Boiling point of hydrocarbons

boiling point depends on intermolecular force ( weak van der waal's force or strong hydrogegn bond) between molecules and molecular mass.

for alkane and alkene, weak van der waal's force are found between molecules, so they usually have lower boiling point.
surface area is also a factor affecting melting point of alkane and alkene. The boiling point of alkane and alkene in a ring shape or branched is higher than that of a straight chain.

as for alkanol and alkanoic acid, hydrogegn bond are found between molecules, so they usually have higher boiling point.
alkanol contain one OH group, which can form 1 hydrogen bond.
alkanoic acid have COOh group. since it has 2 O, it can form 2 hydrogen bond.

although ester has COO group, since O is not link to H, so it cannot form hyfrogen bond and only weak van der waals force are found between molecules.

so the order is :
alkene< alkane < ester < alkanol

2007-01-20 23:29:11 補充：
alkene< alkane < ester < alkanol < alkanoic acid

Assume all the above compounds are in straigh chain form with similar molecular mass.

The order of boiling point should be as follows.

alkane < alkene < ester < alkanol < alkanoic acid

Boiling point is determined by the kind of intermolecular force between molecules.(Strengh of intermolecular force: hydrogen bond > diploe-dipole attraction > van der Waals' force )

Among these five compound, molecules of alkane and alkene are hold mainly by van der Waals' force. Molecules of ester is hold mainly by diploe-dipole attraction while molecules of alkanol and alkanoic acid are hold mainly by hydrogen bond.

Since the pi electrons of alkene are more diffuse, It is easier to be polarized. Temporary diploe is formed more easily. This increase the van der Waals' interaction between alkene molecules. Thus, alkene has higher boiiling point than alkane.

Ester molecules has oxygen atoms which are highly electronegative. Permanent dipoles were formed in oxygen atoms and carbon atoms. This cause ester molecules to have greater intermolecular force than those in alkane and alkene which are formed by temporary dipoles. Thus, ester has higher boiiling point than alkane and alkene.

Molecules of alkanol and alkanoic acid can form hydrogen bond. An alkanol molecule can form one hydrogen bond with another alkanol molecule. On the other hand, an alkanoic acid molecule can form one hydrogen bond with another alkanoic acid molecule. Thus, alkanoic acid molecules can form more extensive intermolecular hydrogen point than alkanol molecules. Therefore, alkanoic acid has higher boiiling point than alkanol.

These five compounds should have the above order of boiling point.

alkanoic acid>alkanol >ester>alkane>alkene