25:["$","title",null,{"children":"Module 02 Optical Isomerism - Chirality and its consequences | OrganChem | Observable"}] 49:T43c,There are two ways by which this can happen. A. One of the ways this happens is when a carbon atom in a molecule has 4 different groups attached. Such a carbon is called a **chiral center** or **asymmetric carbon**. The two molecules are referred to as chiral or optically active. This is because they interact in a unique way with a special type of light known a s plane polarized light. There are only ever two optical isomers formed for each chiral center. If there are two chiral centers, then there will be two pairs of optical isomers. Enantiomers only differ in two characteristics; their interaction with plane polarized light and how they react with other chiral molecules. This image shows the plane polarized light has been rotated when passed through to the isomers. However, the light is rotated in opposite directions for each isomer, but by the same angle either way. A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate. ![](${await FileAttachment('polarimeter.png').url()}) http://www.wiredchemist.com/files/shared/images/figure39.gif4a:T49d,**Check your understanding 1** 1. Which of the following molecules have a chiral carbon? Mark the chiral carbon with an asterisk. a. ![](${await FileAttachment('check 1 a.png').url()}) b. ![](${await FileAttachment('check 1 b.png').url()}) c. ![](${await FileAttachment('check 1 c@1.png').url()}) d. ![](${await FileAttachment('check 1 d.png').url()}) For a complete discussion on chirality of molecules containing asymmetric carbons, see the following presentation. ["Stereoisomerism"](https://www.academia.edu/45033710/Optical_isomerism) by Pallavi Tiwari Roy is licensed under [CC BY-NC 4.0](http://creativecommons.org/licenses/by-nc/4.0). Thus, to summarize, the term **chiral center** refers to an atom in the molecular structure. The term **chiral molecule** refers to the entire molecule. The presence of one chiral center renders the entire molecule chiral. The presence of two or more chiral centers **may or may not** result in the molecule being chiral. To be chiral, a molecule and its mirror image must be nonsuperimposable. The following symmetry elements must be absent in a molecule having more than one asymmetric carbon to make it chiral.4b:T65c,**Elements of symmetry** (a) ***Plane of symmetry:*** A plane which bisects the molecules into two mirror images is called a plane of symmetry. If the plane of symmetry is present then the molecule is optically inactive, if absent then it is optically active.

(b) **Centre of symmetry:** A center of symmetry in a molecule is said to exist if a line is drawn from any atom or group to this point and then extended to an equal distance beyond this point meets the identical atom or group. A center of symmetry is usually present only in an even membered ring. ![](${await FileAttachment('centre of symmetry.png').url()}) *trans 2, 4 - dimethylcyclobutane trans 1, 3- carboxylic (inactive)* (c) ***n-fold alternating access of symmetry:*** If a rotation by 360º/n degrees (n = 1, 2, 3, …) followed by reflection in plane perpendicular to the axis results in identical molecule the compound said to be possess n-fold alternating access of symmetry. If plane of symmetry or center of symmetry is present, then n-fold alternating axis of symmetry is present. If plane of symmetry or center of symmetry are absent, then n-fold alternating axis of symmetry will be absent. If the n-fold alternating axis of symmetry present, then the molecule is optically inactive, if absent then optically active. 4c:T528,**Check your understanding 2** Which of the following molecules are chiral? For each molecule, explain why it is chiral/achiral. (The chiral carbons are indicated by asterisk/s) a. ![](${await FileAttachment('check 2 a.png').url()}) b. ![](${await FileAttachment('Check 2 b.png').url()}) c. ![](${await FileAttachment('check 2 c.png').url()}) *Additional reading* [Polarimetry, specific rotation and optical purity](https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_420_-_Organic_Chemistry_I/Text/06%3A_Stereochemistry_at_Tetrahedral_Centers/6.07%3A_Optical_Activity_and_Racemic_Mixtures#:~:text=The%20%22optical%20purity%22%20is%20a,the%20optical%20purity%20is%2050%25.) B. The second way by which molecules can have a nonsuperimposable mirror image is due to the phenomenon of atropisomerism. Atropisomerism is stereoisomerism due to restricted rotation about a single bond. Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric strain barrier to rotation is high enough to allow for the isolation of the conformers. **Atropisomerism** is also called **axial chirality** and the chirality is not simply a center or a plane but an **axis**. Some of the systems that exhibit this kind of chirality include biphenyls, spirans and cumulenes. 