Welcome to part two of the general
chemistry review for organic chemistry.
I'm Melissa Maribel your personal tutor
and in part one we reviewed Lewis
structures now let's review polarity.
Polarity is like a tug-of-war between
elements if one side is stronger than
it's polar, if both sides are of equal
strength than it's nonpolar. A polar
structure has a dipole moment which is
an unequal pull of electrons. There are
three main factors to look at for
polarity. One symmetry if the structure
is symmetrical then it's nonpolar
because there is an equal pull of
electrons, if the structure is non
symmetrical it is most likely polar
because there is an unequal pull of
electrons. Two electronegativity,
electronegativity tells us which atom is
stronger and will most likely win the
tug-of-war, electronegativity essentially
defines how well an element can attract
electrons. The trend on the periodic
table for electronegativity increases as
you move from left to right and move up
a group. Fluorine is the most
electronegative atom so whatever is
closest to fluorine will be more
electronegative. The noble gases however
are not electronegative since they
already have eight valence electrons and
do not want to attract any more. If we
were comparing oxygen and chlorine
though they are both equidistant to
fluorine the higher atom is more
electronegative so oxygen is more
electronegative than chlorine. Looking at
our examples let's figure out which
structure is the most polar though both
structures are polar this structure is
the most polar since oxygen is closest
to fluorine. Three lone pairs on the
central atom, lone pairs on the central
atom add to polarity. For example NH3 may
seem symmetrical since we can cut it
down the middle and it's surrounded by
the same atoms but since there's a lone
pair on that central atom of Nitrogen
that adds to an unequal pull of
electrons which makes the overall
structure polar onto VSEPR theory for
organic chemistry it's going to be very
important to be able to visualize 2D
structures in 3D which is what VSEPR
theory helps us do.
Here's CH4 in its VSEPR form, remember
that a wedged line means the bond is
coming towards you or out of the plane
while a dashed line is hidden in the back
and a solid line is on the plane of the
paper or think of it as it's in your
line of sight. Let's draw C2H6 in its
VSEPR form so it has a geometry of
tetrahedral since each carbon has four
bonds, each carbon will have one hydrogen
with a solid line, dashed line and wedged
line now let's visualize this in its 3D
form. Here's carbon, we'll attach a
hydrogen that is in the plane
represented by a solid line, a hydrogen
hidden in the back which is a dashed
line and a hydrogen coming towards us
which is a wedged line.
Bond the carbon to another carbon with
the same type of hydrogens, one in the
plane, one hidden in the back and one
coming towards us and this is our 3D
structure. Now for CH3OH here's the lewis
structure we found in part 1 and our
geometry for carbon is still tetrahedral
and for oxygen the molecular geometry is
bent. Drawing this in VSEPR form the
carbon still has one solid line between
hydrogen, one dashed and one wedged while
oxygen has one solid line between
hydrogen and one dashed and one wedged
line for each lone pair. Let's visualize
this in 3D, here's our carbon and the
hydrogen with a solid line meaning it's
in the plane, here's the hydrogen hidden
in the back which is represented with a
dashed line and a hydrogen coming
towards us which is the wedged line. We'll
bond the carbon and oxygen together and
place the hydrogen which is in the plane
represented by a solid line and this is
the structure. Part three is the most
important part of all because we cover
formal charges, resonance and
hybridization which is going to prepare
you a lot for organic chemistry. You can
find that right over here and I'll see
you over there.
