Organic solvents: Likes dissolve likes – Don’t forget this!
This one goes back to your days in freshman general chemistry and it is a simple one, but important to organic chemistry. As we will see below, unlike in general chemistry, this rule now has applicability and means you have a clue to what reaction mechanism will occur by which solvent is used.
In our world, likes dissolve likes. This means that polar molecules are easily dissolved in polar solvents and non-polar solvents are easily dissolved in non-polar solvents.
Now that we have comes to terms with this fact, the next question is: what constitutes polar molecules and solvents?
The chart above very effectively displays where different solvents rank with respect to their polarity. What should you take away from all of this?
1) There is no need to memorize the chart. Just generically learn which groups are more polar than others.
2) Alkanes and aromatics are very non-polar.
3) Water and alcohols are the main polar solvents.
4) POLAR MOLECULES ARE EASILY DISSOLVED IN POLAR SOLVENTS. NON-POLAR MOLECULES ARE EASILY DISSOLVED IN NON-POLAR SOLVENTS.
The polarity of molecules to be dissolved can be estimated using the chart above. Further, within classes of molecules, polarity can also be estimated.
Above, we have four alcohols. As we move from left to right, and the non-polar alkyl substituent is shortened, the overall polarity of the molecule increases. Therefore, it can be concluded that adding non-polar substituents to a molecule will decrease its overall polarity.
Further, we can continue to differentiate between polar solvents by designating them as polar protic or polar aprotic. This is important when working SN1/SN2 problems. (See trick #22)
Some Polar Aprotic Solvents (Favor SN2 reactions)
Some Polar Protic Solvents (Favor SN1 reactions)
Take Home Message: Polar solvents dissolve polar molecules. Non-polar solvents dissolve non-polar molecules.