4d:T591,**Biphenyls** are compounds whereby a phenyl ring is connected to another through a central **σ bond**. The C-C sigma bond (indicated by an arrow) is also known as the pivotal bond. ![](${await FileAttachment('biphenyl.png').url()}) In unsubstituted **biphenyl**, there is enough freedom of rotation around the central single bond to allow for free interconversion between the various conformers or rotamers so that the various rotamers cannot exist independently. However, **biphenyls** with large substituents at the ***ortho*** positions on either side of the **central σ bond** experience restricted rotation along this bond due to **steric hindrance**. If the substituents are different, a chiral molecule existing as a pair of enantiomers called **atropisomers** is obtained. ![](${await FileAttachment('biphenyl enantiomers.png').url()}) In the same way, if the two rings attached to the spiro carbon of spirans are unsymmetrically substituted, then the molecule will exist as enantiomers. Similarly, in a cumulated diene, if each of the two sp2 hybridized carbon atoms of the diene have two different substituents, then it can show optical isomerism. See VIDEOS [Stereochemistry of biphenyls](https://www.youtube.com/watch?v=ws4m6q_Znbs) [Stereochemistry of Spirans](https://www.youtube.com/watch?v=-05EqCWMtaA) [Stereochemistry of cumulenes](https://www.youtube.com/watch?v=W0BS45HranA) 4e:Tae9,**Relative Configuration** Two configurations are possible for a compound having one asymmetric carbon atom which represents two optical isomers. One of them is dextro rotatory and other one is laevo rotatory. Since optical activity is an experimental property, we need a system of nomenclature that indicates the configuration (arrangement) of the atoms or groups about the asymmetric carbon. Relative configuration is a system of assigning a name to a molecule by establishing configurational relationship among the members of a family of compounds. For this, glyceraldehyde is arbitrarily chosen as standard. This system is primarily used for sugars and amino acids. **D, L system of nomenclature** Most of the sugars are genetically related to either D or L-glyceraldehyde and thus their configurations are defined by D or L. According to Fischer projection formula, the main carbon chain of glyceraldehyde is kept vertically in which —CHO group is at the top and CH2OH group is at the bottom. Now, if —OH group of the chiral carbon atom is placed right side, it is D-configuration and if —OH is placed on the left side, it is L – configuration. **Note that D and L have no relation with optical rotation of a molecule.** ![](${await FileAttachment('glyceraldehyde enantiomers.png').url()}) *Additional reading* D,L – Convention for designating stereocenters See VIDEO [Naming of cabohydrates](https://youtu.be/doe_zZDjU5c) **Erythro, threo system of nomenclature** **Erythro** and **threo** are terms used for naming molecules with two stereogenic centers. The names are derived from the saccharides **erythrose** and **threose shown below**. ![](${await FileAttachment('erythrose threose.png').url()}) Similar groups on same side similar groups on opposite side Threose and erythrose exist in two enantiomeric forms which are designated as D and L enantiomers. ![](${await FileAttachment('D,L erythrose threose.png').url()}) Together they are 2 pairs of enantiomers. However, some uncertainty arose when the erythro/threo nomenclature was applied to systems with only one common substituent on the two neighboring chiral centers. ***Syn* and *anti*-system of nomenclature** When the carbon chain is drawn as a zig-zag chain and the substituents on the two adjacent chiral carbons are in the same plane, the stereoisomer is designated as *syn*. When the groups are on the opposite sides of the plane, the stereoisomer is designated as *anti*. ![](${await FileAttachment('syn-anti.png').url()}) 4f:T9d3,**Absolute Configuration** The *precise* arrangement of substituents at a stereogenic center is known as the absolute configuration of the molecule. This is indicated by the R (*Rectus*) and S (*Sinister*) labels. **R** and **S** notations are used only to describe asymmetric molecules following **Cahn-Ingold-Prelog** (**CIP**) sequence rules. **(a) Rule I:** first we assign the priority numbers to the four atoms/groups attached to chiral center according to CIP rules. Priority is assigned based on the atomic number of the atoms bonded directly to the chiral carbon – the larger the **atomic number**, the higher the priority (isotopes with a higher mass number have higher priority). For example: S > O > N > C > H. **(b) Rule 2:** If two or more of the atoms that are bonded directly to the chiral center are the same, then prioritize these groups based on the next set of atoms (i.e., atoms adjacent to the directly bonded atoms). Continue until priorities can be assigned. Priority is assigned at the first point of difference. ![](${await FileAttachment('RS priority.png').url()}) *If two atoms have substituents of the same priority, higher priority is assigned to the atom with more of these substituents.