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CHEM&219 UNIT 3, ORGANIC CHEMISTRY MODULE 3,
FINAL EXAM CHEM 219 LATETST 2025|QS & AS|A +
GRADED
Stereochemistry the study of the three-dimensional structure of molecules Stereoisomers Compounds with the same structural formula and connectivity but with a different 3D arrangement of the atoms in space. 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. 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. 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 trans-stereoisomers have the substituents on opposite sides/faces of the ring or double bond Chirality the concept of "handedness" in an object.
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. 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. 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).
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.
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.
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)/ 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?
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.
- 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. Stereochemistry Examines the 3D structure of organic molecules Stereoisomers Molecules that have the same molecular formla and connectivity between atoms but differ in the 3D arrangement of atoms or groups in space. Stereoisomerism is responsible for...
Significant differences in the chemical and physical properties of organic compouds, including the biochemical behavior of many compounds such as drug molecules , amino acids, carbohydrates, and nucleic acids. Cis/Trans Forms due to the relative positioning of their substituents on the same (cis) or opposite (trans) sides. They are stereoisomers. Chirality Stereoisomerism Concept of handedness in an object. If an object is chiral, it possesses the property of handedness meaning that it is not interchangeable. Chiral Not interchangeable ; possesses the property of handedness, cannot be superimposed on their own mirror image A compound that can exist as two different forms
