Physical Properties of Carboxylic Acid: Carboxylic Acids are the organic compounds that contain the carboxylic group. In the carboxylic functional group, the carbon \(\left( {\rm{C}} \right)\) atom is bonded to an oxygen \((\mathrm{O})\) atom by a double bond and a hydroxyl group \((-\mathrm{OH})\) by a single bond. The general formula of the carboxylic acid homologous series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{{\text{O}}_2}\), where \(\text {n}=1,2,3 \ldots\). This article will discuss about the physical properties related to carboxylic acids in details. Read on to find out more.

Carboxylic Acid: Representation

Carboxylic acids are represented as \(\mathrm{R}-\mathrm{COOH}\), where \(\mathrm{COOH}\) refers to the carboxyl group, and \(\text {R}\) refers to the rest of the molecule to which this group is attached.

One of the most common carboxylic acids is Vinegar, chemical formula \(\mathrm{CH}_{3} \mathrm{COOH}\). It is also known as ethanoic acid or acetic acid. Lemon juice is sour in taste and is acidic due to the presence of a carboxylic acid known as citric acid. The burning sensation due to ant bite is due to a carboxylic acid known as formic acid. Lactic acid or fumaric are used for fermentation in the food industry. Some of the properties of carboxylic acids are listed below.

Carboxylic Acids: Physical Properties

The various physical properties of carboxylic acids are explained below:

Colour and Odour

Many carboxylic acids are colourless liquids with disagreeable odours. The carboxylic acids with \(5\) to \(10\) carbon atoms have the odour of poorly ventilated locker rooms. The acids with more than ten carbon atoms are waxlike solids, and their odour diminishes with increasing molar mass and decreasing volatility.

Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature. At the same time, some aromatic carboxylic acids exist as crystalline solids.

Polarity

The carboxylic acid functional group is considered to be highly polar in nature. This polarity is due to the presence of a strongly polarised carbonyl \((\mathrm{C}=\mathrm{O})\) group and hydroxyl \((\mathrm{O}-\mathrm{H})\) group. The carboxylic moiety consists of two oxygen atoms that are relatively electronegative than carbon and hydrogen atom. This results in a strong permanent dipole in \(\mathrm{C}=\mathrm{O}\) and \(\mathrm{O}-\mathrm{H}\) groups.

The \(\text {O}-\text {H}\) group in the carboxylic group is even more strongly polarised than the \(\text {O}-\text {H}\) group of alcohols. This is due to the presence of the carbonyl moiety adjacent to the \(\text {O}-\text {H}\) group.

The \(-\text {OH}\) bond being polar in nature, develops slight negative and positive charges over the oxygen and hydrogen atoms, respectively. As opposite charges attract, the partially negative oxygen atom of one molecule will interact with the partially positive hydrogen atom of the other molecule. These interactions between the negative oxygen atom and positive hydrogen atom result in the formation of hydrogen bonds. These bonds are about \(1 / 10\) as strong as normal bonds.

Hydrogen Bonding

The dipoles present in carboxylic acids allow these compounds to function as both an \(\text {H}-\)bond donor and acceptor. These compounds readily participate in energetically favourable hydrogen bonding (\(\text {H}-\)bonding) interactions with polar molecules like water.

The dipoles present in carboxylic acids are more in number and stronger than other organic compounds containing \(\text {OH}\) and/or \(\text {C}=\text {O}\) dipoles such as amines, alcohols, phenols, aldehydes, ketones, esters, amides. Thus it can form more and stronger \(\text {H}-\)bonds with other substances capable of \(\text {H}-\)bonding interactions.

Hydrogen Bonding in Alcohol: Impact

1. The hydrogen bonding leads to strong intermolecular forces of attraction, increasing their boiling points compared to their analogous alcohols, aldehydes, and ketones.
2. The presence of hydrogen bonding accounts for the dimerisation in carboxylic acids. 

