Name: carboxylic acid boiling point trends
Uploaded: 2023-04-30T11:31:32.000Z
Duration: 1 min 31 s
Channel: JoVE

Carboxylic acids have polar &#8211;C=O and &#8211;OH groups, ensuring dipole-dipole interactions between two molecules.
Further, the polar groups are involved in two strong intermolecular hydrogen bonds to form stable dimers, effectively doubling the molecular weights.
This results in a higher boiling point for carboxylic acids than for analogous compounds.
In general, lower-molecular-weight carboxylic acids are liquid, while higher-molecular-weight carboxylic acids are solid at room temperature.
Saturated carboxylic acids bearing long nonpolar hydrocarbon chains have higher melting points than unsaturated acids due to increased London forces and close packing.
Dicarboxylic acids, with two additional hydrogen bonds, have relatively higher melting points.
Carboxylic acids also form hydrogen bonds with water. Lower-molecular-weight carboxylic acids are readily soluble in water.
As the hydrophobic hydrocarbon chain increases, water solubility decreases. Such long-chain carboxylic acids can easily dissolve in alcoholic solvents due to the London forces in addition to hydrogen bonding.
If the solvent is nonpolar, acids exist in their dimer form, making them quite soluble.

physical properties of carboxylic acids

Physical Properties of Carboxylic Acids

Carboxylic acids with lower molecular weight exhibit a sharp and unpleasant odor. They also have higher boiling and melting points than analogous compounds, such as aldehydes, ketones, and alcohols.
<img alt="Organic compound boiling points; structural formulas; propanoic acid, 2-butanol, 2-butanone chart." src="/files/ftp_upload/13083/13083_PT_Figure_1.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
In addition to the dipole&#8211;dipole interaction of the polar carbonyl and hydroxyl bonds, strong intermolecular hydrogen bonding between carboxylic acid molecules dictates several physical properties.
<img alt="Acetic acid dimer structure, hydrogen bonding, chemical diagram, organic chemistry concept" src="/files/ftp_upload/13083/13083_PT_Figure_2.svg" style="text-align: center; height: 115px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
The strong interaction causes the carboxylic acid molecules to exist in stable dimers, increasing their molecular weight two-fold. Consequently, they have higher boiling points. These carboxylic acid dimers have much higher equilibrium constants (as large as 106&#8211;107 M&#8722;1) than alcohol dimers (11 M&#8722;1).
A similar trend is observed with the melting points of carboxylic acids. The melting point increases with the increase in hydrocarbon chain length. The saturated carboxylic acids with more than eight carbons are typically solids at room temperature. However, unsaturated carboxylic acids have lower melting points. For example, stearic acid (C18H36O2), a saturated carboxylic acid, melts at 70 &#176;C, while linoleic acid (C18H32O2), bearing two cis double bonds, melts at &#8722;5 &#176;C.
In terms of solubility, low molecular weight carboxylic acids are soluble in water, owing to their hydrogen bonding interaction with water molecules. In contrast, higher molecular weight carboxylic acids are more soluble in alcohols due to the higher van der Waals interaction between the molecules. Notably, carboxylic acids are highly soluble in non-polar solvents like CCl4. Interestingly, some dicarboxylic acids are fairly water-soluble due to their substantial hydrogen bonding interaction.

Carboxylic acids with lower molecular weight exhibit a sharp and unpleasant odor. They also have higher boiling and melting points than analogous ...

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JoVE Core

Organic Chemistry

Unit 1

Carboxylic Acids

KEY TERMS AND CONCEPTS

carboxylic acid boiling point trends

Carboxylic acids have distinct physical properties that change with size and structure. Lower molecular weight carboxylic acids often have a sharp, unpleasant odor. They also tend to have higher boiling and melting points than similar aldehydes, ketones, and alcohols.

