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?
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The differences in melting and boiling points of ethyl alcohol and dimethyl ether can be explained by the differences in intermolecular forces between their molecules. Intermolecular forces are the attractive forces between molecules that determine their physical properties, such as melting and boiling points.
Ethyl alcohol molecules (CH3CH2OH) are polar because the oxygen atom is more electronegative than the carbon and hydrogen atoms. This creates a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. The polar nature of ethyl alcohol gives rise to several intermolecular forces, including hydrogen bonding and dipole-dipole interactions.
Hydrogen bonding is a particularly strong type of intermolecular force that occurs when a hydrogen atom is bonded to a highly electronegative atom (in this case, oxygen) and is attracted to a different electronegative atom (oxygen or nitrogen) in a neighboring molecule. These hydrogen bonds are relatively strong and require more energy to break, leading to higher melting and boiling points. Ethyl alcohol can form hydrogen bonds between its oxygen atom and hydrogen atoms of neighboring molecules.
On the other hand, dimethyl ether (CH3OCH3) has a similar molecular formula, but it lacks the polar hydroxyl group (OH) found in ethyl alcohol. As a result, it does not experience hydrogen bonding. The only intermolecular forces in dimethyl ether are weak dipole-dipole interactions due to the partial positive and partial negative charges on the oxygen and carbon atoms, respectively.
Dipole-dipole interactions are weaker than hydrogen bonding, and therefore, dimethyl ether has lower melting and boiling points compared to ethyl alcohol. The absence of hydrogen bonding in dimethyl ether allows its molecules to be more easily separated from each other, requiring less energy to break the intermolecular forces, resulting in lower melting and boiling points.
Therefore, differences in the intermolecular forces (specifically, the presence or absence of hydrogen bonding) between ethyl alcohol and dimethyl ether can explain the variations in their melting and boiling points.