* ![](${await FileAttachment('CHCl.png').url()}) *A larger group (i.e., more atoms) may not necessarily have a higher priority over another (smaller) group.* ![](${await FileAttachment('small big.png').url()}) **(c) Rule 3:** Atoms participating in double/triple bonds are considered to be bonded to an equivalent number of similar atoms by single bonds. Note: These new atoms are not bonded to other atoms. ![](${await FileAttachment('multiply bonds.png').url()}) **(d) Rule 4:** In Fischer projection representations orient the molecule so that the least priority group must be on lower end of vertical line. If the lower priority group is on horizontal line, then to bring the group on to vertical line do two mutual exchanges of groups so that the least priority group come to lower end of vertical line. **(e) Rule 5:** After giving priority order for the groups at asymmetric center, if priority direction is clockwise the configuration is specified ‘**R**’ (*Latin*: **rectus**, right); if anticlockwise the configuration is specified ‘*S*’ (*Latin*: **sinister**, left). 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For example, fluoromethane and its mirror image are the same molecule as you can rotate the mirror image and get the original molecule.\n\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":312,"value":"![](${await FileAttachment('Module 2 for pictures.png').url()})","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":25,"value":"However, some molecules have a mirror image that is non-superimposable. In other words, you cannot rotate one to get the other, so therefore they are different molecules and have different names. For example, the image in the left-hand side is (-) lactic acid in sour milk and the image on the right is (+) lactic acid in muscles. Such pairs of molecules are called enantiomers.\n![](${await FileAttachment('Lactic acid.png').url()})","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":202,"value":"$49","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":273,"value":"\n![](${await FileAttachment('rotation.png').url()})\n\n*Additional reading*\n\nFundamentals of chirality\n\nSee VIDEO [Introduction to Chirality](https://youtu.be/tk-SNvCPLCE)","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":453,"value":"$4a","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":457,"value":"$4b","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":459,"value":"$4c","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":461,"value":"***Conditions of Atropisomerism:*** \n1. A rotationally stable axis \n2. Presence of different substituents on both sides of the axis \n\t\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":463,"value":"$4d","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":465,"value":"## 2.2 Nomenclature of chiral compounds","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":467,"value":"$4e","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":480,"value":"$4f","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":469,"value":"**Check your understanding 3**\n\n![](${await FileAttachment('check 3 1.png').url()})\n \n![](${await FileAttachment('check 3 2.png').url()})\n \n![](${await FileAttachment('check 3 3.png').url()})","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":471,"value":"*Additional reading* \n\tNaming enantiomers by the R,S system \n\tSee VIDEO \n\t[R,S - System](https://youtu.be/kFD6hzLseVs) \n\t[R,S-system practice](https://youtu.be/9IYTqlVk_ZI) \n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":473,"value":"**Sources** \n1. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Fundamentals/Isomerism_in_Organic_Compounds/Optical_Isomerism_in_Organic_Molecules (public domain)\n2. https://kpu.pressbooks.pub/organicchemistry/chapter/5-3-chirality/ \n [Creative Commons Attribution-Noncommercial-Share Alike 4.0 International License](https://creativecommons.org/licenses/by-nc-sa/4.0/).\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":28,"value":"---","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":7,"value":"cssstyle","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":10,"value":"logo","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":14,"value":"after","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":5,"value":"import {\n front,\n cssstyle,\n MathJax,\n after,\n logo\n} from \"@yuwang/organicchemistry_imports\"","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":103,"value":"MathJax","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W50","stdlib":"1"}} 24:null