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- Look at your left and right hands.
With both palms facing up,
place your left hand on top of your right hand.
You will see that your hands are non-superimposable.
The pinky of your left hand
aligns with the thumb of your right hand,
rather than matching congruently.
Your hands are like chiral molecules.
Chiral molecules are non-superimposable
mirror images of one another.
Chirality depends heavily on symmetry.
Consider how a plane of symmetry can split a molecule
into symmetrical halves.
For instance, boron trifluoride has a plane of symmetry
that can divide the molecule into two exact halves
that mirror each other.
Chiral molecules lack this plane of symmetry.
For instance, lactic acid has no plane of symmetry
where a plane can bisect the molecule to form mirror images.
The mirror images of lactic acid here
are non-superimposable upon each other.
As a result, the molecule is chiral.
A chiral center is an atom
that has four different substituents connected to it.
Generally,
a molecule with at least one chiral center is chiral
and a molecule that is not chiral
is said to be achiral.
In organic chemistry,
chiral centers are often sp3-hybridized carbon atoms.
Recall that sp3 hybridization
simply means that the atom is bonded
to four different groups
using one s orbital and three p orbitals.
When optimizing the structure of one s orbital
and three p orbitals,
a tetrahedron shape forms.
The MCAT may ask you to identify
the number of chiral structures within a molecule.
When you're given a structure,
it might be helpful to first identify
which atoms are sp3-hybridized
to begin the process
of identifying the possible chiral centers.
Note that any sp3-hybridized carbon
may not be a chiral center,
particularly if the atom
does not have four different substituents connected to it.
Consider methanol, COH4, for example.
Although the central carbon is sp3-hybridized
the carbon is not connected to four different substituents
as it is bound to three hydrogens.
Thus, the molecule can create
a self-superimposable mirror image of itself
and is therefore achiral.
This brings us to our first Shem Gem,
knowing that a carbon must have four different groups
to be considered chiral is very important.
The test writers will sometimes make you determine
whether a molecule has R or S stereochemistry
or if it's achiral overall.
As such, it's essential to know
when a molecule is not chiral.
Something unique to chiral molecules
is that they are often optically active,
meaning they are able to interact with and polarize light.
However, not all compounds that contain a chiral center
are optically active.
In fact, a compound that has multiple chiral centers
may not be optically active.
In some cases, a molecule may have multiple chiral centers,
but still be achiral.
Meso compounds, for instance,
contained multiple chiral centers
and an internal plane of symmetry.
As a result, a meso compound
containing multiple chiral centers
can still be achiral overall.
To determine whether a compound is meso,
first, check that it has at least two chiral centers
denoted by the asterisks
in our example to the right.
Then, bisect the structure into halves
via a plane of symmetry.
If the compound reflects the same connectivity
of the substituents on both sides,
then it is meso.
As such, it is not optically active.
Like hands can either be left or right-handed,
the absolute configuration of a chiral center
can be R or S.
To determine whether a chiral center
has an R or S configuration,
first determine the priorities
of the four groups connected to the chiral center.
What does priority refer to?
The Cahn-Ingold Prelog Priority Rules
are conventions that determine the priority
of substituent functional groups.
Each substituent connected to a chiral center
can be assigned a hierarchical priority.
The basics are as follows.
The groups with the higher atomic numbers
receive a higher priority.
If groups are isotopes, however,
the ones with higher atomic mass receive higher priority.
If the groups are the same,
then the priority can be assigned
by the atomic number of mass
at the first point of difference.
Any double bonds should be treated as two single bonds
to an identical substituent.
Any triple bonds should be treated as three single bonds
to an identical substituent.
Let's determine the absolute configuration
of the compound 3-chloro-1-butene.
3-chloro-1-butene has one sp3-hybridized carbon atom
that is connected to H,
Cl,
CH3,
and H2C double bonded to CH,
which are all four different groups.
Thus, this is the chiral center.
Next, the priorities of the attached substituents
can be determined.
Recall that atoms with higher atomic weight
receive higher priority.
In this case,
the chlorine atom receives the highest priority,
number one,
while the hydrogen atom receives the lowest priority,
number four.
The other two substituents
are both connected to the chiral center at a carbon atom.
Thus, the attached bonds must be traced further
to determine the first point of difference.
The CH3 group is attached to three other hydrogen atoms
while the CH double bonded to CH2 group
can be thought of as being attached
to one other hydrogen atom and two other carbon atoms.
Thus the CH3 group receives a lower priority
and will be number three in our example.
We can see the fully prioritized molecule to the right.
Determining the priority of each of the groups
is the first part of determining R or S.
Then, be sure to orient the molecule
so that the group with the lowest priority
is pointed backward,
which is represented as a dash line.
With the lowest-priority substituent pointing backward,
draw an arrow around the chiral center
from the highest priority group
to the second highest priority group,
then to the third highest priority group.
If the arrow travels in a counterclockwise direction,
the molecule is in the S configuration.
If the arrow travels in the clockwise direction,
it is the R configuration.
Here's another tutor tip.
Note that these directions are reversed
if the lowest priority substituent
is pointing out of the page.
In this case, if the arrow travels
in the clockwise direction,
the chiral center is an S configuration.
If the arrow travels in the counterclockwise direction,
the chiral center is an R configuration.
Be careful not to make an error here,
as it is an easy place to lose points on test.
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