Lena Savoie
Carboxylic acids have a higher boiling point than alcohols, aldehydes, ketones, and hydrocarbons with comparable molecular masses. This is due to the presence of intermolecular hydrogen bonding, which is stronger in carboxylic acids. The boiling point of a liquid is the temperature at which it starts boiling, and it is influenced by factors such as molecular weight, pressure, and the strength of intermolecular forces. Carboxylic acids have stronger intermolecular forces due to the formation of hydrogen bonds between the hydrogen of one acid and the oxygen of another. This results in carboxylic acids existing as dimers, requiring higher heat energy to break the molecules apart.
| Characteristics | Values |
|---|---|
| Boiling point of carboxylic acid | Higher |
| Boiling point of alcohol | Lower |
| Reason for higher boiling point of carboxylic acid | Presence of intermolecular hydrogen bonding |
| Reason for lower boiling point of alcohol | Cannot form intermolecular hydrogen bonding |
| Molecular weight | Higher molecular weight leads to higher boiling point |
| Pressure | Higher pressure leads to higher boiling point |
| Non-volatile impurity | Addition of non-volatile impurities leads to higher boiling point |
| Branching | Higher number of branches leads to lower boiling point |
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What You'll Learn
Carboxylic acid has a higher boiling point due to intermolecular hydrogen bonding
Carboxylic acids have a higher boiling point than alcohols, aldehydes, ketones, and hydrocarbons with comparable molecular masses. This is due to the presence of intermolecular hydrogen bonding in carboxylic acids.
The boiling point of a liquid is the temperature at which it starts boiling, and it is unique to every liquid. The boiling point increases with an increase in molecular weight and intermolecular forces. Carboxylic acids and alcohols both contain an O-H bond, but the O-H bond in carboxylic acids is more strongly polarized due to the presence of adjacent electron-withdrawing carbonyl groups. This allows carboxylic acids to form stronger hydrogen bonds.
In carboxylic acids, hydrogen bonding occurs between the hydrogen of one carboxylic acid and the oxygen of the carbonyl group of another carboxylic acid. This leads to the formation of cyclic dimers, with two hydrogen bonds between every two molecules of carboxylic acid. The hydrogen bonds in carboxylic acids are not easily broken, even in the vapour phase, and therefore, they require higher heat energy to break or separate the molecules.
On the other hand, alcohols do not have the same type of strong attractive forces between their molecules. While they do exhibit hydrogen bonding, only one hydrogen bond can be formed between two alcohol molecules. This results in carboxylic acids having stronger intermolecular forces and, consequently, higher boiling points than alcohols.
For example, ethanoic acid and propanol have the same molecular mass of 60. However, the boiling point of ethanoic acid is 391K, while that of propanol is 370K. This difference in boiling points can be attributed to the stronger intermolecular hydrogen bonding in carboxylic acids.
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Carboxylic acid exists as dimers
Carboxylic acid has a higher boiling point than alcohol, aldehyde, ketone, and even hydrocarbons. This is due to the presence of intermolecular hydrogen bonding. The hydrogen bond is a stronger bond between electronegative atoms like oxygen, sulphur, and fluorine, and hydrogen. In carboxylic acid, the hydrogen bonding is established between the hydrogen of one carboxylic acid and the oxygen of the carbonyl group of another carboxylic acid molecule. This is why carboxylic acid exists as dimers.
The dimerisation of carboxylic acid is an important concept in chemistry. Carboxylic acid molecules are held together by two hydrogen bonds, forming cyclic dimers. The hydrogen bonds are not easily broken, even in the vapour phase. Therefore, they require higher heat energy to break or separate the molecules from each other.
The dimerisation of carboxylic acid is a useful model for understanding the interplay of hydrogen bonding, hydrophobic effects, and entropy in self-association and assembly. The hydrophobic interactions and entropy changes upon binding also play a role in the self-association of carboxylic acids in water.
Furthermore, the dimerisation of carboxylic acid provides insights into the thermodynamics of hydrogen bonding and hydrophobic interactions. The changes in total entropy and enthalpy upon dimerization can be negative, as observed in formic acid, or become less negative and more positive with longer alkyl chains. These observations align with our understanding of hydrogen bonding and hydrophobic interactions, where the presence of adjacent electron-withdrawing carbonyl groups results in a stronger hydrogen bond.
In summary, the existence of carboxylic acid as dimers is a crucial aspect of its chemical behaviour and contributes to its higher boiling point compared to corresponding alcohols. The dimerisation of carboxylic acid involves hydrogen bonding, hydrophobic effects, and entropy changes, offering valuable insights into the underlying chemical processes.
