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Organic nomenclature and isomerism Nomenclature Nomenclature review 3,5-dimethylheptane Trans-but-2-ene trichloromethane Pentan-2-ol 2-bromo-3-chloropentane H H Br C C C C H H H H H 4-bromobut-1-ene 2-methylpropan-2-ol 5-chlorohex-5-en-3-ol 3,4,9-trimethyldecane H H Cl H H C H C C C H C H H H H chlorodimethylpropane H (E)-5-bromopent-2-ene H H H H C H C C H H C C C H H H H H cyclohexane Lesson objectives • Understand and be able to work with IUPAC nomenclature extended to more complex compounds including cyclic, aromatic and heterocyclic compounds. • Be able to draw and build structures which show geometrical (E-Z) isomerism • Recognise asymmetrical carbon atoms and understand that they give rise to optical isomers Success criteria • Name and draw the structure of different groups of organic compounds • Give reason why geometrical isomers exist. • Draw molecules that show geometrical isomers • Identify asymmetrical carbon atoms • Draw optical isomers. Keywords • • • • • • • • • • • • Aromatic Ortho Para Meta Heterocyclic Stereoisomers Geometrical isomers Optical isomers Asymmetrical/ chiral carbon Superimpose Non-superimpose Enantiomer Aromatic Hydrocarbons Aromatic Compounds Aromatic compounds contain benzene ring. Benzene has: C6H6 • 6 C atoms and 6 H atoms. • Two possible Lewis structures. Naming of Aromatic Compounds Aromatic compounds are named: • With benzene as the parent chain. • Name of substituent comes in front of the “benzene”. • CH3-methyl Cl-chloro • CH2CH3-ethyl Br-bromo • NO2 –nitro F-fluoro CH3 methylbenzene Cl Chlorobenzene CH2-CH3 ethylbenzene Nomenclature of Benzene •Di-substituted benzene. ID the “parent”. You can use “benzene”, or combine a substituent with benzene as the parent. •Then use the lowest numbering system or ortho, meta, para designations: 1-bromo-3-chloro benzene, or 3-chloro-bromobenzene, or meta-chloro-bromobenzene Naming of Aromatic Compounds If we have three or more substituents: - Start numbering from a special name (if we have). - If we do not have, number them to get the smallest set of numbers. - List them by alphabetical order. Naming of Aromatic Compounds Aryl group: When we remove a H atom from an arene (Ar-). Phenyl C6H5- 1 Nomenclature of Benzene •Common Names (you must learn these): toluene benzoic acid phenol benzaldehyde benzyl alcohol acetophenone aniline benzyl bromide Activity-6 minutes Heterocyclic compounds • Organic compounds that contain a ring structure containing in addition to carbon, such as sulfur, oxygen and nitrogen as the heteroatom Nomenclature of heterocyclic compounds It is based on: • Types of heteroatom (O, N, S, e.t.c) • Ring size • Nature of the ring Naming heterocyclic compounds • IUPAC name = Prefix + suffix • Prefix- from the atom present • Suffix= from the size of the ring Atom Prefix O S N oxa thia aza Priority decreases • Examples H N • This ring contains (N) Prefix is aza • The ring is 3-membered and fully saturated suffix is iridine • By combining the prefix and suffix, two vowels ended up together (azairidine), therefore the vowel on the end of the first part should be dropped. This gives the correct name: Aziridine Heterocyclic Chemistry Oxirane oxole oxetane Most common heterocyclic molecules Pyrrole Furan Pyrrolidine/ tetrahydropyrrole Thiophene Pyridine Isomerism • The existence of two or more molecules with the same molecular formula but different structures or arrangement of atoms in space. 27 ISOMERISM STRUCTURAL ISOMERS = molecules with the same molecular formula but different structural formulas. Chain Functiona l group Position STEREOISOMERS = isomers with the same structural formula but different spatial arrangements geometrical Optical Types of isomers Pentane 2-methylbutane 2,2-dimethylpropane Chain isomers 1-propanol 2-propanol Position isomers Propanal Propanone Functional group isomers Cis-but-2-ene Trans-but-2-ene Geometrical (Cis-Trans) isomers Geometrical isomers • Watch video Geometrical isomers Which of the molecules will show geometrical isomer? B Name the two geometrical isomers. Trans-but-2-ene cis-but-2-ene Explain why they show geometrical isomerism. The presence of a double bond with a restricted rotation Each of the carbon of the double bond is attached to two different group. H H C H E-pent-2-ene Z-pent-2-ene H Chiral and Achiral Molecules • Although everything has a mirror image, mirror images may or may not be superimposable. • Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable. 36 • Optical isomers mirror images. are non-superimposable • Condition for optical isomerism • Involves a tetrahedral sp3 atom i.e carbon atom joined to four different groups. Carbons of this type are called ‘chiral./ asymmetric carbon • No plane of symmetry. H CH3 C CH2 CH3 OH Chiral Centre 2-Butanol CH3 HO H C CH3 H C OH CH2 CH2 CH3 CH3 Stereoisomers whose molecules are nonsuperimposable mirror images of one another are called enantiomers CO2H H H CO2H H3C C C CO2H OH OH Cl C2H5 H3C C C CH3 HO O Cl Cl CO2H H F CH3 H3C C C CH3 OH OH Cl C2H5 C6H5 C C CH3 HO O O Cl C6H5 Cl Naming Enantiomers The R,S system of nomenclature Rank the groups (atoms) bonded to the chirality center Orient the lowest priority (4) away from you Clockwise = R configuration Counterclockwise = S configuration e.g 1. Rank the groups bonded to the asymmetric carbon 1 4 2 3 2. If the group (or atom) with the lowest priority is bonded by hatched wedge, do not reverse, if not reverse • If two atoms on a stereogenic center are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher atomic number determines the higher priority. 43 Figure 5.6 Examples of assigning priorities to stereogenic centers 44 Label each compound as R or S. a) Cl 2 S H Br H3C b) 3 CH2Br ClH2C 2 1 3 1 CH2Br rotate H3C OH H3C HO 2 2 CH2Cl 1 3 1 3 R 45 Label each compound as R or S.