Remember the rules of what gives compounds a higher boiling point (and melting point):
Van der Waals Forces
More electrons = more Van der Waals interactions = higher boiling point
So in general, the more "stuff" in a molecule (the higher the molecular weight), the higher its b.p. Example: C10H22 will have a higher b.p. than C5H12.
How to remember this trend? What do you grill with? propane gas. (C3H8)
What's in a lighter? butane liquid (C4H10)
Hydrogen Bonding
Compounds that can hydrogen bond have higher b.p./m.p/ than those that don't.
Compounds can hydrogen bond is they have a N, O, or F bonded directly to an H.
In organic chemistry, the best examples are alcohols (ROH) and amines (RNH2).
Branching
Branching makes it harder for molecules to pack together, which makes it harder to form Van der Waals interactions, and so tends to lower b.p./m.p.
a) (highest b.p.) n-octane > n-pentane > ethane (lowest b.p.)
Reason: octane has the most "stuff" (higher molecular weight).
b) 1-butanol > 1-chloroethane > n-butane
Reason: 1-butanol can hydrogen bond. 1-chloroethane has a higher molecular weight than n-butane.
c) n-octane > 2-methylheptane > 2,5-dimethylhexane
Reason: Branching. n-octane has no branching. 2,5-dimethylhexane has the most branching. Notice that each compound has the same molecular formua- C8H18. (and therefore the same weight).
