Organic Chemistry: Introductory Topics


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[ Visit ] This is a summary presentation of the introductory topics in Organic Chemistry, prepared according to the Singapore-Cambridge GCE A Level 9647 H2 Chemistry syllabus.

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Organic Chemistry: Introductory Topics

  1. 1.
  2. 2.  Carbon forms a huge number of compounds.  The study of the structures, reactions and properties of these carbon-based compounds is called organic chemistry.  Note: ◦ Carbon monoxide, carbon dioxide, metal carbonates and metal cyanides are not classified as organic
  3. 3. Tetravalent • Carbon forms four bonds. Strong C−C bonds • Due to strong C−C bond, it• C−C bonds are much stronger can bond with itself to form than bonds between atoms of stable straight or branched other elements. chains, or ring structures. Bond Bond energy / kJ mol−1 Bonding Nature C−C 350 of Carbon Si−Si 222 Multiple bonds N−N 160 O−O 150 • Carbon can also form single, double or triple bonds with itself. • Carbon compounds can3 have varying degree unsaturation.
  4. 4. Organic Compounds Aliphatic Aromatic • Saturated • Contains one • Unsaturated or more benzene rings Open-chain Alicyclic Cl • Straight-chain (closed ring) O OH • Branched
  5. 5. Organic compounds in the same Example: alkenehomologous series:• possess the same general formula CnH2n• differ from the previous member in the CH2=CH2, CH2=CHCH2−H, series by a −CH2− group CH2=CHCH2CH3, CH2=CHCH2CH2CH3• possess similar chemical properties, • Alkenes possess a carbon-carbon due to the presence of same functional double bond. group • Alkenes undergo electrophilic addition reactions.• show gradual change in physical Etheneb.p. = −102 °C properties due to increased molecular Propeneb.p. = −48 °C size and mass, caused by longer 1-Butene b.p. = −6.5 °C carbon chains 1-Pentene b.p. = 30 °
  6. 6.  A functional group is an atom or a group of atoms that governs the chemical properties of an organic
  7. 7. Class of compound Functional group Name of functional group RCH2OH Primary alcohol RR1CHOH Secondary alcohol Hydroxy compounds RR1R2COH Tertiary alcohol C6H5OH Phenol RCHO Aldehyde Carbonyl compounds RR1CO Ketone where R, R1, R2 represent alkyl groups, −
  8. 8. Class of compound Functional group Name of functional group Carboxylic acids RCO2H Carboxylic acid RCOCl Acyl chloride Carboxylic acid RCO2R1 Ester derivatives RCONH2 Amide RNH2 Primary amine Amines RR1NH Secondary amine RR1R2N Tertiary amine Nitriles RC N Nitrile9 where R, R1, R2 represent alkyl groups, −
  9. 9.  The structural formulae of organic compounds may be represented using ◦ displayed formulae ◦ condensed structural formulae ◦ skeletal formulae  Note that any structural formula given must give an unambiguous structure. ◦ E.g. 1-propanol CH3CH2CH2OH ✓ C3H7OH ✗
  10. 10.  Displayed Formulae ◦ Show both the relative placing of atoms and the number of bonds between them. ◦ All bonds between atoms must be shown. ◦ Exception: The following convention for representing the aromatic ring is
  11. 11.  Condensed Structural Formulae ◦ Each carbon atom is written separately. ◦ Following each of these carbon atoms,  the other atoms that are bonded to this carbon atom are written.  Substituent groups that are bonded to this carbon atom are enclosed in brackets. ◦ Similarly, the following convention for representing the aromatic ring is
  12. 12.  Skeletal Formulae ◦ Simplified representation derived from a structural formula by removing hydrogen atoms (and their associated bonds) and carbon atoms from alkyl chains, leaving just the carbon-carbon bonds in the carbon skeleton and the associated functional groups. ◦ The following convention for representing the aromatic ring is
  13. 13.  IUPAC Nomenclature is a system of naming chemical substances developed by the International Union of Pure and Applied Chemistry (IUPAC).  The IUPAC sets global standards for names, symbols, and units used in chemical
