Question

Question #10710

Answer 1

(CH4)>(CH3CHCH3CH2CH3)>(CH3CH2CH2CH2CH3)>(CH3CH2CH2CH2OH)

 Vapor pressure is the force exerted by liquid molecules when they are transformed into the gaseous phase. In order to exert vapor pressure, liquid molecules must break away by breaking their [intermolecular bonds](http://socratic.org/chemistry/intermolecular-bonding/intermolecular-bonds) and escape as gas particles. therefore, we have to consider the strength of these intermolecular bonds.
  Several intermolecular bonds exist and these include:

1) Hydrogen bonds - the strongest; formed between polar molecules
2) London dispersion forces (LDF) - the weakest; formed between nonpolar molecules
3) Ion-dipole bonds - formed between polar and ionic compounds

Personally CH4 should not be included in the list because it is naturally in the gaseous state. But let us assume that (CH4) is in the liquid state. Among hydrocarbons (which are usually nonpolar in character), another criteria that we have to consider is the
(a) length of the carbon chain: more carbon = more chances of forming LDFs. Therefore: (CH4)>(CH3CHCH3CH2CH3)
(b) branching in the C chain hinders compact arrangement of the molecules, thus, weaker attractions. Therefore:
(CH3CHCH3CH2CH3)>(CH3CH2CH2CH2CH3). (CH3CHCH3CH2CH3) is branched while (CH3CH2CH2CH2CH3) is linear.
(c) presence of polar portions in the molecule.
One end of the (CH3CH2CH2CH2OH) is nonpolar while the other end is polar (OH). Thus it can form both hydrogen bonds and LDFs. More energy in the form of heat is needed to break the intermolecular bonds and transform it to the gaseous phase and exert vapor pressure.