CHEM219 / CHEM 219 Module 3: (Latest Update
2026 / 2027) Principles of Organic Chemistry
with Lab | Questions & Answers | Grade A | 100%
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2026 / 2027 Academic Year
Q: Characterize each of the following objects as being chiral or achiral:
a. Pencil
b. Wine Glass
c. Baseball Glove
d. Orchestra Conductors' Baton
e. Scissors
Answer
a. achiral
b. achiral
c. chiral
d. achiral
e. chiral
Q: Draw each of the following structures on a piece of paper. Identify any stereo centers in
each indication the carbon number corresponding to a stereo center. If there are no stereo
centers in a compound state none
a. 1,2-dimethycyclohexane
b. 2,3,4-trimethylhexane
c. 1-bromo-1,2-dichloropropane
a. stereocenters at C1 and C2
Answer
b. stereocenters at C3 and C4
Q: Question 2 part 2: 2,2 dichloropropane, 1,2 dichloroputane, 2 methylhexane
Answer
a. none
b. one stereocenter at C2
c. one stereo center at C2
Q: Assign priority order (1-4; is greatest priority) to the following four groups for the
purpose of deterring R and S configurations at a stereocenter
Answer
i. 3, 1, 2, 4
ii. 4, 3, 2, 1
Q: Based on the direction of twist of the following groups, assign an R or S orientation
S orientation
R orientation
Answer
R orientation
Q: question 4 part 2
S orientation
R orientation
Answer
S orientation
Q: The Fischer projection formula of the molecule fructose is shown below. How many
stereocenters are present in the fructose molecule? What is the maximum number of
stereoisomers that could exist for this sugar?
Answer
a) 3
b) 8 stereoisomers
Q: Question 5 part 2. Blood Sugar
Answer
a) 4
b) 16
Q: Assign an R/S configuration for the following chiral molecule . Explain
Answer
R configuration because this is a clockwise rotation
Q: For each of the following molecules, determine if the C=C is in the E or Z configuration
Answer
a. E configuration
b. Z configuration
Q: Question 7 part 2
Answer
a. Z configuration
b. E configuration
a. Which stereoisomers (if any) represents pairs of enantiomers? (list all the pairings that
apply)
Q: The molecule 2,3-dibromobutane has two stereocenters (C2 and C3) and thus can exist
as a maximum of four different stereoisomers. The Fischer projections of these
stereoisomers are shown below. Answer the questions which follow.
b. Which stereoisomers (if any) represents pairs of diastereomers? ( list all the pairings that
apply)
c. Which stereoisomers (if any) represent a mess compound?
Answer
a.C/D are enantiomers
b. the pairs of A/C, A/D, B/C, and B/D are diastereomers
c. memo compound is A
Q: Question 8 part 2
Answer
a. a/c and b/d are enantiomers
b. a/b, c/d, a/d, b/c are diastereomers
c. a,b,c,d are meso compound
Answer
True
Q: Determine if the statements below are true of false regarding pair of molecules shown.
They have different boiling points
Answer
False
Q: Determine if the statements below are true of false regarding pair of molecules shown.
One rotates plane-polarized light in the opposite direction from the other
Q: Determine if the statements below are true of false regarding pair of molecules shown.
They have the same density
Answer
True
Q: Determine if the statements below are true of false regarding pair of molecules shown.
One rotates plane-polarized light by a different number of degrees than the other
Answer
False
Q: Determine if the statements below are true of false regarding pair of molecules shown.
They are non-superimposable mirror images of each other
Answer
True
Q: Briefly explain why it is important for chemists, biologist, doctors and nurses to
understand the principle of chirality in organic molecules.
Answer
It is important for people dealing in medicine because it is very important to know that
chiral compounds can change greatly in their characteristics and if not administered
carefully can lead to abnormalities.
Q: Why is the concept of chirality an important topic to learn to learn in the study of
organic chemistry?
Answer
Chirality is relevant to biology and can be seen in carbohydrates and amino acids. When
humans consume drugs the different potencies and effects can be based on the chirality. So
it is important to know all of the effects.
Q: Assign R/S configuration
Answer
S configuration because it moves in a counter-clockwise way.
Q: Assign priority
Answer
i) 2, 1, 3, 4
ii) 1, 2, 4, 3
Q: Draw each of the following and identify carbon number to stereo center
Answer
a) no stereocenter
b) C3 stereocenter
3) C2 stereocenter
Q: Stereochemistry
Answer
the study of the three-dimensional structure of molecules
Q: Stereoisomers
Answer
Compounds with the same structural formula and connectivity but with a different 3D
arrangement of the atoms in space.
Q: Enantiomers
Stereoisomers that that are non-superimposable mirror images. Enantiomers have the
same connectivity but different orientations (twist) of the four atoms or groups in 3D space.
