Question

Both the melting point and boiling point of acetamide are higher than those of its \(N, N\) dimethyl derivative. How do you account for these differences? CC(N)=O CC(=O)N(C)C \(\begin{array}{cc}\text { Acetamide } & N, N \text {-Dimethylacetamide } \\\ \mathrm{mp} 82.3^{\circ} \mathrm{C}, \mathrm{bp} 221.2^{\circ} \mathrm{C} & \mathrm{mp}-20^{\circ} \mathrm{C}, \mathrm{bp} 165^{\circ} \mathrm{C}\end{array}\)

Short answer

The difference in melting and boiling points between acetamide and its N,N-dimethyl derivative can be attributed to the presence of stronger intermolecular forces (IMFs) in acetamide. Acetamide exhibits hydrogen bonding (a stronger type of IMF) due to its amide group (-NH_2) while N,N-dimethylacetamide lacks hydrogen bonding and only exhibits dipole-dipole interactions. Compounds with stronger IMFs require more energy to overcome these forces, leading to higher melting and boiling points. Therefore, acetamide has higher melting and boiling points compared to its N,N-dimethyl derivative.

Step-by-step solution

Consider each compound's structure

We need to first examine the molecular structures of both acetamide and its N,N-dimethyl derivative, as this will give us insights into the potential IMFs present. For acetamide (CC(N)=O), we have: \(1\) carbonyl group (C=O) \(1\) amide group (-NH_2) For N,N-dimethylacetamide (CC(=O)N(C)C), we have: \(1\) carbonyl group (C=O) \(1\) N-substituted amide group (-N(CH_3)_2)

Identify the intermolecular forces

Now that we have analyzed the structures of both compounds, we can identify the possible IMFs present in each of them. For acetamide, the IMFs include: \(1\). Hydrogen bonding (due to the amide group -NH_2) \(2\). Dipole-dipole interactions (due to the presence of the polar carbonyl group C=O) For N,N-dimethylacetamide, the IMFs include: \(1\). Dipole-dipole interactions (due to the presence of the polar carbonyl group C=O and the partially polar N-substituted amide -N(CH_3)_2)

Compare the intermolecular forces

It's important to note that hydrogen bonding is the strongest type of IMF among dipole-dipole interactions. Since acetamide has the capacity for hydrogen bonding due to its -NH_2 group, it will have stronger intermolecular forces present than N,N-dimethylacetamide, which only has dipole-dipole interactions.

Relate intermolecular forces to melting and boiling points

Substances with stronger intermolecular forces require more energy to overcome these forces and transition from a solid to a liquid (melting) or from a liquid to a gas (boiling). As a result, compounds with stronger IMFs typically have higher melting and boiling points.

Conclusion

The differences in melting and boiling points between acetamide and its N,N-dimethyl derivative can be attributed to the presence of stronger intermolecular forces (hydrogen bonding) in acetamide. This stronger IMF results in acetamide having higher melting and boiling points when compared to N,N-dimethylacetamide, which only has dipole-dipole interactions.

Key concepts

Hydrogen Bonding

Understanding hydrogen bonding is crucial to grasping why certain organic compounds have particular physical properties. Hydrogen bonding is a type of strong dipole-dipole interaction that occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, experiences an attractive force from a lone pair of electrons on another electronegative atom in a nearby molecule.

This electrostatic attraction results in hydrogen bonds, which are stronger than regular dipole-dipole interactions, but weaker than covalent or ionic bonds. In the context of organic compounds, hydrogen bonding significantly influences boiling and melting points. For example, acetamide, with its amide group (-NH₂), can form hydrogen bonds leading to higher melting and boiling points compared to compounds without this capability, such as N,N-dimethylacetamide which lacks hydrogens directly bonded to the nitrogen.

Melting and Boiling Points

Melting and boiling points are physical characteristics that can be greatly affected by intermolecular forces. The melting point is the temperature at which a solid becomes a liquid, and the boiling point is the temperature at which a liquid becomes a gas.

Compounds with strong intermolecular forces, such as hydrogen bonds, typically have higher melting and boiling points because more energy is required to break these interactions. This is why acetamide has a higher melting point (82.3°C) and boiling point (221.2°C) when compared to its N,N-dimethyl derivative, which exhibits lower melting (-20°C) and boiling (165°C) points due to its weaker dipole-dipole interactions and lack of hydrogen bonding. In essence, the stronger the intermolecular forces, the more energy is needed to change the phase of the compound.

Amide Derivatives

Amide derivatives are organic compounds characterized by the presence of an amide group, which includes a carbonyl group (C=O) attached to a nitrogen atom. An important feature of amides is their ability to participate in hydrogen bonding due to the polar nature of the carbonyl and the presence of a hydrogen atom attached directly to the nitrogen.

When looking at derivatives of amides, such as N,N-dimethylacetamide, we observe a modification where the hydrogens on the nitrogen are replaced with methyl groups (CH₃). This substitution hinders the molecule's ability to engage in hydrogen bonding as the direct connection between hydrogen and nitrogen is lost. As a result, amide derivatives that cannot form hydrogen bonds exhibit lower melting and boiling points, demonstrating how even slight structural changes in organic molecules can significantly impact their physical properties.