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Stereochemistry and Chirality February STAV Workshop Penny Commons Brendan Abrahams Copyright issues We have replaced the images with their urls. Please just click on the url and copy the image into the PowerPoint. Any free images have been left in place. Apologies but we really can’t leave the images on the CEA website, although you can use them in the classroom. VCAA Study Design … • • • • • Essential knowledge Structure and nomenclature of organic compounds • the carbon atom with reference to valence number, bond strength, stability of carbon bonds with other elements and the formation of isomers to explain carbon compound diversity, including identification of chiral centres in optical isomers of simple organic compounds and distinction between cis- and trans- isomers in simple geometric isomers • structures including molecular, structural and semi-structural formulas of alkanes (including cyclohexane), alkenes, alkynes, benzene, haloalkanes, primary amines, primary amides, alcohols (primary, secondary, tertiary), aldehydes, ketones, carboxylic acids and non-branched esters • IUPAC systematic naming of organic compounds up to C8 with no more than two functional groups for a molecule, limited to non-cyclic hydrocarbons, haloalkanes, primary amines, alcohols (primary, secondary, tertiary), carboxylic acids and non-branched esters. … the formation of isomers to explain carbon compound diversity, including identification of chiral centres in optical isomers of simple organic compounds and distinction between cis- and trans- isomers in simple geometric isomers VCAA Study Design … • • Metabolism of food in the human body • metabolism of food as a source of energy and raw materials: general principles of metabolism of food involving enzyme-catalysed chemical reactions with reference to the breakdown of large biomolecules in food by hydrolytic reactions to produce smaller molecules, and the subsequent synthesis of large biologically important molecules by condensation reactions of smaller molecules enzymes as protein catalysts: active site; modelling of process by which enzymes control specific biochemical reactions (lock-and-key and induced fit models); consequences of variation in enzyme-substrate interaction (lock-and-key mechanism) due to the behaviour of a particular optical isomer; explanation of effects of changes in pH (formation of zwitterions and denaturation), increased temperature (denaturation) and decreased temperature (reduction in activity) on enzyme activity with reference to structure and bonding; action of enzymes in narrow pH ranges; and use of reaction rates to measure enzyme activity … consequences of variation in enzyme-substrate interaction (lock-and-key mechanism) due to the behaviour of a particular optical isomer Chirality An object which is non-superimposable on its mirror image is said to be chiral https://upload.wikimedia.org/wiki http://chemwiki.ucdavis.ed pedia/commons/thumb/4/4c/Ne u/@api/deki/files/54210/= ptunea_122a3121b7268f235234e _links%26rechts_gewonden.jpg/ 7a9091fc4ea.jpg?revision 200px-Neptunea_=1 _links%26rechts_gewonden.jpg https://encryptedtbn1.gstatic.com/i mages?q=tbn:ANd 9GcSZ_PBHe32AI yd5EccvUKC9Da7 Kfi88Bts4gdHPCC F3gzw_iWId http://www.science.oregonstate.edu/~gablek/CH334/Chapter5/chirality.htm Achiral objects http://chemwiki.ucdavis.edu/@a pi/deki/files/54210/=122a3121b 7268f235234e7a9091fc4ea.jpg? revision=1 http://www.science.oregonstate.edu/~gablek/CH334/Chapter5/chirality.htm Object Mirror image Exercise 1 Indicate whether the following objects are chiral or achiral Tennis racquet Wrist watch Golf club http://static.giantbo mb.com/uploads/sc ale_small/0/140/37 6641tennis_racket.jpg Stapler Car https://upload.wikimedia.or g/wikipedia/commons/2/2d/ Swingline-stapler.jpg https://cdn.vecto rstock.com/i/co mposite/95,07/g olf-club-vector879507.jpg http://stat.homeshop18.co m/homeshop18/images/pro ductImages/500/titanmens-wristwatchnd1445yl01large_95d244780622d0c46 1e5cc581730541b.jpg http://image.cpsimg.com/s ites/carpartsmc/assets/classroom/ima ges/dash_gauges.jpg Chiral Molecules: Bromochlorofluoromethane Contains a tetrahedral carbon with four different substituents (F, Cl, Br and H) Molecules which are non-superimposable mirror images are called ‘enantiomers’ – Examples of stereoisomers Isomers – Molecules with the same molecular formula but with different structure Isomers Constitutional isomers - different atom to atom connectivity Stereoisomers - same atom to atom connectivity but the arrangement in space is different Stereoisomers Same atom connectivity, but have a different 3-D arrangement of atoms • Enantiomers are stereoisomers which are non superimposable mirror images https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSfHnpbZ1Wnx2FGXtMAh53t_LjCEG20O3dm5UcrcxyoLaABuAtHQ Examples of chiral molecules 3-Methylhexane * Denotes asymmetric carbon Exercise 2 Make a chiral model by taking a black tetrahedral centre and attaching four groups, each of a different colour to it. Compare it with your neighbour’s model. Are they enantiomers or are they the same? https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTRJdU6GyoEpvLen4hf14BzHlgkSi5BAtnVgnSOIKy9hX370v