Organic Chemistry I - Stereochemistry - Quick Guides

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Study GuideOrganic Chemistry I–Stereochemistry1.Optical ActivityWhat Is Optical Activity?Optical activity refers to the ability of certain compounds to rotate plane-polarized light. Thisphenomenon was first observed by the French physicist Jean Biot in the 19th century. While studyingplane-polarized light, Biot discovered that some organic compounds caused this light to rotate, andthese compounds were termedoptically active.•Plane-polarized lightis light that has been filtered so that its waves only vibrate in oneplane.•Optically active compoundsare substances that can rotate plane-polarized light, and thisrotation can be measured using a device called apolarimeter.Figure 1Figure 2

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Study Guide1.1How Does Optical Activity Work?Figure 1 shows a simple setup for plane-polarized light. Light passes through a polarizer that allowsonly light vibrating in one direction to pass through, turning it intopolarized light.InFigure 2, we can see how optical activity is measured using a polarimeter. The light source shinesplane-polarized light through a solution of organic molecules. As the light passes through the solution,the molecules rotate the light. The angle of rotation (α)isthen measured with an analyzer to observehow much the light has rotated.1.2Tartaric Acid Crystals and Pasteur's DiscoveryIn 1849, the chemistLouis Pasteurdiscovered that tartaric acid salts could form two types ofcrystals. After examining them under a microscope, Pasteur realized that the crystals werenonsuperimposable mirror images of each other—similar to how your right and left hands are mirrorimagesbut cannot be superimposed on each other.This discovery led to the idea that certain compounds could exist as two forms that are mirror images(calledenantiomers) and have optical activity.•Enantiomersare pairs of molecules that are mirror images of each other but cannot besuperimposed.•Optically activecompounds will rotate plane-polarized light, but the direction of rotation canbe opposite for each enantiomer. This is shown in the polarimeter experiment (Figure 2).Figure 3

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Study Guide1.3The Structure of Tartaric Acid CrystalsFigure 3 illustrates thecrystalline structureof sodium ammonium tartrate, a salt of tartaric acid. Thisfigure shows two different crystal forms (labeled "a" and "b") that are nonsuperimposable mirrorimages of each other, just like your right and left hands.•Figure 3shows thecrystalline structureof the tartrate salt crystals. The crystals of sodiumammonium tartrate exhibitoptical activityas each crystal form rotates light in the oppositedirection.Optical Purity•Optically purecompounds are made up of just one enantiomer, whereas aracemic mixturecontains equal amounts of both enantiomers.•Optical purityis a way to measure the composition of a mixture of enantiomers. It iscalculated by the formula:Aracemic mixture(a 1:1 mixture of two enantiomers) does not show any optical activity because therotations of the two enantiomers cancel each other out.Key Takeaway1.Optical activityrefers to the rotation of plane-polarized light by certain compounds.2.Enantiomersare mirror-image molecules that cannot be superimposed on each other, andthey may have opposite rotations of light.3.Optical puritymeasures how much one enantiomer is present in a mixture, and it is used todetermine the proportion of one enantiomer compared to the racemic mixture.2.ChiralityWhat Is Chirality?Molecules that exist asenantiomers—which are nonsuperimposable mirror images of each other—are calledchiral molecules. To understand chirality, we need to think aboutsymmetry.

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Study GuideAplane of symmetryis an imaginary plane that divides an object into two halves, where each half isa mirror image of the other. If a molecule has a plane of symmetry, then its two halves are identicaland can be superimposed on one another.Achiralmolecules (those without chirality) possess aplane of symmetry, whilechiralmolecules do not.For example,butaneis anachiralmolecule because it can be divided into two identical halves. Incontrast,2-bromobutaneis achiralmolecule because it lacks a plane of symmetry and cannot besuperimposed on its mirror image.2.1Stereogenic CentersThe most common cause of chirality in organic molecules is astereogenic center. This is a carbonatom bonded to four different atoms or groups, making itchiralorasymmetric.•These centers are often marked with an asterisk (*) in formulas and projections.2.2van't Hoff RuleThevan't Hoff rulehelps us predict the maximum number of enantiomers (optically active forms) amolecule can have. The rule states that the maximum number of enantiomers a molecule canpossess is equal to2 raised to the power of n, wherenis the number ofstereogenic centersin themolecule.For example, the molecule2-chlorobutanehas one stereogenic center, so it can have up totwoenantiomers.

