The covalent compounds ethyl alcohol and dimethyl ether have the formula C2H6O. However, the alcohol melts at -117.3 degrees C and boils at 78.5 degrees C the ether melts at -138.5 degrees C and boils at -23.7 degrees C. How could differences in forces between molecules be used to explain these observations?
The alcohol is a more polar molecule and there is hydrogen bonding in ethanol.
The differences in melting and boiling points between ethyl alcohol and dimethyl ether can be explained by examining the types of intermolecular forces present in each compound.
Ethyl alcohol, also known as ethanol, has polar covalent bonds due to the presence of an oxygen atom that is more electronegative than carbon and hydrogen atoms. This polarity gives rise to hydrogen bonding between ethanol molecules, in addition to London dispersion forces.
Hydrogen bonding is a strong intermolecular force that occurs when a hydrogen atom is directly bonded to a highly electronegative atom (such as oxygen or nitrogen) and is attracted to another electronegative atom in a nearby molecule. In the case of ethanol, the oxygen atom of one molecule can form hydrogen bonds with the hydrogen atoms of neighboring molecules. These hydrogen bonds result in a higher intermolecular force of attraction, making it more difficult to break the intermolecular bonds and causing ethanol to have a higher melting and boiling point.
On the other hand, dimethyl ether, with the formula CH3OCH3, does not have hydrogen bonding capabilities. Although it also has polar covalent bonds due to the presence of oxygen, the absence of hydrogen directly bonded to the electronegative oxygen atom restricts the formation of hydrogen bonds. Hence, dimethyl ether exhibits only London dispersion forces, which are relatively weaker than hydrogen bonding.
London dispersion forces, or van der Waals forces, arise from temporary fluctuations in electron distribution around atoms or molecules. These temporary fluctuations can induce temporary dipoles in adjacent molecules, resulting in attractive forces between them. While London dispersion forces exist between all molecules, without hydrogen bonding, dimethyl ether has weaker attractive forces compared to ethanol.
The difference in intermolecular forces explains the difference in boiling and melting points between ethanol and dimethyl ether. The presence of stronger hydrogen bonding in ethanol leads to a higher boiling point (78.5 degrees C) and melting point (-117.3 degrees C) compared to dimethyl ether, which has weaker London dispersion forces, resulting in a lower boiling point (-23.7 degrees C) and melting point (-138.5 degrees C).