Fo http://image.slidesharecdn.com/isomers-120206102033phpapp01/95/isomers-22-638.jpg?cb=1360324779 https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTi55JoCgJEYb5O_90PFTyU7keY2rs0yxzD 6p3uebAJufLLlviAOw Exercise 3 Place an asterisk (*) by each of the asymmetric (stereogenic) centres contained in the molecules shown below: CH3 H3C OH H2N O D Exercise 3 cont… Place an asterisk (*) by each of the asymmetric (stereogenic) centres contained in the molecules shown below: How do the chemical and physical properties of enantiomers differ? Enantiomers have identical physical properties except for the direction of rotation of plane-polarised light (PPL). They will react in the same way with symmetric molecules but differently with asymmetric (often biological) molecules. Polarimeter – An instrument for measuring the rotation of the plane of polarised light https://www.google.com.au/imgres?imgurl=http://wps.prenhall.com/wps/media/ob jects/3313/3393071/imag2403/AAAYPEV0.JPG&imgrefurl=http://wps.prenhall.co m/wps/media/objects/3313/3393071/blb2403.html&h=197&w=495&tbnid=xe1hfryGOTdM:&docid=02qRCyvnP6RXIM&ei=BIPWVpzWLcut0ATj_bFg&tbm=isch&ved=0 ahUKEwic6LyFq6HLAhXLFpQKHeN-DAwQMwhCKBswGw Demonstration of optical rotation using the enantiomers of Limonene Some Terminology: Molecules which rotate PPL are said to be optically active Specific rotation (-) 102° anticlockwise levorotatory (l) Limonene or (-) Limonene (+) 102° clockwise dextrorotatory (d) Limonene (+) Limonene A 1:1 mixture of enantiomers is known as a racemic mixture or racemate. A racemate is optically inactive (rotations of each enantiomer cancel out). Exercise 4 Consider how each of these objects interacts with a right hand compared to a left hand. Tennis racquet Wrist watch http://static.giantbo mb.com/uploads/sc ale_small/0/140/37 6641tennis_racket.jpg http://stat.homeshop18.co m/homeshop18/images/pro ductImages/500/titanmens-wristwatchnd1445yl01large_95d244780622d0c46 1e5cc581730541b.jpg Golf club Stapler https://upload.wikimedia.or g/wikipedia/commons/2/2d/ Swingline-stapler.jpg https://cdn.vectorstock.com/i/compo site/95,07/golf-club-vector879507.jpg Many objects are made for right handed people… https://33.media.tumblr.com/ce82ef29988e9971356a03486c9d26d1/tumblr_i nline_mklhe3Kqqz1qz4rgp.jpg (Simpson’s Leftorium) Artificial Vs Natural Chemical Synthesis When synthetic chemists make chiral molecules it is common to obtain a mixture of enantiomers – a racemic mixture. When nature makes a chiral molecule the products are almost always a pure enantiomer e.g. amino acids and sugars Chiral molecules docking with a biological system Enzymes are chiral molecules that will interact differently with pairs of enantiomers Analogous to a left hand not fitting a right glove. Enantiomers differ in their interactions with other chiral molecules – particularly important in biological systems http://www.herbmuseu m.ca/files/images/artic le-1138955002F8EF600000258941_233x423.jpg Assignment of configuration 1. Assign priorities in order of decreasing atomic number of the immediately attached atom to the asymmetric carbon ie. highest atomic number is ranked first, lowest (often H) is fourth. 2. If a decision can’t be reached by ranking the first atoms consider second, third and fourth atoms outwards until a difference is found. 3. Orient the lowest ranked substituent away from you. 4. If the order of 1, 2 and 3 is clockwise then it is R, if the order is anticlockwise then it is S. Assigning configuration to bromochlorofluoromethane (R)-bromochlorofluoromethane (S)-bromochlorofluoromethane Assigning configuration to 3-methylhexane R-3-methylhexane S-3-methylhexane Lactic acid 2 1 4 3 4 1 S configuration 3 2 R configuration The S configuration is the one of biological importance. Axial Chirality Allenes Where A has higher priority than B Name this structure R S Axial Chirality Axial Chirality M Helicity P Helicity Youtube video: http://www.youtube.com/watch?v=h_DROS_NNx4 Ethene – Overlap of p orbitals to give p-bond https://encryptedtbn1.gstatic.com/images?q=tbn:ANd9GcSNXaTkXIhJ0r PZ3PfAlFkOxDvEYalAPtO7MUG7z6tkMBJBxpXv4g Stereoisomerism in Alkenes Geometry of Ethene No rotation around double bond Consider structure of 2-butene CH3CH=CHCH3 These cannot interconvert Cis (Z) An example of geometric Trans (E) isomerism Stereoisomers which arise due to hindered rotation of bonds are known as geometrical isomers. Alkenes which have two different groups at either end of a C=C double bond display geometrical isomerism. Nomenclature for alkene geometrical isomers For simple, unsubstituted internal alkenes, e.g 2-butene, the cis – trans nomenclature was developed early on. If the carbon substituents are on the same side of the C=C double bond, the geometry is designated as cis. This compound is called cis-2-butene If the carbon substituents are on the opposite side of the C=C double bond, the geometry is designated as trans. This compound is called trans-2-butene cis-hex-2-ene trans-oct-3-ene The early cis-trans nomenclature does not work well for all alkenes, and has been replaced by the more general E / Z nomenclature: (1) (1) (2) (2) (1) (2) Z-3-methyl-hex-2-ene (2) (1) E-3-methyl-hex-3-ene Thank you for your attention. [email protected]