Physical properties of carbonyls

Page 1: Introduction to Aldehydes and Ketones

Overview

• Aldehydes and ketones contain a polar carbonyl group.

• The carbonyl group has:

  • Partially negative charge on the oxygen atom

  • Partially positive charge on the carbon atom

Page 2: Polarity of Carbonyl Group

Behavior at Room Temperature

• Methanal (formaldehyde) and ethanal (acetaldehyde) are gases at room temperature.

• Aldehydes and ketones with 3 to 10 carbon atoms are typically liquids.

Influence of Polarity

• The polar carbonyl group affects:

  • Boiling points

  • Solubility in water

Page 3: Boiling Points of Aldehydes and Ketones

Intermolecular Forces

• Aldehydes and ketones exhibit dipole–dipole interactions due to their polar groups.

Comparison with Other Compounds

• Higher boiling points than alkanes and ethers with similar mass (due to dipole–dipole interactions).

• Lower boiling points than alcohols of similar mass (alcohols form hydrogen bonds).

Page 4: Trends in Boiling Points

Influence of Carbon Chain Length

• Boiling points of aldehydes and ketones increase with:

  • An increasing number of carbon atoms

  • Increased dispersion forces

Page 5: Boiling Points and Solubility Data

Summary of Selected Compounds

Page 6: Solubility of Aldehydes and Ketones

Solubility Characteristics

• Aldehydes and ketones with:

  • 1 to 4 carbons: soluble in water

  • 5 or more carbons: not very soluble in water

Hydrogen Bonding

• Aldehydes and ketones form hydrogen bonds with water between their carbonyl oxygen and the hydrogen atoms of water.

Page 7: Learning Check 1

Question

• Select the compound in each pair that would have the higher boiling point, and explain your choice.

Page 8: Solution to Learning Check 1 (1 of 2)

Comparison of Compounds

Answer A

• Aldehyde has a lower boiling point than alcohol because alcohols can form hydrogen bonds.

Answer B

• The ketone has a higher boiling point than the alkane due to dipole–dipole interactions present in carbonyl groups.

Page 9: Solution to Learning Check 1

Explanation for Answer C

• Ketones exhibit higher boiling points compared to alkanes because of increased dipole–dipole interactions in carbonyl groups.

Page 10: Learning Check 2

Question

• Determine the solubility in water for each given compound (A, B, C, D).

Page 11: Solution to Learning Check 2

Solubility Highlights

Answer A

• Not soluble in water.

Answer B

• Forms hydrogen bonds; soluble in water.

Answer C

• Soluble; aldehydes with less than five carbon atoms are soluble.

Answer D

• Soluble; ketones with four or fewer carbon atoms are soluble in water.