Carboxylic Acids Flashcards
Define:
Carboxylic Acid
A carboxylic acid is an organic acid that contains a carboxyl (-COOH) group.
Carboxylic acids have IUPAC names that end in “-oic acid” (i.e. propanoic acid).
Which of the following acids (top or bottom) is a carboxylic acid?
The top acid is a carboxylic acid due to the presence of a carboxyl (-COOH) group.
An acid must contain at least one carboxyl group in order to be classified as a carboxylic acid.
Which of the following acids is a carboxylic acid?
- Phosphoric acid
- Benzenesulfonic acid
- Pentanoic acid
Pentanoic acid is a carboxylic acid.
Carboxylic acids are named by converting the IUPAC name of the base chain, and adding the suffix “-oic acid.” Pentanoic acid consists of a five carbon chain (pentane) that contains a carboxyl group (oic acid).
What is the IUPAC name of this molecule?
This molecule is named ethanoic acid.
Carboxylic acids are named by converting the IUPAC name of the base chain, and adding the suffix “-oic acid.” Ethanoic acid is commonly known as acetic acid.
What is the IUPAC name of this molecule?
This molecule is named ethanedioic acid.
Dicarboxylic acids are named by adding the suffix of “-dioic acid” to the IUPAC name, to indicate that two carboxyl groups are attached. Ethanedioic acid is commonly known as oxalic acid.
What is the IUPAC name of this molecule?
This molecule is named benzoic acid.
Carboxylic acids are typically named by dropping the “e” from the IUPAC alkane that contains the carboxyl group and adding the suffix of “-oic acid.” In this case the benzene ring is the base.
How do the boiling points of carboxylic acids compare to the boiling points of other types of molecules with similar mass and branching?
Carboxylic acids tend to have higher boiling points relative to other molecules of similar mass and branching.
Carboxylic acids contain O-H groups that allow them to form hydrogen bonds, resulting in high boiling points. Hydrogen bonding is a stronger force than dipole-dipole or dispersion forces.
Why do carboxylic acids have high boiling points compared to other similar sized acids?
Carboxylic acids can form hydrogen bonds with each other, resulting in higher relative boiling points than other acids without the ability to hydrogen bond.
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, resulting in the formation of cyclic dimers and very high boiling points.
Rank the following from highest to lowest BP:
- Butanone
- Butanol
- Butanoic acid
- Butane
Butanoic acid > butanol > butanone > butane
Butanoic acid is able to form dimers (a double hydrogen bond pair) and has the highest boiling point. Hydrogen bonding in butanol is stronger than the dipole-dipole in butanone. And butane has only dispersion forces.
Define:
Hydrogen bonding
Hydrogen bonding is the strongest dipole-dipole interaction between molecules that contain hydrogen bound to highly electronegative atoms (F, O, N).
Carboxylic acids contain O-H bonds and are able to participate in hydrogen bonding, including the formation of dimers.
Define:
Dimer
A dimer is a pair of two structurally similar molecules linked together by chemical bonds.
For the MCAT, dimers are most commonly carboxylic acids linked together by hydrogen bonds.
What features of carboxylic acids allow for dimerization to occur between two molecules?
Carboxylic acids contain O-H bonds and are able to form two hydrogen bonds between molecules.
Dimerization is usually only tested on the MCAT when discussing a homogeneous carboxylic acid. Technically, if water is mixed in, hydrogen bonds can instead form between water and the acid molecules.
What would it look like if acetic acid (ethanoic acid) molecules came together as a dimer?
Acetic acid molecules would form a homodimer linked by two hydrogen bonds.
A homodimer implies that both molecules are the same substance. It’s possible to form heterodimers between different carboxylic acids, though this is rarely tested on the MCAT.
What would the dimer look like if benzoic acid molecules came together?
Benzoic acid molecules would form a homodimer linked by two hydrogen bonds.
A homodimer implies that both molecules are the same substance. It’s possible to form heterodimers between different carboxylic acids, though this is rarely tested on the MCAT.
In what type of solvent are carboxylic acids most soluble?
Carboxylic acids are most soluble in polar protic solvents.
Recall: “like dissolves like” and carboxylic acids are highly polar due to the presence of the carboxyl group and can form hydrogen bonds.
Which of the following solvents will a carboxylic acid dissolve in most readily?
- Water
- Ethane
- Cyclohexane
A simple carboxylic acid will dissolve most readily in water, a polar solvent that forms hydrogen bonds.
Recall: “like dissolves like”.
How is the acidity of a carboxylic acid affected by the inductive effects of subtituents?
Acidity will increase if substituent groups can withdraw electron density through induction and delocalize the negative charge of the carboxyl group.
