P roperties
Chemical/Physical Properties
The boiling points for aldehydes/ketones are generally higher when compared to hydrocarbons but are lower than alcohols. The hydrocarbons have relatively low boiling points due to their structure being non-polar and their intermolecular forces only being weak London Dispersion Forces which makes it require less energy to overcome these forces. Aldehydes/ketones on the other hand have both LDF and stronger Dipole-Dipole intermolecular forces with each other due to the polar carbonyl groups on the molecules. These stronger forces is what makes its boiling point relatively high compared to normal hydrocarbons due to the increase in energy needed to overcome the stronger forces. Since alcohols have LDF, dipole-dipole, and stronger hydrogen bonding intermolecular forces due to the hyrdoxyl grouops on the molecule, they have a generally higher boiling point than aldehydes and ketones. Ketones have generally higher boiling points than aldehydes due to the location of the carbonyl group in the middle of the molecule making the overall molecule more polar (stronger dipole-dipole intermolecular forces), while the carbonyl groups on aldehydes are on the ends resulting in a larger portion of the molecule being non-polar and the polarity/dipole-moment being focused on one end (weaker dipole-dipole intermolecular force).
The melting point for aldehydes/ketones follows the same trend as the boiling points, the melting points are higher than normal hydrocarbons and lower than alcohols. Aldehydes and ketones have LDF and dipole-dipole intermolecular forces while normal hydrocarbons only have weaker LDF intermolecular forces (which is described in more detail above). The strong dipole-dipole intermolecular forces make it so that more energy has to be added in order to break the forces holding the aldehydes/ketones molecules together as a solid comared to normal hydrocarbons. Alcohols have higher melting points compared to aldehydes and ketones, for the same reason as described in the boiling points. The difference in boiling points between ketones and aldehydes also follows the same trends mentioned in the boiling points section.
Since solubility is based on the similarities in intermolecular forces of the solute and the solvent, aldehydes/ketones are generally soluble in water if the molecule is relatively small. This is due to water having LDF, Dipole-Dipole, and Hydrogen Bonding intermolecular forces, while aldehydes/ketones have LDF, Dipole-Dipole, and Hydrogen Bonding (they can form hydrogen bonding IMFs with water but not themselves) intermolecular forces. These hydrogen bonding intermolecular forces that aldehydes and ketones form with water are not as strong as those in alcohols (that have hydroxyl groups instead of carbonyl groups, meaning they have hydrogens that can form hydrogen bonding IMFs with similar molecules). This means as the carbon chain increases on a ketone/aldehyde the solubility will decrease more dramatically than with increasing the carbon chain on an alcohol. In terms of solubility with other aldehydes/ketones molecules, they are soluble. This is due to aldehydes/ketones molecules sharing the same intermolecular forces.
The density of aldehydes/ketones is consistent enough for them to be less dense than water and alcohols (but much denser than most alkanes, alkenes, and alkynes), this is due to their intermolecular forces with each other, as they only have LDF and dipole-dipole intermolecular forces with each other, meaning they have a lower attraction to each other compared to water and alcohols that have hydrogen bonding (stronger) intermolecular forces meaning an increased attraction with each other. This attraction of aldehydes and ketones means they will be more compact (dense) than normal hydrocarbons but less compact (dense) than water and alcohols in equal measurements.
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