25:["$","title",null,{"children":"Module 04 Topicity and Stereochemistry of reactions | OrganChem | Observable"}] 49:T435,**Reactions that form a new chiral center** If the starting material is achiral, then we will always get a racemic mixture of product enantiomers, unless some other chiral agent is involved. This is for the same reason that the S_N1 reaction above gives a racemic product – there is an equal chance of attacking the achiral starting material from either side. Thus **Achiral** $\to$ **Chiral** will give a racemic product. If the starting material is already chiral, and we introduce a new chiral center, the situation is more complicated because it will produce a mixture of diastereomers, which have differing stabilities. *Additional reading* Stereoselective and stereospecific reactions See VIDEO [Regiospecificity, Stereoselectivity and stereospecificity](https://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/e1-e2-tutorial/v/regioselectivity-stereoselectivity-and-stereospecificity) 4a:[{"id":"4e5d9fc3bb4fa1d193188e92817cf723d638533e04ced6fefda805262c85a9052d49abf9e4ac1720cecd7f6f1fb4bc8e517af5544f056bdf3b5dec9275e69946","href":"https://static.observableusercontent.com/files/4e5d9fc3bb4fa1d193188e92817cf723d638533e04ced6fefda805262c85a9052d49abf9e4ac1720cecd7f6f1fb4bc8e517af5544f056bdf3b5dec9275e69946","name":"1. homotopic 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29:{"id":"630d2b38328814a0","slug":null,"title":"Module 04 Topicity and Stereochemistry of reactions","thumbnail":"989309619bdf27be4e7bb4df47f7c150db86ad883ec21c629c24c3101a69a4e4","access_level":"public","create_time":"$D2023-05-18T14:47:12.692Z","update_time":"$D2023-05-20T14:43:15.627Z","view_time":null,"first_public_time":"$D2023-05-18T14:47:12.692Z","has_unpublished_edits":null,"fork":{"id":"a2c3ea3d45e5d2bc","slug":null,"title":"Module 03 Geometric Isomerism and conformational isomerism Restricted vs free rotation","owner":{"id":"413d80f47db57e44","login":"organchem","name":"OrganChem"}},"stars":0,"starred":false,"owner":{"type":"team","id":"413d80f47db57e44","login":"organchem","name":"OrganChem","avatar_url":"https://avatars-next.observableusercontent.com/a6bcbd733d16171eb1b0884f56a848d39052524bbdeac3f0f495d866031181f0","tier":"starter_2024","bio":"Organic Chemistry education team","followed_by_you":false,"follows_you":false},"version":708,"publish_version":null,"role":null,"authored":false,"authors":[{"type":"individual","id":"1a0fd0abc8743a0b","login":"gomathi","name":"Gomathi Shridhar","avatar_url":"https://avatars-next.observableusercontent.com/bc990308552c107165a9afda1a8b24f7fb0823d6bfb7a93f767aa09a58df006d","tier":null,"bio":"","followed_by_you":false,"follows_you":false}],"members":[{"type":"individual","id":"1a0fd0abc8743a0b","login":"gomathi","name":"Gomathi Shridhar","avatar_url":"https://avatars-next.observableusercontent.com/bc990308552c107165a9afda1a8b24f7fb0823d6bfb7a93f767aa09a58df006d","tier":null,"role":"editor"},{"type":"team","id":"413d80f47db57e44","login":"organchem","name":"OrganChem","avatar_url":"https://avatars-next.observableusercontent.com/a6bcbd733d16171eb1b0884f56a848d39052524bbdeac3f0f495d866031181f0","tier":"starter_2024","role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":0,"collection":{"id":"1199aa0ef269999f","slug":"organchem","title":"Stereochemistry","owner":{"id":"413d80f47db57e44","login":"organchem","name":"OrganChem"}},"type":"notebook","body":{"titling":"auto","title":"Module 04 Topicity and Stereochemistry of reactions","cells":[{"id":531,"value":"# Module 04 Topicity and Stereochemistry of reactions","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":12,"value":"front","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":552,"value":"## 4.1 Topicity of ligands\n\nThe **stereochemical equivalence** or **non- equivalence** of chemically identical atoms or groups (ligands) in a molecule is called as topicity. They are classified as","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":652,"value":"**i.\tHomotopic ligands**\n\nThese ligands are chemically and stereochemically equivalent. If replacing the two groups with a different group (X) **gives the same compound**, then the groups are **homotopic**.