Strong intermolecular hydrogen bonding helps explain these trends. The polar carbonyl and hydroxyl bonds create dipole-dipole interactions, and carboxylic acid molecules can form stable dimers. A dimer is a pair of molecules held together by attraction. This doubling effect raises the apparent molecular weight and leads to higher boiling points. These dimers are much more stable than alcohol dimers, with equilibrium constants as large as 10^6 to 10^7 M^-1, compared with about 11 M^-1 for alcohols.

Melting points also follow clear patterns. As hydrocarbon chain length increases, the melting point usually increases. Saturated carboxylic acids with more than eight carbons are typically solids at room temperature. Unsaturated carboxylic acids have lower melting points. For example, stearic acid, with 18 carbons, melts at 70 °C, while linoleic acid, which has two cis double bonds, melts at -5 °C.

Solubility depends on molecular size and the solvent. Low molecular weight carboxylic acids are soluble in water because they can hydrogen bond with water molecules. Higher molecular weight carboxylic acids are more soluble in alcohols because of stronger van der Waals interactions. Carboxylic acids are also highly soluble in non-polar solvents such as CCl4. Some dicarboxylic acids are fairly water-soluble because they can form strong hydrogen bonds.

Carboxylic Acid Boiling Point Trends

iupac nomenclature of carboxylic acids

Naming Carboxylic Acids with IUPAC Rules

Carboxylic acids are named by IUPAC rules that focus on the carboxyl group, –COOH. For acyclic saturated monocarboxylic acids, the longest hydrocarbon chain that includes the –COOH carbon is chosen as the parent chain. The final -e in the parent hydrocarbon name is replaced with -oic acid, and numbering starts at the –COOH carbon. Substituents are then named and placed as prefixes in alphabetical order. In the presence of a carboxyl group, other groups such as –CHO, –CO–, –OH, –OR, –NH2, and...

Carboxylic acids have distinct physical properties that change with size and structure. Lower molecular weight carboxylic acids often have a sharp, unpleasant odor. They also tend to have higher boiling and melting points than similar aldehydes, ketones, and alcohols. Strong intermolecular hydrogen bonding helps explain these trends. The polar carbonyl and hydroxyl bonds create dipole-dipole interactions, and carboxylic acid molecules can form stable dimers. A dimer is a pair of molecules held...

acidity of carboxylic acids

Why Carboxylic Acids Are More Acidic

Carboxylic acids are the strongest organic acids, but they are still much weaker than mineral acids such as HCl. In water, they ionize by losing the hydroxyl proton. This forms a carboxylate ion, which is stabilized by resonance. The strength of this ionization is shown by the acid dissociation constant, or Ka, and the pKa value. For simple carboxylic acids, Ka is around 10^-5, and pKa is usually 4 to 5. These values show that carboxylic acids are moderately acidic. Carboxylic acids are more...

substituent effects on acidity of carboxylic acids

substituent-effects-on-acidity-of-carboxylic-acids

How Substituents Change Carboxylic Acid Strength

Carboxylic acid strength depends on the substituents attached to the acid group. The key factor is the stability of the carboxylate anion, which is the conjugate base formed when the acid loses a proton. An electron-withdrawing substituent makes the carboxylate anion more stable. It pulls electron density away through the inductive effect, which lowers the electron density on the carboxylate ion and increases acidity. An electron-donating substituent does the opposite. It pushes electron...

ir and uv&ndash;vis spectroscopy of carboxylic acids

ir-and-uvvis-spectroscopy-of-carboxylic-acids

Carboxylic Acid Bands in IR and UV-Vis

Carboxylic acids show clear signals in both IR and UV-Vis spectroscopy. Their IR spectra help identify the C=O bond and the O-H bond, while UV-Vis spectra show a weak absorption from an n→π* transition. In IR spectroscopy, the C=O bond usually appears as a characteristic band between 1710 and 1760 cm⁻¹. The O-H bond gives a broad band between 2500 and 3300 cm⁻¹. The exact C=O stretching frequency changes with structure. Free carboxylic acids show a higher stretching frequency at 1760 cm⁻¹.

