Question

Ethylene glycol (HOCH \(\left._{2} \mathrm{CH}_{2} \mathrm{OH}\right)\), the major substance in antifreeze, has a normal boiling point of \(198^{\circ} \mathrm{C}\). By comparison, ethyl alcohol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right)\) boils at \(78^{\circ} \mathrm{C}\) at atmospheric pressure. Ethylene glycol dimethyl ether \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{2} \mathrm{O} \mathrm{CH}_{3}\right)\) has a normal boiling point of \(83{ }^{\circ} \mathrm{C},\) and ethyl methyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\right)\) has a normal boiling point of \(11^{\circ} \mathrm{C}\). (a) Explain why replacement of a hydrogen on the oxygen by a \(\mathrm{CH}_{3}\) group generally results in a lower boiling point. (b) What are the major factors responsible for the difference in boiling points of the two ethers?

Short answer

(a) Replacing hydrogen with CH extsubscript{3} decreases hydrogen bonding. (b) Differences in molecular size and van der Waals interactions.

Step-by-step solution

Analyze bond compositions

Ethers, such as ethylene glycol dimethyl ether and ethyl methyl ether, have replaced one of the hydrogens in the hydroxyl group (OH) of alcohols with a methyl group (CH extsubscript{3}). This replacement reduces the ability to form hydrogen bonds, as oxygen in an ether is less exposed compared to the hydroxyl group in alcohol, which leads to a decreased boiling point.

Assess the role of hydrogen bonding

Hydrogen bonding in molecules significantly affects their boiling points. Ethylene glycol is capable of forming more extensive hydrogen bonds compared to ethyl alcohol due to its two hydroxyl groups, whereas ethers form fewer hydrogen bonds, reducing their boiling points. When a hydrogen atom is replaced with a methyl group, the molecule can form fewer hydrogen bonds, leading to a lower boiling point.

Consider molecular structure and forces

The differences in boiling points between ethylene glycol dimethyl ether and ethyl methyl ether are due to molecular weight and the capacity to form van der Waals interactions. Ethylene glycol dimethyl ether is larger and has more extensive electron cloud interactions compared to ethyl methyl ether, resulting in a higher boiling point.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction that occurs between molecules. It happens when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen or nitrogen, interacts with another electronegative atom. This interaction is crucial because it contributes significantly to the physical properties of substances, such as boiling points.
For instance, ethylene glycol possesses two hydroxyl groups, each capable of forming hydrogen bonds with neighboring molecules. This extensive hydrogen bonding network elevates ethylene glycol’s boiling point, reaching as high as 198°C.
On the other hand, when a hydrogen atom in a hydroxyl group is replaced by a methyl group, as seen in ethers, the ability to hydrogen bond is drastically reduced. This is because the nonpolar methyl group does not interact favorably with electronegative atoms, resulting in a lower boiling point.

Ethers

Ethers contain an oxygen atom bonded to two alkyl or aryl groups. They are distinct from alcohols because the hydroxyl group's hydrogen is replaced by an organic substituent. The key feature here is the ether's oxygen atom, which can participate in hydrogen bonding but to a lesser extent than alcohols.
Ethers such as ethyl methyl ether and ethylene glycol dimethyl ether do not create hydrogen bonds between their molecules as effectively as alcohols do because they lack hydrogen connected directly to the oxygen. This significantly lowers their boiling points compared to alcohols. For example, ethyl methyl ether has a boiling point of only 11°C.
Although ethers can engage in dipole-dipole interactions due to the polar C–O–C linkage, these are not as strong as the hydrogen bonds found in alcohols.

Molecular Structure

The molecular structure determines many physical properties of a compound, including its boiling point. The structure impacts how easily different types of intermolecular forces can be established.
For instance, ethylene glycol is simpler in structure, allowing it to form extensive hydrogen bonds. Its linear chain and two hydroxyl groups contribute to a higher boiling point of 198°C. In contrast, the molecular structure of ethers, like ethylene glycol dimethyl ether and ethyl methyl ether, includes oxygen bound to more bulky groups, reducing the molecule's ability to form intermolecular interactions like hydrogen bonds.
The replacement of hydrogen atoms by methyl groups—common in ethers—adds bulk and reduces hydrogen bonding. This change mainly leads to weaker interactions and, consequently, lowers the boiling points of the substances.

Van der Waals Interactions

Van der Waals interactions are weak forces that contribute to the overall intermolecular attractions between molecules. They are generally present in all molecular interactions but become especially notable when other stronger forces like hydrogen bonding are absent.
In the case of ethers, their boiling points are influenced primarily by these Van der Waals forces. Larger molecules can have more significant Van der Waals interactions due to a more substantial electron cloud that can fluctuate, temporarily inducing dipoles.
For instance, ethylene glycol dimethyl ether, being larger than ethyl methyl ether, experiences stronger Van der Waals attractions. This explains its higher boiling point of 83°C compared to ethyl methyl ether's 11°C. Thus, even when hydrogen bonding is not substantial, the size and shape of a molecule can greatly influence its boiling point through these interactions.