ORG CHEM LAB - STRUCTURAL EFFECTS ON MELTING POINT AND BOILING POINT

Melting and Boiling Points

• Physical properties of a compound’s structure

• Depend largely on which bonds can hold three atoms together in a molecule

• Useful for synthesizing a new compound

Melting Point

• The range of temperature at which a substance changes from solid to liquid phase.

• Frequently followed by decomposition

  • The value may not be an equilibrium temperature but a temperature of transition from solid to liquid only.

Boiling Point

• Corresponds to the temperature at which thermal energy of the molecule is great enough to overcome the cohesive forces that hold them in the liquid state.

• Employed for compounds having low melting points and compounds that are usually liquid at room temperature.

• The range of this constant should not exceed 5 C except for extremely high boiling compounds.

• Thee constant may also be decreased or increased depending on the volatility of the impurity present.

Factors Affecting Melting and Boiling Points

• Molecular Weight

• Structure and Intermolecular Forces

• Impurities

Hydrogen Bonding

• Attraction between a H atom that is bound to an electronegative atom and another molecule.

• Functional groups: alcohols, amines (primary and secondary), carboxylic acids, amides (primary and secondary), and thiols

Dipole-Dipole Interactions

• Stronger than Van der Waals Interaction, but not as strong as H bonds

• The positive part of one molecule is attracted to the negative part of another molecule

• Functional groups: all polar compounds (alcohols, ethers, aldehydes, ketones, carboxylic acids, amides, esters, alkyl halides, and thioethers)

Induced Dipole-Induced Dipole (Van der Waals/London Dispersion Forces)

• Weakest of all the intermolecular forces

• A temporary polarized molecular causes its neighbor to become temporary polarized as well

• Every molecule exhibits this attraction

• Increase in chain length increases the attraction, whereas increase in branching decreases the attraction.

Hydrogen Bonding > Dipole-Dipole Interaction > Van der Waals

Intermolecular Forces Ranking

B. 1-propanol

Exercise

Determine the intermolecular forces of attraction involved. Identify which of the given compounds has a higher boiling point.

A. propanone

B. 1-propanol

B. 1-butanol

Exercise

Determine the intermolecular forces of attraction involved. Identify which of the given compounds has a higher boiling point.

A. Ethanol

B. 1-butanol

B. 1-butanol

Exercise

Determine the intermolecular forces of attraction involved. Identify which of the given compounds has a higher boiling point.

A. t-butyl alcohol

B. 1-butanol

B. 1-propanol

Exercise

Determine the intermolecular forces of attraction involved. Identify which of the given compounds has a higher boiling point.

A. 1-propanamine

B. 1-propanol

B. butanedioic acid

Exercise

Determine the intermolecular forces of attraction involved. Identify which of the given compounds has a higher boiling point.

A. propanedioic acid

B. butanedioic acid