急!! chem 有關boilng point 問題

The arrangement:
Propane < Methoxymethane < Ethanol
Explanation:
Can be in terms of the polarity of individual molecules. First, Since ethanol contains O-H bond which implies that between the molecules, there exists HYDROGEN BONDS which can much strengthen the intermolecular forces.
Then for propane and methoxymethane, the latter molecule is polar in nature while the former is not. Therefore the overall intermolecular forces in methoxymethane is of a greater magnitude than those in propane and hence, methoxymethane has a higher boiling point than propane.

Boiling point:
CH3CH2OH(ethanol,78° C) =&gt; CH3oCH3 (methoxymethane,-23°C=&gt; CH3CH2CH3(propane,-42.1 C)

ABOVE THREE COMPOUND are having similar molecular mass, so we need to focus on the INTER-molecular forces among their molecules.
Ethanol contains O-H bond which implies that between the molecules, there exists HYDROGEN BONDS which can much strengthen the intermolecular forces.
Methoxymethane is a greater magnitude than those in propane and methoxymethane has a higher boiling point than propane.

B.P.:
ethanol (78° C) &gt; methoxymethane (-23°C) &gt; propane (-42.1 C)

Since the three compounds are having similar molecular mass, we may only focus on the INTER-molecular forces among their molecules.

[methoxymethane &gt; propane]
Both methoxymethane and propane have simple molecular structure. However the presence of electronegative oxygen atom makes the methoxymethane molecules having PERMANENT - PERMANENT dipole moment among themselves. On the other hand, the intermolecular force of propane molecules is only caused by SIMULTANEOUS - INDUCED dipole moment which is much weaker than the PERMANENT - PERMANENT one.

[ethanol &gt; methoxymethane &gt; propane]
For ethanol, the -OH group contributes to the HYDROGEN BOND among the molecules. This hydrogen bond is stronger than all of those simple molecular forces as a result from the large electronegativity difference between the oxygen atom and the hydrogen atom of the -OH group. Therefore ethanol has a highest B.P. among the three.

Remarks: Dipole is formed by the electronegativity difference and the spacial arrangement of the elements of a molecule. You may regard the hydrogen bond as a strong dipole.

Arrange the following compounds in increasing order of boiling points.
CH3CH2OH (ethanol)
CH3CH2CH3 (propane)