Answer
The two different forms of the chiral molecule (the "left-handed" form or a "right-handed"
forms)
How do the differences caused by steroisomers differ than the differences caused by
constitutional isomers.
Answer
trans-stereoisomers
have the substituents on opposite sides/faces of the ring or double bond
While the structural differences between stereoisomers are more subtle than those between
constitutional isomers, stereoisomerism is often responsible for significant differences in
the chemical and physical properties of organic compounds, including the biochemical
behavior of many compounds such as drug molecules, amino acids, carbohydrates, and
nucleic acids. Several of the human senses, especially smell, are greatly affected by
stereoisomerism in organic compounds.
cis-stereoisomers
have the substituents on the same side/face of the ring or double bond
Chirality
the concept of "handedness" in an object.
Consider the difference between a pair of socks and a pair of gloves. A sock can be worn
(interchangeably) on either the left or right foot, but the same is not true for the gloves. A
left-handed glove cannot be worn on the right hand and vice-versa. The gloves possess the
property of "handedness" (chirality)
Observation will reveal that chiral objects are typically not symmetrical (asymmetric), while
achiral objects typically exhibit multiple planes of symmetry within the object itself.
chiral
a molecule that is not superimposable on its mirror image (possesses the property of
handedness)
achiral
A molecule that is superimposable on its mirror image (does not possess the property of
handedness)
What test can be applied to determine if an object is chiral or achiral?
A simple test involves comparing the object and its mirror image for superimposability.
Superimposability
The ability to align (overlap) two objects so that every unique part of each is in direct
alignment with the same unique part on the other.
What does it mean when a molecule is a chiral molecule?
An organic compound that can exist as two different forms ("left-handed" form or a "right
handed" form)
Is lactic acid structure chiral or achiral?
Attempting to superimpose lactic acid and its mirror image reveals that the two forms
cannot be superimposed, and thus, the molecule is chiral, and it exists as two separate
forms known as enantiomers (Figure 3.4). No matter how one of the structures is rotated, it
is impossible to get all of the atoms/groups in one to align with the same atoms/groups in
the mirror image structure.
Is the molecule 2-chloropropane chiral or achiral?
When the 3D structure of this molecule is drawn, a tetrahedral geometry is observed
around the central carbon, similar to that of lactic acid. At first glance, it may not be clear
that the molecule is achiral. It is only through the application of the superimposability test
that the molecule reveals its achiral nature.
The absence of a stereocenter is the reason why molecules like 2-chloropropane are achiral.
An analysis of the 3D structure of 2-chloropropane reveals a tetrahedral geometry around
the central carbon but only three different types of atoms or groups attached (there were 4
bonds to the carbon but 2 of the groups were identical). The symmetrical nature of the 2
chloropropane molecule comes from having two of the same group on the central carbon -
this allows the structure to be superimposed on its own mirror image, and thus, makes the
molecule achiral.
stereocenter
A carbon atom with tetrahedral geometry that is bonded to four DIFFERENT atoms or
groups (4 single bonds connected to four different atoms or groups).
What is the most common cause of chirality in an organic molecule?
The presence of a tetrahedral stereocenter.
Identifying a stereocenter in an organic molecule is a way to quickly determine if an organic
molecule may possess the property of chirality.
This type of carbon is known as a stereocenter, a stereogenic center, or a chiral center.
A chiral molecule will almost always possess one (or more) stereocenters. In contrast, most
molecules without a stereocenter are not chiral.
Stereocenter vs Chirality
The stereocenter is the cause of the chirality, while chirality is a property of the molecule as
a whole.
What are two things that rule out a carbon from being a stereocenter? - if the carbon has more than one hydrogen on it (makes it so there can't be four different
groups/atoms) - if the carbon has a double or triple bond (makes it so there cant be four separate bonds
and therefore can't have tetrahedral geometry)
How is the presence of a stereocenter in an organic structure is denoted?
With an asterisk next to the stereogenic carbon
Look at the image of 3-methylhexane and explain whether it has a stereocenter and why?
Yes, it is chiral because it has a stereocenter. The substituents attached to carbon #3 are a
methyl, an ethyl, a propyl, and a lone hydrogen (not pictured). While methyl, ethyl, and
propyl are all carbon-based and very similar, they are not identical and that creates a
stereocenter.
Look up an image of 1-bromo-2-iodopropane and explain whether it has a stereocenter and
why?
Yes it does have a stereocenter. Only carbon #2 is a stereocenter because the #1 carbon that
bears the bromine also bears two identical hydrogens (and thus is not a stereocenter). The
presence of the bromine on carbon #1 makes this carbon different from the methyl group
(carbon #3) and thus creates the stereocenter on carbon #2.
Look at the image of cyclohex-2-ene-1-ol and explain whether it has a stereocenter and
why?
