Noah Vinter
Amines and alcohols are both organic compounds with comparable molar mass, yet amines have lower boiling points than alcohols. This is because the boiling point of a substance depends on the strength of the bonding present. The more the intermolecular force of hydrogen bonding, the higher the boiling point. In amines, the hydrogen bond is formed between the nitrogen atom and the hydrogen atom of water, while in alcohols, the hydrogen bond is formed between the oxygen atom and the hydrogen atom. Since the $O-H$ bond in alcohol is stronger than the $N-H$ bond in amines, the boiling point of alcohols is higher.
| Characteristics | Values |
|---|---|
| Boiling point of substance depends on | Bonding present between them or the hydrogen bonding |
| Reason for lower boiling point of amines | The \(O - H bond in alcohol is stronger than the\) N - H bond in amine |
| Oxygen is more electronegative than nitrogen | |
| Oxygen has two lone pairs of electrons to act as hydrogen acceptors, whereas nitrogen has only one | |
| Amines can only participate as a single hydrogen bond acceptor and donor (except for tertiary amines) |
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What You'll Learn
Hydrogen bonding
The boiling point of a substance depends on the strength of the hydrogen bonding present in the molecule. The stronger the intermolecular force of hydrogen bonding, the higher the boiling point.
Primary and secondary amines have higher boiling points than alkanes or ethers of similar molar mass because they can engage in intermolecular hydrogen bonding. Amines have hydrogen atoms bonded to a nitrogen atom and are therefore capable of hydrogen bonding.
However, the boiling points of amines are lower than those of alcohols. This is because alcohol molecules have hydrogen atoms bonded to an oxygen atom, which is more electronegative than nitrogen. The $\ce{O-H}$ bond in alcohols is more polar than the $\ce{N-H}$ bond in amines and can therefore form stronger hydrogen bonds.
Additionally, the fact that an alcohol has two lone pairs of electrons is the main reason for its higher boiling point compared to amines. Although a primary amine has two hydrogen atoms that can act as hydrogen bond donors, their weaker $\delta +$ charge is spread among the two, making those hydrogen bonds very weak. As a result, a primary amine can effectively act as a single hydrogen bond donor.
In summary, the difference in boiling points between amines and alcohols can be attributed to the relative strengths of their hydrogen bonds, which are influenced by the electronegativity of the atoms involved and the number of lone electron pairs available for bonding.
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Electronegativity of atoms
The boiling points of amines are lower than those of corresponding alcohols due to the difference in electronegativity of atoms. Amines contain nitrogen and hydrogen atoms bonded together, while alcohols contain oxygen and hydrogen atoms bonded together. Nitrogen has a lower electronegativity compared to oxygen. This results in weaker N-H bonds in amines than O-H bonds in alcohols.
Electronegativity is a fundamental concept in chemistry that describes the tendency of an atom to attract a shared pair of electrons within a chemical bond. It is key to understanding the behaviour of molecules during reactions and in different states of matter. On the periodic table, electronegativity increases from left to right and decreases from top to bottom. This means that atoms like fluorine, oxygen, and nitrogen are among the most electronegative.
The higher electronegativity of oxygen in alcohols leads to a stronger attraction between molecules. Oxygen has two lone pairs of electrons that can act as hydrogen acceptors, while nitrogen has only one. This results in a stronger partial charge in oxygen. The difference in electronegativity between nitrogen and oxygen determines the strength of intermolecular hydrogen bonding. Hydrogen bonding is a strong intermolecular force that occurs between molecules containing a highly electronegative atom bonded to a hydrogen atom. The strength of hydrogen bonds depends on the electronegativity of the atoms involved. A higher electronegativity indicates a stronger ability to attract electrons, leading to a stronger partial charge and enhancing the hydrogen bond's strength.
Since oxygen is more electronegative than nitrogen, the hydrogen bonding in alcohols is generally stronger than that in amines. This stronger hydrogen bonding holds alcohol molecules more tightly together, leading to higher boiling points than amines. Boiling points are determined by the strength of the intermolecular forces between molecules. The stronger the intermolecular forces, the more energy is required to separate the molecules, and thus the higher the boiling point.
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Tertiary amines
Amines are usually compared with alcohols because of their structural similarities. Tertiary amines have a lower boiling point than alcohols of similar molecular mass. This is because tertiary amines cannot form hydrogen bonds. They do not have a hydrogen atom bound to a nitrogen, oxygen or fluorine atom. They can only accept hydrogen bonds from water, so they have much lower solubility than alcohols.
