Question

Part A 

List the following compounds in order of decreasing boiling point:

 Rank from largest to smallest. To rank items as equivalent, overlap them. 

CH3COH    CH3CH2OH    CH3CH3 

Answer 1

One of the main factors that affect the boiling point of a substance is the intermolecular forces present in them. The stronger the intermolecular forces between the molecules of a substance, higher will be its boiling point. This is because stronger intermolecular bonds require more energy to break and this energy is supplied in the form of heat during boiling.

The order of strength of intermolecular forces is :

ionic bonding hydrogen bonding dipole-dipole interactions van der Waals dispersion force

Based on this observation, we can compare the boiling points of the given 3 molecules - acetaldehyde, ethanol, ethane.

The only intermolecular forces present in alkanes is the weak van der Waals dispersion force. In alcohols, hydrogen bonds exist between the partially-positive hydrogen atoms and the lone pairs on oxygen atoms of other molecules. Hydrogen bonding is much stronger than van der waals dispersion forces, and thus more energy is required to separate alcohol molecules than to separate alkane molecules. So, alcohols have a higher boiling point than alkanes.

In aldehydes, the carbon to oxygen double bond is quite polar. The electronegative oxygen atom has a greater attraction for the bonding electron pairs than the carbon atom and this makes the oxygen atom partially negatively charged and carbon atom gains a partial positive charge. This charge separation leads to dipole-dipole interactions among the aldehyde molecules. Comparing with hydrogen bonding, dipole-dipole interactions are weak. Dipole-dipole interactions are weaker than hydrogen bonding, but stronger than van der Waals forces. Hence, aldehydes have a lower boiling point than alcohols and a higher boiling point than alkanes (provided alcohol,aldehyde and alkane contain the same number of carbon atoms) .

So we rank the given 3 molecules from largest to smallest in terms of their boiling points as :

ethanol acetaldehyde ethane

CH₃CH₂OH CH₃COH CH₃CH₃

Answer 2

CORRECT ORDER OF BP IS CH3CH2OH >CH3COH >CH3CH3