carboxyl carbon is a classic electrophile.

Question 29

In a reaction, what role would likely be played by the oxygen atom in the -OH of a carboxyl group?

Answer 29

The oxygen atom would most likely act as a nucleophile.

Nucleophiles are usually negatively charged, or electron-rich. They donate electrons in a reaction and attack electrophiles, such as the carbon atom in a carbonyl group. With its lone pairs, -OH is a classic nucleophile.

Question 30

Why is the alpha carbon of a carboxylic acid especially reactive?

Answer 30

A carboxylic acid has the ability to form an enol (and an enolate ion) at the alpha position. This renders that carbon much more reactive than the others in the chain.

Carboxylic acids often require conversion to an enolizable form before reactions at the alpha carbon can occur.

Question 31

Briefly describe the nucleophilic attack of a carboxylic acid.

Answer 31

The nucleophile attacks the carboxyl carbon atom, producing an intermediate with a negative charge on its carbonyl oxygen.

To regain a low final energy state, nucleophilic substitution must occur. This includes the reformation of the carbonyl double bond and the loss of a leaving group.

Question 32

What intermediate will most likely form when a nucleophile attacks the carboxylic acid below?

Answer 32

The nucleophile would attack the electron-deficient carboxyl carbon atom, yielding the intermediate shown here.

This type of attack and intermediate is common in many reactions involving carboxylic acids, including reduction, ester formation, and acyl halide formation.

Question 33

Define:

α-halogenation

Answer 33

α-halogenation is the substituion of a halide atom for a hydrogen at the alpha carbon of a carboxylic acid.

This reaction commonly involves Br₂ and a catalytic amount of PCl₃ or PBr₃. It proceeds through the formation of an acid bromide enol.

Question 34

What product will form when the carboxylic acid below is α-halogenated with bromine?

Answer 34

The product below will form, in which bromine substitutes for a hydrogen at the carboxylic acid’s alpha carbon.

An acid bromide intermediate is produced and converted to its enol form, which reacts with bromine.

Question 35

What name is given to the most common mechanism by which substitution reactions of carboxylic acids occur?

Answer 35

Most substitution reactions involving carboxylic acids occur through nucleophilic acyl substitution.

In this mechanism, the nucleophile attacks the carbon of the carboxyl group and substitutes for the leaving group (a hydroxyl group, in the case of a carboxylic acid).

Question 36

What product will be formed by the nucleophilic acyl substitution shown below?

Answer 36

An acyl chloride (specifically, acetyl chloride) will form.

Acyl chlorides are very reactive carboxylic acid derivatives that are useful for the synthesis of other, less reactive molecules. SOCl₂, or thionyl chloride, is often used in their synthesis.

Question 37

What type of molecule will be formed by the strong reduction of a carboxylic acid?

Answer 37

Strong reducing agents can convert carboxylic acids into their corresponding primary alcohols.

A common strong reducing agent on the MCAT is LiAlH₄. NaBH₄, another reductant, is not as strong and generally reduces carboxylic acids to aldehydes.

Question 38

What product will be formed by the reduction of the following carboxylic acid with LiAlH₄?

Answer 38

A primary alcohol (isobutanol) will form.

LiAlH₄ is a strong reducing agent. It can convert carboxylic acids (here, isobutanoic acid) to their corresponding alcohols.

Question 39

Define:

decarboxylation

Answer 39

Decarboxylation is the loss of a CO₂ molecule when the carboxyl group is effectively removed from a carboxylic acid.

When carboxylic acids undergo decarboxylation reactions, the number of carbon atoms decreases by one. A hydrogen atom attaches in the place of the original carbon.

Question 40

What product will form when the following carboxylic acid undergoes a decarboxylation reaction?

Answer 40

Benzoic acid will undergo decarboxylation to form benzene.

As with all decarboxylation reactions, a CO₂ group is lost from the molecule.

Question 41

What product will form when the following carboxylic acid undergoes a decarboxylation reaction?

Don’t worry about naming this product; simply describe it.

Answer 41

The product will resemble the original molecule, but with a CO₂ removed. The NH₂ group will extend into the newly open space, though that is not important to know.

This reaction happens to also have biological significance, as it is the decarboxylation of tryptophan to form tryptamine.

Question 42

What product will form when the following carboxylate ion (pyruvate) undergoes a decarboxylation reaction in the presence of HS-CoA?

Answer 42

The decarboxylation of pyruvate produces acetyl-CoA. This reaction is an example of enzyme-catalyzed oxidative decarboxylation.

This happens during aerobic cellular respiration, just prior to the Krebs cycle.

Question 43

Define:

esterification

Answer 43

Esterification is the synthesis of an ester from a carboxylic acid and an alcohol. It is a dehydration reaction.

This reaction is commonly carried out by heating the carboxylic acid and alcohol in the presence of an acid catalyst.

Question 44

In the IUPAC convention, what functional group is denoted by the suffix “-oate”?

Answer 44

The “-oate” ending denotes an ester. Specifically, esters are named as follows: the alcohol substituent is given the suffix “-yl,” while the carboxylic acid substituent is given the “-oate” ending.

A common ester on the MCAT is ethyl ethanoate, which is formed from ethanol and ethanoic acid.

Question 45

What is the IUPAC name for the ester formed from the molecules below?

Answer 45

This ester is called methyl ethanoate, due to its methanol and ethanoic acid parent groups.

Note that this reaction shows the removal of a water molecule, since esters are synthesized via dehydration.

Question 46

What is the IUPAC name for the ester formed from the molecules below?

Answer 46

This ester is called ethyl propanoate, due to its ethanol and propanoic acid parent groups.

Note that this reaction shows the removal of a water molecule, since esters are synthesized via dehydration.

Question 47

When an alcohol and a carboxylic acid are combined in solution and heated, what reaction is expected to occur? What product(s) will form?

Answer 47

An esterification reaction will occur between the alcohol and carboxylic acid. The expected products include an ester and water.

Esterification reactions are commonly conducted in the presence of an acid catalyst.