The boiling points of tertiary amines are comparable to those of alkanes and ethers of similar molar mass. This reflects the dipole/dipole intermolecular attractions. Amines have higher boiling points than hydrocarbons of comparable molecular mass because they are polar compounds. However, with the exception of tertiary amines, they form associate molecules due to intermolecular hydrogen bonding between the nitrogen of one molecule and the hydrogen of another.
The order of boiling points of isomeric amines is: primary amines have the highest boiling point, followed by secondary amines, and then tertiary amines which have the lowest boiling point. This is because primary amines can form more hydrogen bonds between molecules than secondary amines. Secondary amines have a smaller molecular dipole due to the presence of nitrogen within the carbon chain.
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Molar mass
The boiling point of a substance is determined by the strength of the bonding present between the molecules. The stronger the intermolecular forces, the higher the boiling point. Amines and alcohols can both form hydrogen bonds, but the strength of these bonds differs due to the differing electronegativity of the atoms involved.
Amines contain a nitrogen atom, which has a lower electronegativity than the oxygen atom found in alcohols. This results in the $\ce{N-H}$ bond in amines being less polar than the $\ce{O-H}$ bond in alcohols. The oxygen atom in alcohols has two lone pairs of electrons, which can act as hydrogen bond acceptors, whereas the nitrogen atom in amines has only one. This results in a stronger hydrogen-bonding network in alcohols, leading to a higher boiling point.
The type of amine also plays a role in its boiling point. Primary and secondary amines have hydrogen atoms bonded to a nitrogen atom, allowing them to form hydrogen bonds. However, tertiary amines lack a hydrogen atom bonded to nitrogen and, therefore, cannot participate in hydrogen bonding. Consequently, the boiling points of tertiary amines are comparable to those of alkanes and ethers, which have similar molar masses.
In summary, the difference in boiling points between amines and alcohols of comparable molar masses can be attributed to the differing strengths of their intermolecular hydrogen bonds. The higher electronegativity of oxygen in alcohols results in stronger hydrogen bonds and, consequently, a higher boiling point compared to amines.
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Intermolecular forces
The boiling point of a substance is determined by the strength of the intermolecular forces present within it. These intermolecular forces are influenced by the type of bonding that occurs between the molecules. In the case of amines and alcohols, hydrogen bonding is the predominant intermolecular force.
Amines and alcohols differ in the atom involved in hydrogen bonding with other molecules. Amines form hydrogen bonds through the nitrogen atom (N-H bond), while alcohols involve the oxygen atom (O-H bond). Oxygen is more electronegative than nitrogen, resulting in a stronger polar bond in alcohols compared to amines. This increased polarity in alcohols leads to stronger intermolecular forces, which, in turn, raises the boiling point.
Additionally, the number of hydrogen bond donors and acceptors plays a role in the strength of intermolecular forces. Alcohols have two lone pairs of electrons on the oxygen atom, allowing them to act as strong hydrogen bond acceptors. In contrast, amines have only one lone pair of electrons on the nitrogen atom, making them weaker hydrogen bond acceptors. Consequently, alcohols can participate in a more extensive hydrogen bonding network, further contributing to their higher boiling points.
The strength of the intermolecular forces within a substance directly impacts its boiling point. With stronger intermolecular forces, a higher temperature is required to break these forces and reach the boiling point. Conversely, substances with weaker intermolecular forces have lower boiling points because less energy is needed to overcome these forces and transition to a gas phase.
In summary, the difference in boiling points between amines and alcohols of comparable molar mass can be attributed to the varying strengths of their intermolecular forces, which are influenced by the type of bonding and the number of available hydrogen bond donors and acceptors. Alcohols, with their stronger O-H bonds and superior hydrogen bonding capabilities, exhibit higher boiling points compared to amines.
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Frequently asked questions
The boiling point of a substance depends on the strength of the bonding present. The more the intermolecular force of hydrogen bonding, the higher the boiling point. Amines have an $ N - H $ bond which is weaker than the $ O - H $ bond in alcohols, as the oxygen atom is more electronegative than nitrogen.
Methylamine (CH3NH2) has a boiling point of −6°C, whereas methanol (CH3OH) has a boiling point of 65°C, despite their similar molar masses.
No, primary and secondary amines have higher boiling points than alkanes or ethers of similar molar mass due to their ability to form hydrogen bonds. However, their boiling points are still lower than those of alcohols. Tertiary amines, which lack an $ N - H $ bond, have boiling points comparable to alkanes and ethers.