\n\n![](${await FileAttachment('1. homotopic ligands.png').url()})\n\nThese groups have identical physical and chemical properties including reaction with chiral molecules.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":656,"value":"**ii.\tHeterotopic ligands**\n\nChemically equivalent ligands that are stereochemically non-equivalent are heterotopic. Heterotopic ligands are further classified as\n\n**a.\tEnantiotopic ligands**\n\nEnantiotopic ligands are those atoms or groups which on replacement by another ligand one at a time gives a pair of enantiomers.\n\n![](${await FileAttachment('Enantiotopic.gif').url()})\n\n**b.\tDiastereotopic ligands**\n\nDiastereotopic ligands are those atoms or groups which on replacement by another ligand one at a time gives a pair of diastereomers.\n\n![](${await FileAttachment('Diastereotopic.gif').url()})\n![](${await FileAttachment('Diastereotopic2.png').url()})\n\nHeterotopic ligands differ in their reaction with chiral molecules.\n\n**Prochiral centre**\n\nA prochiral centre is a carbon atom to which enantiotopic ligands are attached as the replacement of one of these ligands which will convert the carbon atom into chiral centre.\n\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":658,"value":"**Consequences of topicity of ligands**\n\ni.\t Molecules that have heterotopic ligands can form mixture of stereoisomers as products during a reaction.\n\n*NOTE*:\n* *Homotopic groups cannot be differentiated by chiral reagents.* \n* *Enantiotopic groups can be differentiated by chiral reagents.* \n* *Diastereotopic groups are differentiated by achiral and chiral reagents.* \n\nii.\tIn NMR spectroscopy,\n\n**Homotopic** protons have the exact same chemical shift. \n**Enantiotopic** protons have the same chemical shift in the vast majority of situations. However, if they are placed in a chiral solvent they will have different chemical shifts. \n**Diastereotopic** protons have different chemical shifts in all situations. \n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":660,"value":"**Check your understanding 1**\n\nFor each of the following compounds, determine whether the two protons shown in \nred are homotopic, enantiotopic, or diastereotopic.\n\n![](${await FileAttachment('Check1.png').url()})","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":662,"value":"## 4.2 Topicity of faces in flat or partially flat molecules","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":664,"value":"Flat or partially flat molecules have faces. Faces may be homotopic or heterotopic. Heterotopic faces may be either enantiotopic or diastereotopic. Addition or symmetry criterion serve to test the equivalency (homotopicity) and non-equivalency (heterotopicity) of faces.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":666,"value":"**i.\tHomotopic faces**\n\nTwo corresponding faces of a molecule (usually faces of a double bond) are homotopic when addition of the same reagent to either face gives the same product. \n\nAddition of $\\ce{HCN}$ to acetone will give the same cyanohydrin, no matter to which face addition occurs The two faces of the $\\ce{C=O}$ bond of acetone are thus homotopic.\n\n![](${await FileAttachment('homotopic_faces.png').url()})\n \nThe faces in acetone, ethylene, 1,1-dichloroethylene, cis-1,2- dichloroethylene, and allene ($\\ce{H2C=C=CH2}$) are homotopic.\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":668,"value":"**ii.\tHeterotopic faces** \n\nTwo corresponding faces of a molecule (usually faces of a double bond) are heterotopic when addition of the same reagent to either face gives a mixture of stereoisomers of the product. Heterotopic (non-equivalent) faces are further classified into enantiotopic and diastereotopic moieties.\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":670,"value":"**a.\tEnantiotopic faces**\n\nTwo corresponding faces of a molecule are enantiotopic if addition of the same achiral reagent to either one or the other face will give rise to enantiomeric products.\n\nAddition of $\\ce{HCN}$ to the two faces of acetaldehyde gives rise to two enantiomers of lactonitrile.\n\n![](${await FileAttachment('Enantiotopic_faces.png').url()})\n \n*NOTE: The added atom/group must be different from any group already there.*\n\n**b.\tDiastereotopic faces**\n\nTwo corresponding faces of a molecule are diastereotopic if addition of the same achiral reagent to either one or the other face will give rise to a pair of diastereomers.