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

Compound

Formula

Carbon Atoms

Boiling Point (°C)

Solubility in Water

Methanal (formaldehyde)

H-C-H=O

1

-21

Soluble

Ethanal (acetaldehyde)

CH₃-C-H=O

2

21

Soluble

Propanal (propionaldehyde)

CH₃-CH₂-C-H=O

3

49

Soluble

Propanone (acetone)

CH₃-C=O-CH₃

3

56

Soluble

Butanal (butyraldehyde)

CH₃-CH₂-CH₂-C-H=O

4

75

Soluble

Butanone

CH₃-CH₂-C=O-CH₂-CH₃

4

80

Soluble

Pentanal

CH₃-CH₂-CH₂-CH₂-C-H=O

5

103

Slightly soluble

2-Pentanone

CH₃-C=O-CH₂-CH₃

5

102

Slightly soluble

Hexanal

CH₃-CH₂-CH₂-CH₂-CH₂-C-H=O

6

129

Insoluble

2-Hexanone

CH₃-C=O-CH₂-CH₂-CH₂-CH₃

6

127

Insoluble

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.