Boiling Points and Melting Points

The strong intermolecular \(\text {H}-\)bonding interactions between acid molecules increase the boiling points than corresponding non-polar hydrocarbons.

They even have higher boiling points than other compounds with weaker or fewer dipoles like amines, alcohols, phenols, aldehydes, ketones, esters, amides, and isosteric compounds.

As a result of the high degree of intermolecular hydrogen bond, Carboxylic acids usually self-associate and exist as dimeric pairs in non-polar media. This tendency to self-associate gives them increased stability as well as higher boiling points relative to the acid in an aqueous solution.This is illustrated in the figure below :

For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds.

Solubility

Carboxylic acids readily interact with water and other polar protic solvents through hydrogen bonding. These acids have high water solubilities compared to comparable organic compounds. For example, butyric acid is infinitely “soluble” in water, while the solubility of the alcohol \(1\)-butanol in water is \(7.3 \,\text {g}/100 \,\text {mL}\):

Carboxylic acids with more than six carbon atoms are sparingly \((1 \,\text {g}/100 \,\text {mL})\) soluble to insoluble in water. However, alcohols such as ethanol, protic organic solvents can dissolve carboxylic acids containing more than ten carbon atoms. This is because ethanol has the ability to form a hydrogen bond with the polar carboxyl group and participates in van der Waals interactions with the non-polar hydrocarbon’ tail’. Thus, ethanol (alcohols and similar solvents) can dissolve considerably “larger” carboxylic acids than water:

Condensed Structural Formula	Name of Acid	Melting Point \(\left( {^\circ {\text{C}}} \right)\)	Boiling Point \(\left( {^\circ {\text{C}}} \right)\)	Solubility \({\text{g}}/100\,{\text{g}}\) of water)
\(\text {HCOOH}\)	Formic acid	\(8\)	\(100\)	miscible
\(\mathrm{CH}_{3} \mathrm{COOH}\)	Acetic acid	\(17\)	\(118\)	miscible
\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\)	Propionic acid	\(–22\)	\(141\)	miscible
\(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{COOH}\)	Butyric acid	\(–5\)	\(163\)	miscible
\(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{COOH}\)	Valeric acid	\(–35\)	\(187\)	\(5\)
\(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{COOH}\)	Caproic acid	\(–3\)	\(205\)	\(1.1\)
\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\)	Benzoic acid	\(122\)	\(249\)	\(0.29\)

As the hydrocarbon chain gets longer (after butanoic acid), the greasy alkyl chain starts interfering with water solubility.

The \(-\text {OH}\) end of the carboxyl moiety forms new hydrogen bonds with water molecules, but the hydrocarbon “tail” does not form hydrogen bonds. Only van der Waals dispersion forces exist between the water molecule and the hydrocarbon “tails.” Hence, as the length of the hydrocarbon chain increases, the process becomes less feasible, and the solubility decreases. 

The solubility of isomeric carboxylic acids increases with branching because the surface area of the hydrocarbon part decreases with branching.

Acidity of Carboxylic Acids

Carboxylic acids are considered “weak acids” because they partially dissociate in water to furnish \({{\text{H}}^ + }\) ions. As proton donors, carboxylic acids are characterised as Brønsted-Lowry acids.

The carboxyl moiety consists of a carbonyl group that has an electron-deficient carbon atom bonded to electronegative oxygen through pi bonding (double bond). This carbonyl carbon is directly linked to and conjugated with the second electronegative oxygen atom, which bears a hydrogen atom. 

