Question

N-Methylpyrrolidine has a boiling point of${\mathbf{81}}^{\mathbf{o}}\mathbf{C}$, and piperidine has a boiling point of${\mathbf{106}}^{\mathbf{o}}\mathbf{C}$.

1. Explain the large difference (${\mathbf{25}}^{\mathbf{o}}\mathbf{C}$) in boiling point for these two isomers.
2. Tetrahydropyran has a boiling point of ${\mathbf{88}}^{\mathbf{o}}\mathbf{C}$, and cyclopentanol has a boiling point of ${\mathbf{141}}^{\mathbf{o}}\mathbf{C}$. These two isomers have a boiling point difference of ${\mathbf{53}}^{\mathbf{o}}\mathbf{C}$.Explain why the two oxygen-containing isomers have a much larger boiling point difference than two amine isomers.
3. N,N-Dimethylformamide has a boiling point of ${\mathbf{150}}^{\mathbf{o}}\mathbf{C}$, and N-methylacetamide has a boiling point of ${\mathbf{206}}^{\mathbf{o}}\mathbf{C}$, for a difference of ${\mathbf{56}}^{\mathbf{o}}\mathbf{C}$.Explain why these two nitrogen-containing isomers have a much larger boiling point difference than the two amine isomers. Also explain why these two amides have higher boiling points than any of the other four compounds shown (two amines, an ether, and an alcohol).

Short answer

(a) Piperidine has an$N-H$bond so it can hydrogen bond with other molecules of itself. N-methylpyrrolidine has no $N-H$bond so it cannot form hydrogen bond and will require less energy to separate one molecule from another, hence will have less boiling point.

(b) Cyclopentanol is involved in hydrogen bonding with other molecules of cyclopentanol whereas tetrahydropyran does not and that’s the reason why boiling point of cyclopentanol is more than that of tetrahydropyran .

(c) N,N-Dimethylformamide has a boiling point of ${\mathbf{150}}^{\mathbf{o}}\mathbf{C}$, and N-methylacetamide has a boiling point of ${\mathbf{206}}^{\mathbf{o}}\mathbf{C}$, for a difference of ${\mathbf{56}}^{\mathbf{o}}\mathbf{C}$. This is due to the resonance structures of these two compounds. The first structure, that is of N,N-dimethylformamide has no hydrogen bonding whereas the second structure, that is of N-methylacetamide has hydrogen bonding in addition to the strong dipole-dipole interaction reflected in its higher boiling point.

Both amides boil higher than the four other compounds because of the dipole-dipole interactions that exist due to the resonance forms.

Step-by-step solution

Step-1. Explanation of part (a):

N-Methylpyrrolidine has a boiling point of${\mathbf{81}}^{\mathbf{o}}\mathbf{C}$, and piperidine has a boiling point of${106}^{\mathrm{o}}\mathrm{C}$, this is because piperidine has an $N-H$bond so it can hydrogen bond with other molecules of itself. N-methylpyrrolidine has no $N-H$bond so it cannot form hydrogen bond and will require less energy to separate one molecule from another, hence will have less boiling point. Intermolecular hydrogen bonding increases boiling point of the molecule.

Step-2. Explanation of part (b):

Tetrahydropyran has a boiling point of${88}^{\mathrm{o}}\mathrm{C}$, and cyclopentanol has a boiling point of${141}^{o}C$. These two isomers have a boiling point difference of ${53}^{o}C$.This is because, the OH group present in cyclopentanol is involved in intermolecular hydrogen bonding with other molecules of cyclopentanol whereas tetrahydropyran does not and that’s the reason why boiling point of cyclopentanol is more than that of tetrahydropyran. Due to hydrogen bonding, the force of attraction between the molecules increases, and hence, more energy will be required to separate the two molecules, thereby increasing the boiling point of the molecule.

Step-3. Explanation of part (c):

N,N-Dimethylformamide has a boiling point of${150}^{\mathrm{o}}\mathrm{C}$, and N-methylacetamide has a boiling point of${206}^{o}C$, for a difference of${56}^{\mathrm{o}}\mathrm{C}$. This is due to the resonance structures of these two compounds.The first structure, that is of N,N-dimethylformamide has no hydrogen bonding because it has no $\mathrm{O}-\mathrm{H}$or $\mathrm{N}-\mathrm{H}$ bond but it is a highly polar structure and has large dipole moment. In the second structure, that is of N-methylacetamide, hydrogen bonding exists in addition to the strong dipole-dipole interaction reflected in its higher boiling point.

Both amides boil higher than the four other compounds because of the dipole-dipole interactions that exist due to the resonance forms of the amide due to which boiling point increases.