Yes it does have a stereo center on carbon #1. In the molecule cyclohex-2-ene-1-ol,
stereocenters can exist in cyclic structures as well. The #1 carbon in this molecule is a
stereocenter because there is a difference in connectivity (different groups) as we go
counterclockwise vs clockwise around the ring from the number one carbon.
How does the chirality of ibProfin impact the effects of the drug?
When ibuprofen is synthesized, the two enantiomeric forms are produced in equal
amounts, so you ingest both enantiomers. Only one of the enantiomeric forms is active at
relieving pain and the other is inactive. The non-active enantiomer is slowly converted by
enzymatic processes in the body to the active form, allowing for an fast and slow effects of
the drug.
How does the chirality of Thalidomide impact the effects of the drug?
One of the enantiomers is therapeutic (treats nausea symptoms) and one of the
enantiomers is toxic/has other harmful effects. Thalidomide was given to pregnant women
in the late 1950's and early 1960's as a treatment for nausea. While one of the enantiomeric
forms was active as a therapeutic, it was not known that the other form caused severe birth
defects (teratogenic) in many of the children born to mothers who used the drug.
Thalidomide was removed from the market for pregnant women shortly after this
discovery.
Many of the human senses are subject to the influence of chirality, which sense in particular
is sensitive to chirality?
Our sense of smell
Describe the ways humans can sense the different enantiomers of the Carvone compound?
Carvone (a molecule with one stereocenter) forms two mirror-image forms or enantiomers.
One form has a sweetish, minty smell, like spearmint leaves. Its mirror image has a spicy
aroma with notes of rye, like caraway seeds. Spearmint leaves and caraway seeds are two of
the largest natural sources for the extraction of the two isomeric forms of carvone.
What does the ability of humans to smell different enantiomers tell about human bodies?
The fact that the two enantiomers are perceived as smelling differently is evidence that
olfactory receptors in the human nose contain chiral groups, allowing them to respond
more strongly to one enantiomer than to the other. Many, but not all enantiomers have
unique and distinguishable odors.
How do different enantiomers behave compared to each other with melting points, boiling
points, density, and refractive index, etc?
These physical properties would be the same between the enantiomers.
While the behavior of enantiomers in biological systems can be quite different, an
interesting note is that the physical properties of pairs of enantiomers are identical.
Why can mixtures of enantiomers of a given compound be very challenging to separate
from one another?
Because the physical properties of pairs of enantiomers are identical (can't boil them to
separate etc.)
What device is used to determine the percent composition of an unknown mixture of
enantiomers?
A polarimeter
How does a polarimeter work?
A polarimeter uses a beam of plane-polarized light to interact with a sample of a chiral
compound of unknown enantiomeric composition. The chiral molecules cause the plane of
the polarization of the beam of light to rotate. Based on the direction and magnitude of
rotation, the % composition (what % R and % S) of the sample can be determined. The
rotation of light by individual enantiomers is always equal but opposite. Under carefully
controlled and standardized conditions, the measurement of the optical activity of a pure
single enantiomer of a compound gives a value known as the specific rotation.
What is specific rotation?
Specific rotation is the amount a compound rotates light per concentration of the molecule
and length of the tube through which the light passes.
It is the measurement of the optical activity of a pure single enantiomer of a compound.
The specific rotation is a unique property of each enantiomer that can be used to help
identify a compound, just as with physical constants like melting and boiling points
What is the R/S system or the Cahn-Ingold-Prelog(CIP) system? What specifically does it
tell us?
The nomenclature system used to name enantiomeric forms of a chiral molecule.
This system determines the direction of the "twist" of the groups attached to a stereocenter
and incorporates a description of that direction into the name of the compound.
What are the R/S designations?
One enantiomer will be known as the "R" enantiomer (from the Latin "rectus"), while the
mirror image will be known as the "S" enantiomer (from the Latin "sinister").
The R/S designation describes the configuration of the groups around a given stereocenter.
What happens to the number of stereoisomers when a molecule has more than one
stereocenter?
Molecules that possess more than one stereocenter become more complex since increasing
the number of stereocenters is associated with increasing numbers of stereoisomeric forms
in which the compound can exist.
With molecules that possess multiple stereocenters, an important skill is to be able to
determine how many stereoisomers will exist for that compound and what the relationships
are (how the individual stereoisomers are related to each other).
Each stereocenter has a pair of enantiomers. So if a compound has two stereocenters there
will be 4 stereoisomers possible.
What formula is used to quickly determine the maximum number of individual
stereoisomers possible for a chiral compound?
2^n (where n is the number of stereocenters in a compound)
The rule says "maximum number", as some structural feature of certain molecules will
result in not all of the predicted number of individual stereoisomers existing.
2-bromo-3-chlorobutane stereoisomers
This molecule contains two stereocenters.
Each stereocenter can exist in either an R or S configuration. Since there are two
stereocenters, the permutations are, R, R; S, S; R, S; or S, R,.