Electron-donating groups will decrease the acidity, since they destabilize the carboxyl group.
Which of the following (top or bottom) is a better acid, and why?
The bottom acid (acetic acid) is a better acid.
The additional alkyl group of the top acid (propanoic acid) is electron-donating and destabilizes the negative charge of the carboxylate ion.
How does resonance contribute to the acidity of a carboxylic acid?
Resonance allows the negative charge of the carboxylate ion to be delocalized/stabilized between the oxygen atoms of the carboxyl group.
Each oxygen atom receives a partial negative charge, and a resonance 1.5 bond strength to the central carbon.
Which of the following two molecules (top or bottom) is a better acid, and why?
The top (carboxylic acid) is much more acidic than the bottom (alcohol) due to the resonance stabilization of the carboxylate ion.
Resonance delocalizes the negative charge and increases acidity. An alcohol should also have relatively low acidity in general.
Which of the following two groups is a better acid, and why?
The carboxylic acid with two chlorine substituents is a better acid because the additional electron-withdrawing group increases the stability of the carboxylate ion.
Which of the following two groups is a better acid, and why?
The carboxylic acid with fewer alkyl substituents (isobutyric acid) is a better acid because it has fewer electron-donating groups.
Additional electron-donating groups decrease the stability of the carboxylate ion.
Which of the following (top or bottom) is a better acid, and why?
The top acid containing chlorine (chloroacetic acid) is a better acid. Chlorine is more electronegative and better able to delocalize the negative charge of the carboxylate ion than bromine.
Which of the following (top or bottom) is a better acid, and why?
The top acid containing fluorine (fluoroacetic acid) is a better acid. Fluorine is more electronegative and better able to delocalize the negative charge of the carboxylate ion than chlorine.
Which of the following carboxylic acids (top or bottom) is a better acid, and why?
The top acid is a better acid due to the closer electron-withdrawing group.
The inductive effects of electron-withdrawing groups decrease with distance. A group on the alpha carbon will have a greater effect than one on the beta carbon.
What feature of a carboxylic acid allows for the reactivity of the alpha carbon?
Reactivity is present at the alpha carbon due to the ability of the acid to form an enol and enolate ion.
Carboxylic acids often require conversion to an enolizable form before reactions at the alpha carbon can occur.
What intermediate would most likely be formed by a nucleophile attack on the following carboxylic acid?
The nucleophile would attack the electron-deficient carbon atom of the carboxyl group.
This type of attack and intermediate is formed in many reactions with carboxylic acids, including reduction, ester formation, and acyl halide formation.
What is the product of the α-halogenation of the following carboxylic acid with bromine?
The bromine replaces a hydrogen at the alpha carbon of the carboxylic acid in a substitution reaction.
An acid bromide intermediate is produced and converted to its enol form, which reacts with bromine.
What is the product of the nucleophilic acyl substituion involving the following carboxylic acid and thionyl chloride (SOCl2)?
The carboxylic acid (ethanoic acid) is converted to an acyl chloride (acetyl chloride).
Acyl chlorides are very reactive carboxylic acid derivatives that are useful for the synthesis of other, less reactive derivatives.
What is the product of the reduction of the following carboxylic acid with LiAlH4?
The carboxylic acid (isobutanoic acid) is reduced to a primary alcohol (isobutanol).
What is the product when the following carboxylic acid undergoes a decarboxylation reaction?
Benzoic acid undergoes decarboxylation to form benzene.
As with all decarboxylation reactions, there is the loss of CO2 from the molecule.
What is the product when the following carboxylic acid undergoes a decarboxylation reaction?
The product will have a CO2 removed from the molecule, and the NH2 group will flex into that open space.
This reaction happens to also have biological significance, as it is the decarboxylation of tryptophan to form tryptomine.
What is the product when the following carboxylate ion (pyruvate) undergoes a decarboxylation reaction in the presence of HS-CoA?
Pyruvate undergoes enzyme-catalyzed oxidative decarboxylation with the addition of HS-CoA to produce acetyl-CoA.
This happens during the second stage of aerobic cellular respiration in the Krebs cycle.
Assume that these two molecules react to form an ester, what would it look like and what is its IUPAC name?
The ester is called methyl ethanoate, from the methanol and ethanoic acid parent groups.
Note: the dehydration reactions creating esters should show a water molecule being removed.
Assume that these two molecules react to form an ester, what would it look like and what is its IUPAC name?
The ester is called ethyl propanoate, from the ethanol and propanoic acid parent groups.
Note: the dehydration reactions creating esters should show a water molecule being removed.
