Ethers, the organic compound with the second least boiling point, are classified by their ether group which consists of an oxygen bonded to two alkyl groups. Much like the alkanes, ethers are unable to form hydrogen bonds with each other which results in their relatively low boiling point. The difference however, is that ethers are slightly polar as their C-O-C bond do not cancel each other out. This creates a slight dipole-dipole force between them, attaining their higher boiling point than alkanes. Additionally, while being unable to hydrogen bond amongst themselves, the two lone pair on the oxygen allow ethers to hydrogen bond with water. However, since they only have the oxygen, their known only to be hydrogen bond acceptors and not donors. Ethers can be soluble in water, but significantly less than that of alcohols.
Amines, can be classified into three different categories – primary, secondary and tertiary. Each category differs by the number of hydrogens in their functional group. Primary amines consist of NH2 bonded to an alkyl. Secondary amines consist of NH bonded to 2 alkyl groups. And lastly, tertiary amines consist of N being bonded to three alkyl groups. Primary amines, do experience hydrogen bonding within each other, hence their boiling point is significantly higher than that of alkanes and ethers. On top of that, they also experience strong dipole-dipole forces due the high electronegativity difference between hydrogen and nitrogen. Secondary amines, also experience hydrogen bonding, however the situating of the nitrogen in the middle of the compound as opposed to the ends, reduces the permanent dipole. Tertiary amines, however are unable to form hydrogen bonds and hence have a significantly lower boiling point than the primary amine and secondary amine.
Aldehydes and ketones share much of the same physical properties, with the slight difference of their functional group. Aldehydes are always found terminally on a chain and also consist of a carbonyl group, O=CH with the C bonded with an alkyl group. Ketones are found in the middle of the chain and consist of a carbonyl group O=C, bonded with two alkyl groups. In terms of boiling points, both ketones and aldehydes experience dipole-dipole forces due to their carbonyl group as oxygen is more electronegative than carbon, creating a partial dipole, making them stronger than ethers and alkanes. Much like ethers, they are able hydrogen bond with water but only as acceptors and not donors.
Alcohols are organic compounds are classified by their hydroxyl group –OH. Much like the primary and secondary amines, alcohols can hydrogen bond with each other as shown in the picture below
However, alcohols have a higher boiling point than amines due to their hydrogen bonds being more electronegative than those of amines (larger electronegative differences between OH than NH). Hence they OH group induces stronger dipoles resulting in stronger bonds. Due to their ability to hydrogen bond, alcohols have a higher boiling point than alkanes, ethers, aldehydes and ketones as well, because hydrogen bonds require more energy to break. Additionally the boiling point of the alcohol increases as the number of carbons much like alkanes, as the dispersion forces increase. As the number of hydroxyl groups increase in the alcohol, the boiling point increases in proportion as there is more surface area being hydrogen bonded.
The compound with highest boiling point from these noted organic compounds are the carboxylic acids. Carboxylic acids are identified by their carboxyl group, C(=O)OH. Much like alcohols they share the ability of hydrogen bond, resulting in a higher boiling point than ethers, alkanes, amines, aldehydes and ketones. However, in terms of comparing the boiling points of carboxylic acids and alcohols, carboxylic acids have a slightly higher boiling point due to presence of an extra oxygen in carboxylic acids. Hence giving to molecules another opportunity to hydrogen bond
The two hydrogen bonds (represented in the red) result in a dimer, which is molecule formed by two identical compounds. Hence when this (as a molecule) interacts with its’ neighbours the size is much larger which results in stronger dispersion forces.
