does molecule symmetry really affects polarity?

Question

with respect to a charged rod, they say that symmetrical molecules aren't affected. is this true because hexane is a symmetrical molecule but it is very large so the electronegativities of the carbon and hydrogen slightly differs causing slight deflection in an electric field.... so is the symmetrical statement just a pattern observed by other small molecules with negligable charges? i think its the size of the charges responsible... not he symmetry... like unsaturated oils, they aren't symmetrical and they don't have charges...... see what i mean?? is this another prediction in chemistry???

Answer 1

When we talk of symmetry in molecules (as it relates to polarity) we are talking about the concentration of partial positive and partial negative charges on opposite sides of the molecule. These partial positives and negatives are the result of unequal sharing of electrons in covalent bonds.

Take H2O as an example. This is a bent molecule with the oxygen at one end and the two hydrogen atoms at the other end. The oxygen atom has two unshared pairs of electrons all its own as well as being more electronegative than hydrogen so it keeps the shared electrons in both O-H bonds closer to it. This effectively leaves hydrogn a bare proton (positive charge) most of the time. Therefore, oxygen would be said to be slightly negative, while the hydrogens would be slightly positive. Because both sides have opposite charges, this molecule is polar.

In the case of unsaturated oils (triglycerides): Triglycerides are composed of one glycerol molecule and three fatty-acid molecules. If you look at the structure, there are hydrogens on all sides. Since all the hydrogens are attached to more electronegative elements (alway unless attached to another hydrogen), then all sides have a positive charge. This makes this molecule nonpolar.

When you dissolve triglycerides in water, they do not mix. Nonpolar substances will not dissolve in polar substances. Remember, "Like dissolves like."

If you take the triglyceride and replace one of the fatty acids with a phosphate bearing chain (phospholipid) then the molecule becomes polar because the phospholipid is negative while the hydrogens on the other fatty acids are positive. This is the lipid of the cell membrane and its polarity is what gives the membrane its ability to be selectively permeable.

Hope this helps.

BIOCHEMISTRY/BIOLOGY TEACHER