As a result, the electron density is “pulled” from the hydroxyl hydrogen through the conjugated carboxyl group, and a proton’s loss occurs. The loss of proton results in carboxylate ion (conjugate base) formation, which is stabilised by resonance delocalisation. The resonance stabilisation of carboxylate ion (conjugate base) is shown below :

Alcohols are similar to carboxylic acids due to the presence of an \(-\text {OH}\) group. In alcohols, ionisation of the \(\text {OH}\) group yields an alkoxide (anion) as the conjugate base. In alkoxide anions, the oxygen atom alone bears the negative charge and is not stabilised through resonance delocalisation. The carbon adjacent to the alkoxide oxygen is \({\text{s}}{{\text{p}}^3}\) hybridised. However, in carboxylic acids, the negative charge on the conjugate base (carboxylate ion) is stabilised through resonance delocalisation. Thus alcohols are less acidic than carboxylic acids. These properties are demonstrated in the following figure.

Summary

Carboxylic acids are an essential class of organic compounds that has a wide range of applications. The acidic property of these organic compounds makes them a vital component in our homes as well as in the food industry. The pharmaceutical industry benefits largely from the physical properties of carboxylic acids. Through this article, we learned the physical properties of carboxylic acids. We also learned how carboxylic acids rank a grade higher than their hydrocarbons.

FAQs on Physical Properties of Carboxylic Acids

Frequently asked questions related to physical properties related to carboxylic acids are listed as follows:

Q.1. Why do carboxylic acids have higher boiling points?
Ans: The presence of intermolecular hydrogen bonding in carboxylic acids accounts for their high boiling points. Carboxylic acids self-associate among themselves and exist as dimers. The hydrogen bonds are not broken easily and completely even in the vapour phase.

Q.2. Why are carboxylic acids acidic?
Ans: The carboxylic acids are acidic due to the presence of a hydrogen atom in the carboxyl \((-\text {COOH})\) moiety. The carbonyl carbon is directly linked to and in conjugation with the second electronegative oxygen atom bearing a hydrogen atom. As a result, the electron density is “pulled” from the hydroxyl hydrogen through the conjugated carboxyl group, and a proton’s loss occurs. The loss of proton results in carboxylate ion (conjugate base) formation which is stabilised by resonance delocalisation.

Q.3. Are carboxylic acids strong or weak?
Ans: The strength of an acid is determined by its ability to produce \(\mathrm{H}^{+}\) in solutions. Unlike mineral acids, carboxylic acids (of the form \(\mathrm{RCOOH}\)) do not completely ionise. They partially ionise to give \(\text {H}^{+}\) and \(\text {RCOO}^{-}\). Therefore, they are called weak acids.

Q.4. Why do carboxylic acids dimerise?
Ans: Due to the small size and strong intermolecular hydrogen bonding interaction, carboxylic acids form a dimer in aqueous conditions. – In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.
This tendency to self-associate among themselves gives them increased stability as well as higher boiling points. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds.

Q.5. Carboxylic acids contain the carbonyl group but do not show the nucleophilic addition reaction like aldehydes or ketones. Why?
Ans: Carboxylic acids ionise to form carboxylate ion, which is stabilised through resonance. Due to resonance, the partial positive charge on the carbonyl carbon atom is delocalised and reduced. Hence, carboxylic acids do not show nucleophilic addition reactions like aldehydes or ketones.

Q.6. Why are carboxylic acids more acidic than alcohols, although both of them have a hydrogen atom attached to an oxygen atom (—O—H)?
Ans: Alcohols are similar to carboxylic acids due to the presence of an \(-\text {OH}\) group. In alcohols, ionisation of the \(\text {OH}\) group yields an alkoxide (anion) as the conjugate base where the oxygen alone bears the negative charge and is not stabilised through resonance delocalisation. The carbon adjacent to the alkoxide oxygen is \(\text {sp}^{3}\) hybridised. However, in carboxylic acids, the negative charge on the conjugate base (carboxylate ion) is stabilised through resonance delocalisation. Hence, carboxylic acids are more acidic than alcohol. These properties are demonstrated in the following figure.

The more stable the anion formed, more easy will be the dissociation of \(\text {O}—\text {H}\) bond, stronger will be the acids.

Study Acidity of Carboxylic Acids

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