The structural formulae can be drawn to represent the four structures using wedge-and
dash notation. Typically, when representing molecules with more than one stereocenter, the
structures are presented with the longest carbon chain written vertically and each
stereocenter presented in an eclipsed conformation (using wedge and dash notation) to
make the comparisons between the individual stereoisomers more convenient
Stereoisomers one and two form a pair of enantiomers. Stereoisomers three and four also
form a pair of enantiomers. Thus far, it has been determined that 2-bromo-3-chlorobutane
exists as two pairs of enantiomers (four total stereoisomers).
What formula is used to find the number of enantiomer pairs for a compound that has
multiple stereocenters?
(2^n)/2
Where n is the number of stereocenters
Cross-pairings of stereoisomers
Relationships between the stereoisomers outside of their enantiomer pairs. Example: What
is the relationship between stereoisomers one and three? What about one and four? How
would we describe any cross-pairing, 1/3,1/4, 2/3,2/4?
Diastereomers
stereoisomers that are not mirror images of one another (they are cross-pairs, not
enantiomers.
This can be stated as the relationship between stereoisomers 1 and 3, 2 and 3, 1 and 4, and 2
and 4 is diastereomeric, or that these compounds represent pairs of diastereomers.
Compare this to compounds 1 and 2, as well as 3 and 4, which represent pairs of
enantiomers (or that their relationship is enantiomeric).
Which word would describe the relationship between cis/trans pairs of molecules?
They are diastereomers
If either the cis or the trans stereoisomer was held up to a mirror, the mirror image would
be the same (the mirror image of cis is not trans, and the mirror image of trans is not cis).
Thus, since they are stereoisomers that are not mirror images of one another, cis-trans pairs
are diastereomers.
How do physical and chemical properties of diastereomer pairs differ from properties of
enantiomer pairs?
While pairs of enantiomers will have the same physical and chemical properties as one
another, diastereomers (because they are not mirror images) will not.
Diastereomers may have vastly different physical and chemical properties from one
another. They may have drastically different melting and boiling points, solubilities, etc. In
short, diastereomers behave as two different chemical substances. In practical applications,
the separation of mixtures of diastereomers is much easier using common laboratory
methods, which take advantage of differences in chemical and physical properties between
compounds.
In general why does the 2^n rule for predicting maximum stereoisomers not always work?
The rule says "maximum number" as there are some structural features of certain molecules
that result in not all of the predicted number of individual stereoisomers existing in reality.
Why does the 2^n formula not correctly predict the number of stereoisomers for 2,3
dichlorobutane?
This molecule has two stereocenters (carbons 2 and 3). Using the rule to predict the
maximum number of individual stereoisomers, a maximum possibility of four (2^n = 2^2 =
4) would be determined.
After drawing out the 4 stereoisomers we can see that stereoisomers one and two represent
a pair of enantiomers, but stereoisomers three and four are different. Although three and
four are indeed mirror images of one another, if one is rotated by 180° in the plane of the
paper, it becomes superimposable on the other (all the atoms and groups line up exactly on
both molecules), so they are the same compound.
meso compound
A molecule that contains stereocenters but is superimposable on its own mirror image. Due
to this, meso compounds are achiral. The compound 2,3-dichlorobutane exists as three
different stereoisomers: a pair of enantiomers and a meso compound.
What makes a meso compound possible?
An internal plane of symmetry. Meso compounds typically have an internal mirror plane of
symmetry within their molecular structure. In this molecule, the presence of the two
chlorine atoms allows for a plane of symmetry to be created when the chlorines are present
on the same side of the carbon chain when written vertically.
This internal plane of symmetry allows for this particular stereoisomer to be
superimposable on its own mirror image (after a rotation of 180 degrees).
Meso compounds would be diastereomers to the enantiomers, and would therefore have
different physical and chemical properties.
How do the physical and chemical properties of a meso compound compare to the
properties of the enantiomers?
Meso compounds would be diastereomers to the enantiomers, and would therefore have
different physical and chemical properties.
Fischer projection
A method for representing the structures of chiral molecules in 2D
- The carbon representing the stereocenter is usually omitted and is represented instead by
the crossing point of horizontal and vertical lines. - The vertical and horizontal lines represent things bonded to the stereocenter. - Horizontal bonds are equivalent to "wedge bonds" (from wedge-and-dash notation).
These bonds represent things that project towards the reader (out of the plane of the
paper). - Vertical lines are equivalent to "dash bonds" and represent things that project away from
the reader (behind the plane of the paper).
What is the rule when using Fischer projections for moving a projections?
The only "allowed" move is a rotation of 180 degrees in the plane of the paper. No "flips" or
rotations other than 180 degrees. Unallowed moves will change the horizontal and vertical
bonds and give an incorrect representation of the Fischer projection.
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