Ethylene glycol (HOCH2CH2OH), the major substance in antifreeze, has a normal boiling point of 198 °C. By comparison, ethyl alcohol (CH3CH2OH) boils at 78 °C at atmospheric pressure. Ethylene glycol dimethyl ether (CH3OCH2CH2OCH3) has a normal boiling point of 83 °C, and ethyl methyl ether (CH3CH2OCH3) has a normal boiling point of 11 °C.
a) Explain why replacement of a hydrogen on the oxygen by CH3 generally results in a lowering boiling point.
Note that part b) of this question follows below.
A By replacing the -OH group with an -OCH3 group, the molecule has lost its ability to form H-bonds.
B By replacing the -OH group with an -OCH3 group, the molecule has gained the ability to form more effective London dispersion forces.
C By replacing the -OH group with an -OCH3 group, the molecule has gained the ability to form H-bonds, which is the weakest of the IMF, replacing the stronger London dispersion forces and dipole-dipole intermolecular forces that were on the molecule before this change.
D This is a trick question. The boiling points should actually be swapped for ethylene glycol and ethylene glycol dimethyl ether, as well as the ethyl alcohol and ethyl methyl ether. Replacing the -OH group with an -OCH3 group should actually cause an increase in boiling point.
The correct answer and explanation is :
Correct Answer: A
“By replacing the -OH group with an -OCH3 group, the molecule has lost its ability to form H-bonds.”
Explanation:
Boiling point is largely influenced by the strength of intermolecular forces (IMFs) within a substance. The stronger these forces, the more energy (in the form of heat) is required to break them and allow the molecules to escape into the gas phase.
Hydrogen bonding is one of the strongest types of IMFs. It occurs when a hydrogen atom is directly bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. In alcohols like ethyl alcohol (CH₃CH₂OH) and ethylene glycol (HOCH₂CH₂OH), the presence of -OH (hydroxyl) groups allows for strong hydrogen bonding between molecules. Ethylene glycol, with two -OH groups, forms extensive hydrogen bonding networks, which significantly raise its boiling point to 198 °C.
When a -OH group is replaced by a -OCH₃ (methoxy) group to form an ether, such as ethyl methyl ether or ethylene glycol dimethyl ether, the molecule can no longer donate hydrogen bonds (since it lacks the -OH hydrogen). While ethers still have a polar C–O bond and can accept hydrogen bonds (e.g., from water), they cannot form hydrogen bonds among themselves the way alcohols can. This greatly reduces the overall strength of IMFs in the liquid phase, resulting in much lower boiling points.
For instance, ethyl methyl ether (CH₃CH₂OCH₃) has a boiling point of just 11 °C—dramatically lower than ethyl alcohol (78 °C). The difference arises solely from the loss of hydrogen bonding capability upon replacing -OH with -OCH₃.
In short, the key factor in the boiling point difference is hydrogen bonding. Losing -OH groups and replacing them with -OCH₃ groups means losing the molecule’s ability to form these strong intermolecular forces, thus lowering the boiling point.