Predicting Polarity

Bond Table:

Background:

When you have a molecule different parts of the molecule behave differently. This is because the majority of molecules have different parts. Some parts have O-H bonds or C-C bonds. These bonds don't simply snap together and get called "covalent" or "ionic."

Instead, covalent bonds can have some degree of polarity, Polarity means that electrons spend more time around one of the two atoms involved instead of the other. This means that one of the atoms will tend to be more negative atom ( the atom that tends to hog the electron more often) and a more positive side (the atom that tends to loose out on the electron sharing partnership).

Example: Find the bond polarities for C₃H₈O

It is helpful to first draw the lewis dot structure (Lewis Dot below)

Identify the different bonds we have in this molecule are:

C--H bonds

C--C bonds

H--O bonds

O--C bonds

...we're just looking to see what different elements are bonded to, now here are the steps to figure out what kinds of bonds these are:

❶ Look up the electronegativity of each atom on the electronegativity chart (first table above). Subtract the two values (subtract the smaller value from the larger value so your answer is positive). Try to answer the question marks below and then click for the answer.

(C--H) = 2.55 - 2.00 = .55

(C--C) =2.55-2.55 = 0

(H--O) = - =?

(O--C) = - =?

❷ Next, take your answer and compare it on the Bond table to interpret your value. This will tell you what is going on with that bond. Try to first answer the question marks then click them for the answers.

(C--H) = .55 = moderately polar covalent

(some difficulties mixing with polar molecules)

(C--C) = 0 = non-polar covalent

(will absolutely not mix with polar molecules)

(H--O) = 1.24 =?

(O--C) = .89 =?

Non-polar parts of molecules (or molecules that are only made up of non-polar bonds will not dissolve in water (which is polar). Example: Oils and fats

Polar molecules like water will not dissolve in oils and fats

Alcohols (have --O--H 's attached somewhere) like the example above, isopropyl rubbing alcohol (C₃H₈O) can mix with both water and dissolve oils. This is because part of the molecule has non-polar bonds, and the other part has polar bonds.

How is this stuff relevant??

If you want molecules to cut grease and wipe away you need a molecule that can exist with polar (water) and non-polar (greases and fats) molecules. If you are trying to make a drug that helps patients, you need to know how the properties will act with other molecules. You don't want to inject someone with something that is non-polar beacues that will clog veins (remember humans are mosly water!)