Question

Give plausible explanation for each of the following: (i) Why are amines less acidic than alcohols of comparable molecular masses? (ii) Why do primary amines have higher boiling point than tertiary amines? (iii) Why are aliphatic amines stronger bases than aromatic amines?

Short answer

Amines are less acidic due to weaker stabilization of the conjugate base. Primary amines have higher boiling points due to extensive hydrogen bonding. Aliphatic amines are stronger bases as their nitrogen lone pairs are more available due to less delocalization.

Step-by-step solution

Understanding Acidity of Amines and Alcohols

Amines have a nitrogen atom with a lone pair of electrons capable of accepting protons, while alcohols have an -OH group. The difference in acidity is due to the ability of the nitrogen or oxygen to donate the lone pair or stabilize the resulting conjugate base. Oxygen, being more electronegative, stabilizes the negative charge better than nitrogen, making alcohols more acidic than amines.

Boiling Points in Primary vs Tertiary Amines

Boiling points are influenced by hydrogen bonding. Primary amines can form more hydrogen bonds due to having two hydrogen atoms attached to the nitrogen, making them capable of stronger intermolecular forces compared to tertiary amines, which lack hydrogen atoms bonded to the nitrogen, leading to weaker intermolecular forces and lower boiling points.

Base Strength of Aliphatic vs Aromatic Amines

The base strength of amines is determined by the availability of the lone pair on the nitrogen atom to accept protons. In aliphatic amines, the electron-donating alkyl groups increase electron density on the nitrogen, enhancing its basicity. However, in aromatic amines, the lone pair is delocalized into the aromatic ring, reducing availability to accept protons, thus making aliphatic amines stronger bases.

Key concepts

Acidity comparison in Amines and Alcohols

Amines are less acidic compared to alcohols, and this difference can root back to the molecular structures and electronegativities of oxygen and nitrogen. In alcohols, the -OH group features an oxygen atom, which is highly electronegative. This means oxygen is adept at drawing electronic density towards itself, thereby stabilizing the negative charge in the conjugate base when a proton is lost. This stabilization translates to increased acidity.
In contrast, amines have a nitrogen atom. Although it also possesses a lone pair that can lead to proton donation, nitrogen is less electronegative compared to oxygen. Its lesser ability to stabilize the negative charge renders amines less acidic than alcohols with comparable molecular masses.

Boiling Points in Primary versus Tertiary Amines

The boiling points of amines, like many substances, are largely determined by the presence and strength of intermolecular forces such as hydrogen bonding. Primary amines have two hydrogen atoms bonded to the nitrogen, allowing them to form stronger hydrogen bonds. This increases the cohesion between molecules, necessitating more heat energy to break these interactions when increasing temperature.
On the other hand, tertiary amines lack the hydrogen atoms attached to nitrogen, leading to fewer opportunities for hydrogen bonding. With weaker intermolecular forces, tertiary amines have lower boiling points compared to primary amines.

Base Strength in Aliphatic versus Aromatic Amines

Amines are known for acting as bases because they can accept protons through their lone electron pair on nitrogen. However, the environment around nitrogen significantly influences its basicity. Aliphatic amines, for example, have alkyl groups which are electron-donating. This increases the electron density on nitrogen, making it more effective at accepting protons, thus strengthening its basic nature.
Aromatic amines introduce a complexity due to electron delocalization. Here, the nitrogen’s lone pair can resonate into the aromatic ring, decreasing its availability to bind with protons. This electron sharing reduces the base strength of aromatic amines compared to their aliphatic counterparts.

Role of Hydrogen Bonding in Amines

Hydrogen bonding plays a crucial role in determining the physical properties of amines, notably their boiling points and solubility in water. Amines with hydrogen atoms directly connected to nitrogen, like primary and secondary amines, have the ability to partake in hydrogen bonding. This leads to stronger intermolecular attractions in liquids, contributing to higher boiling points. Furthermore, this hydrogen bonding capability aids in the solubility of amines in water.
Tertiary amines, lacking hydrogen atoms bonded to nitrogen, have limited hydrogen bonding abilities, resulting in lower boiling points and varying solubility depending on their molecular structure and size.

Electron Delocalization in Aromatic Amines

Electron delocalization is paramount in understanding the reactivity and base strength of aromatic amines. In such aromatic structures, the lone pair of electrons on the nitrogen atom can resonate with the pi system of the aromatic ring. This process involves the spreading out or delocalization of electrons across several atoms or bonds and adds an element of stability to the molecule.
While this stability is beneficial, it also means that the lone pair is less available to interact with protons, diminishing the base strength of the amine. The aromatic system effectively "shares" nitrogen's electrons, unlike in straight-chain or branched aliphatic amines. This electron sharing has profound effects on the chemical behavior of aromatic amines, influencing their reactivity patterns in organic synthesis.