Problem: The table shown here lists the molar heats of vaporization for several organic compounds.Compound Heat of vaporization (kJ/mol)CH3CH2CH319.0CH3CH2CH2CH2CH327.6CH3CHBrCH331.8CH3COCH332.0CH3CH2CH2Br33.6CH3CH2CH2OH47.3Use specific examples from this list to illustrate how the heat of vaporization varies with molecular shape.

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We’re being asked to identify the compound with the highest boiling point among the given choices.


Recall that there are several types of intermolecular forces (IMF):

1. Ion-dipole interaction – occurs between an ion and a polar covalent compound; strongest IMF

2. Hydrogen bonding – occurs in compounds where hydrogen is directly connected to an electronegative element such as N, O, or F; 2nd strongest IMF

3. Dipole-dipole interaction – occurs between two polar covalent compounds; 3rd strongest IMF

4. Dispersion forces – occurs in all compounds and is the primary IMF exhibited by nonpolar compounds; weakest IMF


Compounds with strong intermolecular forces have high boiling points. This is because they require more energy to be able to break the bonds during the phase transition.


So, we need to identify which among the given compounds has the strongest intermolecular forces.


For CH3CH2CH3: The total number of valence electrons present in CH3CH2CH3 is:

            Group            Valence Electrons

C         4A                   3 × 4 e12 e

H         1A                   8 × 1 e8 e

Total:  20 valence e


The Lewis structure of CH3CH2CH3 is:




The electronegativity difference between C (EN = 2.5) and H (EN = 2.2) is 0.3 so the C–H bond is nonpolar, which means CH3CH2CH3 is a nonpolar molecule. Since CH3CH2CH3 is nonpolar, it exhibits dispersion forces.


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Problem Details
The table shown here lists the molar heats of vaporization for several organic compounds.


Compound Heat of vaporization (kJ/mol)
CH3CH2CH319.0
CH3CH2CH2CH2CH327.6
CH3CHBrCH331.8
CH3COCH332.0
CH3CH2CH2Br33.6
CH3CH2CH2OH47.3


Use specific examples from this list to illustrate how the heat of vaporization varies with molecular shape.

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