CHAPTER 13: Unit 4. Physical properties

Learning Objectives

1. Explain why the boiling points of aldehydes and ketones are higher than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols.
2. Compare the solubilities in water of aldehydes and ketones of four or fewer carbon atoms with the solubilities of comparable alkanes and alcohols.

Aldehydes and ketones are polar molecules. Their boiling points of aldehydes and ketones are intermediate between those of alcohols and alkanes. The polarity of the carbonyl groups enables aldehyde and ketone molecules to interact with each other through dipole-dipole interactions.They cannot, however form hydrogen bond with each other. Lower molecular mass aldehydes and ketones are soluble in water. Some ketones are known for attractive odor and found in nail polish remover, melted butter, perfume etc.The carbon-to-oxygen double bond is quite polar, more polar than a carbon-to-oxygen single bond. The electronegative oxygen atom has a much greater attraction for the bonding electron pairs than does the carbon atom. The carbon atom has a partial positive charge, and the oxygen atom has a partial negative charge:  Table: Boiling Points of Compounds Having Similar Molar Masses but Different Types of Intermolecular ForcesCompoundFamilyMolar MassType of Intermolecular ForcesBoiling Point (°C)CH3CH2CH2CH3alkane58dispersion only–1CH3OCH2CH3ether60weak dipole6CH3CH2CHOaldehyde58strong dipole49CH3CH2CH2OHalcohol60hydrogen bonding97Formaldehyde is a gas at room temperature. Acetaldehyde boils at 20°C; in an open vessel, it boils away in a warm room. Most other common aldehydes are liquids at room temperature.he oxygen atom of the carbonyl group engages in hydrogen bonding with a water molecule.The solubility of aldehydes is therefore about the same as that of alcohols and ethers. Formaldehyde, acetaldehyde, and acetone are soluble in water. As the carbon chain increases in length, solubility in water decreases. The borderline of solubility occurs at about four carbon atoms per oxygen atom. All aldehydes and ketones are soluble in organic solvents and, in general, are less dense than water.