nmr and mass spectroscopy of carboxylic acids

nmr-and-mass-spectroscopy-of-carboxylic-acids

Carboxylic Acid Signals in NMR and MS

Carboxylic acid signals in NMR and mass spectrometry help identify this functional group. In proton NMR, the acidic –COOH proton is strongly deshielded and usually appears far downfield at about 9–12 ppm. Its exact position can change with concentration and solvent. The nearby carbons also give useful clues. The α protons of carboxylic acids usually appear at 2–2.5 ppm, while the β protons show up farther upfield. A simple test uses deuterium oxide, or D2O, which swaps with the acidic proton...

preparation of carboxylic acids: overview

Making Carboxylic Acids from Oxidation

Carboxylic acids can be made by oxidation of several organic compounds. A primary alcohol or an aldehyde can be converted into a carboxylic acid with a strong oxidizing agent. Aldehydes can also be oxidized with a mild oxidizing agent. Alkyl benzenes with at least one benzylic hydrogen can also form benzoic acid. In this reaction, KMnO4 or Na2Cr2O7 is used under vigorous conditions such as heat and a high oxidant concentration. The product is benzoic acid exclusively. Alkynes can undergo...

preparation of carboxylic acids: hydrolysis of nitriles

preparation-of-carboxylic-acids-hydrolysis-of-nitriles

Nitrile Hydrolysis in Carboxylic Acid Synthesis

Nitrile hydrolysis is a useful way to make carboxylic acids from nitriles. A nitrile has the formula R-CN, and it can be converted into a carboxylic acid, R-COOH, with aqueous acid. Under base-catalyzed conditions, the product is a carboxylate anion, R-COO−. This reaction is also part of a two-step route from haloalkanes to carboxylic acids. First, a haloalkane reacts with sodium cyanide in an SN2 reaction to form a nitrile. Then the nitrile is hydrolyzed to give a carboxylic acid with one...

preparation of carboxylic acids: carboxylation of grignard reagents

preparation-of-carboxylic-acids-carboxylation-of-grignard-reagents

Adding Carbon to Grignard Reagents

Carboxylic acids can be made by adding carbon dioxide to a Grignard reagent. This reaction is useful because it turns alkyl, vinyl, benzyl, and aryl halides into carboxylic acids with one extra carbon atom. The process happens in two steps. First, the Grignard reagent attacks carbon dioxide and forms the magnesium salt of the carboxylate ion. Then an aqueous acid is added to protonate that salt and produce the carboxylic acid. The acid is added only after the first step is complete. A proton...

reactions of carboxylic acids: introduction

reactions-of-carboxylic-acids-introduction

Carboxylic Acid Reactions and Derivatives

Carboxylic acids are organic compounds with an acidic –COOH group. Their acidity comes from resonance stabilization of the conjugate base, which spreads the negative charge over both oxygen atoms. The carbonyl group is also polar because oxygen is highly electronegative. That makes the carbonyl carbon reactive and open to nucleophilic attack. Because of this reactivity, carboxylic acids can undergo nucleophilic acyl substitution. In these reactions, the –COOH group can be converted into acid...

carboxylic acids to esters: acid-catalyzed (fischer) esterification overview

carboxylic-acids-to-esters-acid-catalyzed-fischer-esterification

Driving Fischer Esterification to Esters

Fischer esterification makes esters from carboxylic acids and alcohols in an acidic medium. The reaction also produces water. Emil Fischer developed this condensation reaction in 1895. Hydroxy-functionalized carboxylic acids can react within the same molecule by intramolecular Fischer esterification. This forms lactones, which are cyclic esters. Five-membered and six-membered lactones form spontaneously. The reaction speed depends strongly on steric factors, or how crowded the molecules are...