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Carboxylic acid has a higher boiling point than aldehyde, ketone, and alcohol
Comparing carboxylic acid with alcohol, aldehyde, and ketone, which have comparable molecular masses, carboxylic acid has a higher boiling point. This is because, in carboxylic acid, the $\text{O-H}$ bond is more strongly polarised due to the presence of adjacent electron-withdrawing carbonyl groups. Therefore, carboxylic acids can form stronger hydrogen bonds. The molecules of carboxylic acids are held together by two hydrogen bonds and form cyclic dimers. These types of attractive forces do not exist between alcohols, aldehydes, or ketones.
The boiling point of a substance depends on the strength of the intermolecular forces in a molecule. The stronger the intermolecular forces, the higher the boiling point. Carboxylic acid has stronger intermolecular forces than alcohol, aldehyde, or ketone, which is why it has a higher boiling point.
For example, let's consider ethanoic acid and propanol. They have the same molecular mass of 60. However, the boiling point of ethanoic acid is $391\text{ K}$, while that of propanol is $370\text{ K}$. Acetic acid and ethanol are another example of a carboxylic acid and an alcohol with similar molecular compositions. Acetic acid has a boiling point of $117.9\text{ °C}$, while ethanol's boiling point is $78.3\text{ °C}$.
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Carboxylic acid has a stronger O-H bond
Carboxylic acids have a higher boiling point than alcohols due to the presence of intermolecular hydrogen bonding. Hydrogen bonds are the stronger bonds formed between electronegative atoms like oxygen and hydrogen.
In the case of carboxylic acid, the hydrogen bonding is established between the hydrogen of one carboxylic acid and the oxygen of the carbonyl group of another carboxylic acid molecule. This is also known as dimerization. The hydrogen bonds in carboxylic acids are stronger than those in alcohols because the O-H bond in carboxylic acids is more strongly polarized due to the presence of adjacent electron-withdrawing carbonyl groups.
The electronegativity of the oxygen atom in the carboxyl group is greater than that of the hydroxyl group in alcohols, resulting in a more polarized O-H bond in carboxylic acids. This polarization makes the oxygen atom partially negative and the hydrogen atom partially positive, creating a stronger hydrogen bond.
Additionally, carboxylic acids have two hydrogen bonds holding their molecules together, while alcohols do not have these additional attractive forces. The presence of these extra hydrogen bonds in carboxylic acids increases the strength of the intermolecular forces, leading to a higher boiling point.
The boiling point of a substance is influenced by several factors, including molecular weight, pressure, and the presence of non-volatile impurities. However, the primary factor contributing to the higher boiling point of carboxylic acids is the presence of strong intermolecular hydrogen bonds.
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Carboxylic acid has a higher molecular weight
Carboxylic acid has a higher boiling point than alcohol, despite both molecules containing an O-H bond. This is due to the presence of intermolecular hydrogen bonding in carboxylic acid, which is a stronger bond between electronegative atoms such as O, S, and F. The hydrogen bonding in carboxylic acid is established between the hydrogen of one carboxylic acid and the oxygen of the carbonyl group of another carboxylic acid molecule. This type of bonding does not exist in alcohol.
The boiling point of a liquid is the temperature at which it starts boiling and is dependent on the type of bonding present in the molecule. Carboxylic acid exists as dimers due to hydrogen bonding, and these bonds are not easily broken, even in the vapour phase. Therefore, a higher amount of heat energy is required to break or separate the molecules from each other.
The O-H bond in carboxylic acids is more strongly polarised than in alcohols due to the presence of adjacent electron-withdrawing carbonyl groups. This allows carboxylic acids to form stronger hydrogen bonds. The molecules of carboxylic acids are held together by two hydrogen bonds, forming cyclic dimers, whereas two alcohol molecules can only form one hydrogen bond between each other. This results in stronger intermolecular forces and higher boiling points in carboxylic acids compared to alcohols of comparable molecular masses.
An example of this can be observed when comparing ethanoic acid and propanol, which have the same molecular mass of 60. The boiling point of ethanoic acid is 391K, while that of propanol is 370K. Acetic acid, for instance, is more than 1011 times more acidic than ethanol, despite both being oxygen acids. This is due to the resonance contributor, which localizes the negative charge on the second oxygen of the group in acetic acid, resulting in a greater polarization of the O-H bond.
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Frequently asked questions
Carboxylic acid has a higher boiling point than alcohol.
The higher boiling point is due to the presence of intermolecular hydrogen bonding. The hydrogen bond is a stronger bond between electronegative atoms like O, S, F, etc. and H atoms.
The boiling point of ethanoic acid is 391K, while that of propanol is 370K.
The boiling point increases with an increase in molecular weight.
Water has a boiling point of 100° C.