\n\n![](${await FileAttachment('Diastereotopic_faces.png').url()}) \n\n*Additional reading*\n\nTopicity\n\nSee VIDEO [Topicity](https://www.youtube.com/watch?v=48Pr2UXWk_A)\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":672,"value":"## 4.3 Stereochemistry of organic reactions","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":674,"value":"Based on stereochemistry, organic reactions can be classified as\n\n**i.\tStereospecific reactions**\n\nA stereospecific reaction is one which of the different stereochemical outcomes give different stereochemical outcomes. One stereoisomer of the reactant gives one stereoisomer of the product, while a different stereoisomer of the reactant gives a different stereoisomer of product. 'Stereospecific' relates to the ***mechanism*** of a reaction.\n\n![](${await FileAttachment('stereospecific1.png').url()})\n\n\n\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":700,"value":"**ii.\tStereoselective reactions**\n\nA stereoselective reaction is one which of the different stereochemical outcomes possible one is preferred to a greater extent. It is a reaction in which a single reactant can give two or more stereoisomeric products, and one or more of these products is preferred over the others-even if the preference is very small. If the products are enantiomers, the reaction is ***enantioselective***; if they are diastereoisomers, the reaction is ***diastereoselective***. Stereoselectivity is dominated by the ***structural features*** of the reactants.\n\n![](${await FileAttachment('stereospecific2.png').url()})\n\nIn order to understand stereochemistry of reactions, let’s examine some specific reactions.\n\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":702,"value":"**Reactions at a single chiral carbon, leading to a chiral product (chiral $\\to$ chiral)**\n\nIf a bond to a chiral carbon breaks during a reaction, there are several possibilities: \n* (a) Bond is broken and new bond is made in a single step, or in an ordered way. In this situation, the stereochemistry will be either retained or inverted.\n* (b) Bond is broken and a new bond is made in two or more steps (i.e., via intermediates), or in a less ordered way. In this case, the stereochemistry may become less distinct, and it may lead to a racemic mixture (50:50 mixture of enantiomers).\n\nTo illustrate each of these cases, we will look at the stereochemistry of some reactions we will be learning later in the semester.","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":677,"value":"**Example: S_N2 reaction**\n\n![](${await FileAttachment('example_SN2.png').url()})\n \nHere, iodide ($\\ce{I^-}$) attacks the alkyl halide from the back and forms a product where the chiral center has been inverted. Because the process happens all in one step, there is no scrambling of the stereochemistry, and the product would remain fully optically active.\n\n**Example: S_N1 reaction**\n\n![](${await FileAttachment('Example_SN1.png').url()})\n \nAgain, iodide ion is attacking the alkyl halide, but in this case, the reaction takes place in two steps. After the first step an achiral intermediate is formed, and this will lead to a racemic (50:50) mixture of enantiomers for the chiral product.\n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":679,"value":"$49","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":574,"value":"**Check your understanding 2**\n\n1. Identify if the following molecules have enantiotopic or diastereotopic faces\n \t\n![](${await FileAttachment('check2_1.png').url()})\n \t\n2.\tIdentify if the following reactions are stereoselective or stereospecific.\n \t\n![](${await FileAttachment('check2_2.png').url()}) \n","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","tool":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":576,"value":"**Sources**\n\n**1.\thttps://en.wikipedia.org/wiki/Topicity#:~:text=In%20stereochemistry%2C%20topicity%20is%20the,homotopic%2C%20enantiotopic%2C%20or%20diastereotopic. (public domain)** \n**2.\thttps://en.wikipedia.org/wiki/Stereoselectivity (public domain)**\n","pinned":false,"hidden":false,"mode":"md","author":"$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":"$W4a","stdlib":"1"}} 24:null