carboxylic acids to esters: acid-catalyzed (fischer) esterification mechanism

Fischer Esterification: Building Esters from Acids

Fischer esterification is an acid-catalyzed reaction that turns a carboxylic acid and an alcohol into an ester. This condensation reaction is a common way to make esters in organic chemistry. It also involves nucleophilic acyl substitution, which means one group is replaced on the acid carbonyl. The reaction proceeds through a tetrahedral intermediate. During this step, the molecule briefly has a tetrahedral shape before the reaction moves forward. Water is then eliminated as the leaving group.

carboxylic acids to methylesters: alkylation using diazomethane

carboxylic-acids-to-methylesters-alkylation-using-diazomethane

Making Methyl Esters with Diazomethane

Carboxylic acids can be converted into methyl esters using diazomethane in an ether solvent. This reaction gives the methyl ester of the same acid and often does so in excellent yield. It is a useful esterification method in organic chemistry. Diazomethane is a yellow gas with a boiling point of minus 23 degrees Celsius. It is commonly prepared by treating N-methyl-N-nitrosourea or N-methyl-N-nitrosotoluenesulphonamide with a base. During the reaction, the carboxylic acid first protonates...

carboxylic acids to acid chlorides

Making Acid Chlorides from Carboxylic Acids

Carboxylic acids can be converted into acid chlorides using SOCl2 or PCl5. Acid chlorides are the most reactive carboxylic acid derivatives. They are also important synthetic reagents. In some aromatic compounds, they are useful for Friedel-Crafts acylation. Phosphorus pentachloride is one alternative reagent for this conversion. The carboxylic acid first attacks the phosphorus center of PCl5. This step removes a chloride ion. The chloride ion then attacks the carbonyl center and forms the...

carboxylic acids to primary alcohols: hydride reduction

carboxylic-acids-to-primary-alcohols-hydride-reduction

Selective Reduction of Carboxylic Acids

Carboxylic acids can be reduced to primary alcohols with the right reagent. Strong reducing agents, such as lithium aluminum hydride, first react with the acid and then are followed by hydrolysis, which gives the primary alcohol product. Not every hydride reagent works on carboxylic acids. Weaker reducers like lithium tri-tert-butoxyaluminum hydride and diisobutylaluminum hydride do not reduce carboxylic acids to primary alcohols. Borane in tetrahydrofuran, or THF, is another way to make this...

loss of carboxy group as co2: decarboxylation of &beta;-ketoacids

loss-of-carboxy-group-as-co2-decarboxylation-of-ketoacids

Beta-Ketoacids and Carbon Dioxide Loss

Beta-ketoacids can lose a carboxyl group as carbon dioxide gas when they are heated in an acid medium. This reaction is called decarboxylation. Monocarboxylic acids, which have only one carboxyl group, do not decarboxylate easily under the same conditions. Carboxylic acids can also undergo decarboxylation in another way. A silver salt of a carboxylic acid can react with bromine or iodine at high temperature. This Hunsdiecker reaction releases carbon dioxide gas and forms a halide with one...

loss of carboxy group as co2: decarboxylation of malonic acid derivatives

loss-carboxy-group-as-co2-decarboxylation-malonic-acid

Malonic Acid Decarboxylation and CO2 Release

Malonic acid derivatives can lose a carboxyl group as carbon dioxide during decarboxylation. This reaction is a useful way to turn a β-dicarboxylic acid into a monocarboxylic acid. It follows a pattern similar to the thermal decarboxylation of β-keto acids, which form ketones. The mechanism begins with an internal shift of electrons. That shift helps form a cyclic six-membered transition state, a short-lived arrangement of atoms during the reaction. The process then breaks a carbon-carbon bond...

key terms and concepts

Explore how hydrogen bonding, chain length, and structure affect carboxylic acid boiling point, melting point, and solubility in high school chemistry.

Carboxylic Acid Boiling Point Trends | JoVE K12

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Carboxylic Acid Boiling Point Trends

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