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Study GuideKey Takeaway1.Chiralityoccurs in molecules that are nonsuperimposable mirror images (enantiomers).2.Astereogenic centeris a carbon atom bonded to four different groups, making the moleculechiral.3.Thevan't Hoff rulepredicts the maximum number of enantiomers based on the number ofstereogenic centers.3.Projections in Organic ChemistryTo better understand the 3D structure of molecules, especially in the context ofenantiomersanddiastereomers, chemists use various drawing styles calledprojections. These projections helpvisualize the spatial arrangement of atoms and groups in a molecule. Let’s explore some commonprojection types used to represent molecules.3.1Fischer ProjectionTheFischer projectionis one of the simplest ways to represent the three-dimensional structure of amolecule. In this representation:•Thebackboneof a carbon chain is drawn as a straight line.•Terminal carbon atoms(carbon atoms at the ends of the chain) are written as groups.•Theatomsorgroupsattached to each carbon atom are shown as perpendicular lines, whichhelps indicate their positions in space.

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Study GuideFor example, comparing thestructural formulaandFischer projectionof2-bromo-3-chlorobutanehelps illustrate how the Fischer projection works.3.2Sawhorse ProjectionAsawhorse projectionprovides a better visualization of thethree-dimensional geometrybetweenadjacent carbon atoms. It is often used to show how groups interact on adjacent carbons, which isespecially useful for understanding theconformationsof molecules in reactions.In a sawhorse projection:•Thebackbone carbon atomsare shown as adiagonal line.•Terminal carbon atomsare represented as groups, just like in the Fischer projection.For example, in the2-bromo-3-chlorobutanemolecule, the top carbon of the Fischer projectionbecomes the back carbon in the sawhorse projection.This projection can also help identifystaggeredandeclipsed conformers.

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Study Guide3.3Staggered and Eclipsed ConformersIn thesawhorse projection, we can visualizestaggeredandeclipsed conformations:•Staggered conformers: The atoms or groups attached to each carbon fit into the spaces (orvoids) around the adjacent carbon atoms, minimizing steric strain.•Eclipsed conformers: In these, the atoms and groups on adjacent carbons are in line witheach other, which increases steric strain.For example, in2-bromo-3-chlorobutane, the sawhorse projection shows the molecule in astaggered conformer, where the groups are spaced out to minimize strain.Newman ProjectionTheNewman projectionis another type of representation used to visualize thethree-dimensionalinteractionsbetween atoms or groups. It is particularly useful for understandingsteric crowdinginmolecules.In a Newman projection:•The molecule is viewed from oneend.•Thefront carbonof the backbone is represented by adot, and theback carbonis shown asacircle.•The bonds to the groups on the carbon atoms emanate from the dot and circle at120°angles.The2-bromo-3-chlorobutanemolecule can be shown in Newman projections to comparestaggeredandeclipsed conformers, just like in the sawhorse projection.

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Study GuideKey Takeaway1.Fischer projection: Represents a molecule with the backbone as a straight line and attachedgroups as perpendicular lines.2.Sawhorse projection: A more 3D view, showing the relative positions of adjacent carbonatoms and their groups.3.Staggered and eclipsed conformers: Staggered conformers minimize steric strain, whileeclipsed conformers have atoms aligned, causing strain.4.Newman projection: A view of a molecule from one end, used to assess steric interactionsand stress between atoms/groups.4.Enantiomers and DiastereomersWhat Are Enantiomers?Enantiomers are molecules that are nonsuperimposable mirror images of each other, like your rightand left hands. These molecules are created by therotation of one of the stereogenic centers(asshown in the Fischer projections for 2-chlorobutane). By rotating a structure in the plane of the paper(180°), we do not get a superimposable image. For example, in the diagram with structures(a)and(b)of 2-chlorobutane, rotating(b)by 180° does not lead to a form that can be superimposed onto(a).Thus,(a)and(b)areenantiomers.

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