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Organic Chemistry I - Stereochemistry

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    Study Guide Organic Chemistry I – Stereochemistry 1. Optical Activity What Is Optical Activity? Optical activity refers to the ability of certain compounds to rotate plane - polarized light. This phenomenon was first observed by the French physicist Jean Biot in the 19th century. While studying plane - polarized light, Biot discovered that some organic c ompounds caused this light to rotate, and these compounds were termed optically active . • Plane - polarized light is light that has been filtered so that its waves only vibrate in one plane. • Optically active compounds are substances that can rotate plane - polarized light, and this rotation can be measured using a device called a polarimeter . Figure 1 Figure 2

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    Study Guide 1.1 How Does Optical Activity Work? Figure 1 shows a simple setup for plane - polarized light. Light passes through a polarizer that allows only light vibrating in one direction to pass through, turning it into polarized light . In Figure 2 , we can see how optical activity is measured using a polarimeter. The light source shines plane - 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) is then measured with an analyzer to observe how much the light has rotated. 1.2 Tartaric Acid Crystals and Pasteur's Discovery In 1849, the chemist Louis Pasteur discovered that tartaric acid salts could form two types of crystals. After examining them under a microscope, Pasteur realized that the crystals were nonsuperimposable mirror images of each other — similar to how your right and left hands are mirror images but cannot be superimposed on each other. This discovery led to the idea that certain compounds could exist as two forms that are mirror images (called enantiomers ) and have optical activity. • Enantiomers are pairs of molecules that are mirror images of each other but cannot be superimposed. • Optically active compounds will rotate plane - polarized light, but the direction of rotation can be opposite for each enantiomer. This is shown in the polarimeter experiment (Figure 2). Figure 3

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    Study Guide 1.3 The Structure of Tartaric Acid Crystals Figure 3 illustrates the crystalline structure of sodium ammonium tartrate, a salt of tartaric acid. This figure shows two different crystal forms (labeled "a" and "b") that are nonsuperimposable mirror images of each other, just like your right and left hands. • Figure 3 shows the crystalline structure of the tartrate salt crystals. The crystals of sodium ammonium tartrate exhibit optical activity as each crystal form rotates light in the opposite direction. Optical Purity • Optically pure compounds are made up of just one enantiomer, whereas a racemic mixture contains equal amounts of both enantiomers. • Optical purity is a way to measure the composition of a mixture of enantiomers. It is calculated by the formula: A racemic mixture (a 1:1 mixture of two enantiomers) does not show any optical activity because the rotations of the two enantiomers cancel each other out. Key Takeaway 1. Optical activity refers to the rotation of plane - polarized light by certain compounds. 2. Enantiomers are mirror - image molecules that cannot be superimposed on each other, and they may have opposite rotations of light. 3. Optical purity measures how much one enantiomer is present in a mixture, and it is used to determine the proportion of one enantiomer compared to the racemic mixture. 2. Chirality What Is Chirality? Molecules that exist as enantiomers — which are nonsuperimposable mirror images of each other — are called chiral molecules . To understand chirality, we need to think about symmetry .

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    Study Guide A plane of symmetry is an imaginary plane that divides an object into two halves, where each half is a mirror image of the other. If a molecule has a plane of symmetry, then its two halves are identical and can be superimposed on one another. Achiral molecules (those without chirality) possess a plane of symmetry, while chiral molecules do not. For example, butane is an achiral molecule because it can be divided into two identical halves. In contrast, 2 - bromobutane is a chiral molecule because it lacks a plane of symmetry and cannot be superimposed on its mirror image. 2.1 Stereogenic Centers The most common cause of chirality in organic molecules is a stereogenic center . This is a carbon atom bonded to four different atoms or groups, making it chiral or asymmetric . • These centers are often marked with an asterisk (*) in formulas and projections. 2.2 van't Hoff Rule The van't Hoff rule helps us predict the maximum number of enantiomers (optically active forms) a molecule can have. The rule states that the maximum number of enantiomers a molecule can possess is equal to 2 raised to the power of n , where n is the number of stereogenic centers in the molecule. For example, the molecule 2 - chlorobutane has one stereogenic center, so it can have up to two enantiomers .

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    Study Guide Key Takeaway 1. Chirality occurs in molecules that are nonsuperimposable mirror images (enantiomers). 2. A stereogenic center is a carbon atom bonded to four different groups, making the molecule chiral. 3. The van't Hoff rule predicts the maximum number of enantiomers based on the number of stereogenic centers. 3. Projections in Organic Chemistry To better understand the 3D structure of molecules, especially in the context of enantiomers and diastereomers , chemists use various drawing styles called projections . These projections help visualize the spatial arrangement of atoms and groups in a molecule. Let’s explore some common projection types used to represent molecules. 3.1 Fischer Projection The Fischer projection is one of the simplest ways to represent the three - dimensional structure of a molecule. In this representation: • The backbone of a carbon chain is drawn as a straight line. • Terminal carbon atoms (carbon atoms at the ends of the chain) are written as groups. • The atoms or groups attached to each carbon atom are shown as perpendicular lines, which helps indicate their positions in space.

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    Study Guide For example, comparing the structural formula and Fischer projection of 2 - bromo - 3 - chlorobutane helps illustrate how the Fischer projection works. 3.2 Sawhorse Projection A sawhorse projection provides a better visualization of the three - dimensional geometry between adjacent carbon atoms. It is often used to show how groups interact on adjacent carbons, which is especially useful for understanding the conformations of molecules in reactions. In a sawhorse projection: • The backbone carbon atoms are shown as a diagonal line . • Terminal carbon atoms are represented as groups, just like in the Fischer projection. For example, in the 2 - bromo - 3 - chlorobutane molecule, the top carbon of the Fischer projection becomes the back carbon in the sawhorse projection. This projection can also help identify staggered and eclipsed conformers .

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    Study Guide 3.3 Staggered and Eclipsed Conformers In the sawhorse projection , we can visualize staggered and eclipsed conformations : • Staggered conformers : The atoms or groups attached to each carbon fit into the spaces (or voids ) around the adjacent carbon atoms, minimizing steric strain. • Eclipsed conformers : In these, the atoms and groups on adjacent carbons are in line with each other, which increases steric strain. For example, in 2 - bromo - 3 - chlorobutane , the sawhorse projection shows the molecule in a staggered conformer , where the groups are spaced out to minimize strain. Newman Projection The Newman projection is another type of representation used to visualize the three - dimensional interactions between atoms or groups. It is particularly useful for understanding steric crowding in molecules. In a Newman projection: • The molecule is viewed from one end . • The front carbon of the backbone is represented by a dot , and the back carbon is shown as a circle . • The bonds to the groups on the carbon atoms emanate from the dot and circle at 120° angles. The 2 - bromo - 3 - chlorobutane molecule can be shown in Newman projections to compare staggered and eclipsed conformers , just like in the sawhorse projection.

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    Study Guide Key Takeaway 1. Fischer projection : Represents a molecule with the backbone as a straight line and attached groups as perpendicular lines. 2. Sawhorse projection : A more 3D view, showing the relative positions of adjacent carbon atoms and their groups. 3. Staggered and eclipsed conformers : Staggered conformers minimize steric strain, while eclipsed conformers have atoms aligned, causing strain. 4. Newman projection : A view of a molecule from one end, used to assess steric interactions and stress between atoms/groups. 4. Enantiomers and Diastereomers What Are Enantiomers? Enantiomers are molecules that are nonsuperimposable mirror images of each other, like your right and left hands. These molecules are created by the rotation of one of the stereogenic centers (as shown 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) a nd (b) are enantiomers .

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