chemistry -organic molecules-trends-solution to question 9

Solution
9) Consider the three alcohols shown on the right. Place them in order of increasing boiling point and give reasons.
There are three main factors that influence boiling points. 1) The intermolecular forces acting between molecules 2) Molecular mass 3) Molecular symmetry and branching. All three molecules have the same functional group and so are capable of exhibiting some form of hydrogen bonding and have the same molecular mass. So it comes down to symmetry and branching. Pentan-1-ol is the most assymetrical, molecule and so is the most polar, hence, it will have the highest boiling point. Pentan-2-ol has the hydroxy group in between a methyl and a propyl group so it is relatively surrounded by non-polar alkyl groups. Pentan-3-ol has the hydroxy group in a relatively inaccesible place, surrounded by two ethyl groups right in the middle of the molecule. This also gives this molecule a degree of symmetry.
A space fill model of each molecule better shows how the hydroxy group is shielded in each molecule.
The hydroxy group in pentan-1-ol is not shielded and can interact fully with other molecules.
The hydroxy group in pentan-2-ol is shielded by a methyl group and a propyl group. We can see that interaction with the dipoles of other molecules will be hampered.
The hydroxy group in pentan-3-ol is shielded by two ethyl groups on either side. Its interaction with other dipoles on other molecules is severely reduced by such shielding.