Organic Chemistry 101: Carboxylic Acids

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

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

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.

pentanoic 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”).

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. You should know both the IUPAC and common names for this molecule.

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)

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.

It 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.

Two hydrogen bonds:

• one from the polar C=O group to a new hydrogen

• one from the O-H group to an oxygen

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

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.

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.

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

Here, dimers refer to carboxylic acids linked by hydrogen bonds. Dimers can relate to a variety of proteins and other molecules.

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.

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.

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.

water

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.

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.

The bottom acid (acetic acid).

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.

It 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.

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.

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.

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

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

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

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.

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.

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.

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.

• The oxygens carry a partial negative charge.

• The carbon carries a partial positive charge.

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.

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.

It 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.

The attach will produce 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.

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.

It 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.

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.

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).

acyl chloride (specifically, acetyl chloride)

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.

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

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

primary alcohol (isobutanol)

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

This 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.

Benzoic acid will undergo decarboxylation to form benzene.

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

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.

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.

This 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.

The "-oate" ending denotes an ester. Specifically, esters are named as follows:

• alcohol substituent is given the suffix "-yl"

• carboxylic acid substituent is given the "-oate" ending

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

This 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.

This 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.

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.