Knowing
the different effects that determine solubility between
compounds is of great importance in organic chemistry
since solvent choice can directly determine recrystallization
and reaction yields. More fundamentally, if reagents
are insoluble in the solvent, no reaction will even
occur! Therefore, it is important to be able to determine
whether two compounds will be soluble in each other
just from looking at their structures.
One
easy way of determining solubilities is to remember
that "like dissolves like." In other words,
compounds with similar properties are generally soluble
in each other, and compounds with different properties
are generally insoluble in each other. Polar compounds
are generally soluble in other polar compounds; non-polar
compounds are generally soluble in other non-polar compounds.
Molecules that can form hydrogen bonds are generally
soluble with other molecules that can form hydrogen
bonds (see picture below), and so forth.
Oil
and water don't mix. This is because alkyl chains (which
make up the bulk of oil structures) can be thought of
as hydrophobic (water hating). Interestingly enough,
hydrocarbons and water do in fact have weak attractive
interactions through London Forces;
however, these forces are very weak compared to hydrogen
bonding that occurs between the water molecules. By
dissolving a hydrocarbon, such as propane, a compound
that has no hydrogen bond forming capabilities, water
must give up some of its hydrogen bonds that it has
to other water molecules to form a cage-like structure
around the propane. Doing so would be thermodynamically
unfavorable because the energy of the system due to
the broken hydrogen bonds would be increased (remember,
nature is lazy and will always choose the lowest energy
state possible). Therefore, to minimize the energy,
oil and water separate, with oil generally remaining
on the surface because of its lower density.
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Ethanol
is completely miscible in water because its
-OH group can be involved in hydrogen bonding
with water |
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Propane
is insoluble in water because it is not polar,
nor capable of forming hydrogen bonds |
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