  14. 14.  The IUPAC name for an organic compound consists of the following parts: No. of carbon atoms in Nature of longest continuous carbon parent chain chain, i.e. parent chain Prefix(es) Infix Root Suffix(es) 1° suffix + 2° suffix Side chains & functional groups of lower priority Degree of saturation or unsaturation Functional group of17 highest priority molecule
  15. 15. Locants indicate positions ofsustituent groups, bonds etc. in molecule 3-ethyl-4-methylhexane locant 3-oxohexanal side-chains & infix root 1° suffix 2° suffixfunctional groups of lower (nature of (no. of carbon atoms (degree of (main functional group, priority, arranged in parent chain) in parent chain) saturation) of highest priority) alphabetical order locant 3-methylcyclopentene locant 2-hydroxypropanoic acid 2 1
  16. 16. (Source: Locants indicate positions of root 1° suffix sustituent groups, bonds etc. in (no. of carbon atoms (degree of molecule in parent chain) saturation) 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricos-6,13-diene-19-yne-3,9-dione tricos 23 carbons side-chains & 2° suffix functional groups of lower priority, (main functional group,19 arranged in alphabetical order of highest priority)
  17. 17.  Punctuation rules: ◦ Di-, tri- etc. are not taken into consideration when arranging side-chains and functional groups alphabetically. ◦ Commas are placed between numbers. ◦ Hyphens are placed between a number and a letter. ◦ All parts are connected either by merging successive names, including commas or hypens to form a one-word
  18. 18.  Functional groups control the chemistry of organic molecules.  They contain reactive sites: ◦ Electron deficient sites  E.g. a carbon atom bonded to a more electronegative bromine atom ◦ Electron rich sites  E.g. a π-system in an alkene or a benzene
  19. 19. Types of Bond Fission Heterolytic Homolytic• When a covalent bond between • When a covalent bond between two two atoms breaks, one of the atoms breaks, each atom retains one atoms retains both the bonding bonding electron. electrons to bear a negative • Free radicals are formed. charge. • A free radical is an electrically neutral• The other atom bears a positive atom or group of atoms that has one charge. unpaired electron. It is unstable and highly reactive. H−Cl→ H+ + Cl:− Cl−Cl→Cl + Cl
  20. 20. Types of Reagents Electrophile Nucleophile • A reagent that is attracted to • A reagent that is attracted to regions of negative charge regions of positive charge or or high electron density electron deficient sites • An electron pair acceptor • An electron pair donor • Good bases are generally Positive ions: NO2+ good nucleophiles δ+ end of an induced dipole: δ+Br−Brδ− δ+ end of a permanent dipole: δ+H−Clδ− Negative ions: H−O:−,N C:− Neutral molecules: H2O:,
  21. 21.  Isomers are molecules that have the same molecular formula but different arrangement of atoms in space. Isomerism Stereoisomerism Structural isomerism • Same molecular formula • Same molecular formula • Same structural formula • Different structural formulae • Different spatial arrangement of atoms in 3D space Cis-trans Chain Functional Positional isomerismisomerism isomerism isomerism25 isomerism
  22. 22.  Chain Isomerism ◦ Arises due to the different arrangement of carbon atoms in a carbon chain. ◦ E.g. isomers of hexane,
  23. 23.  Positional Isomerism ◦ Arises due to the different positions assumed by a functional group on a carbon chain or ring. OH OH OH pentan-1-ol pentan-2-ol pentan-3-ol CH3 CH3 CH3 Cl Cl 2-chloromethylbenzene 3-chloromethylbenzene Cl
  24. 24.  Cis-trans (a.k.a. geometric) isomerism [Use of E-Z nomenclature is not required by the 9647 syllabus.] ◦ Criteria:  Restricted rotation about a double bond, e.g. carbon-carbon double bond, or  Restricted rotation due to a rigid ring structure  Two different substituent groups attached to each atom in a double bond, e.g. to each carbon atom in a carbon-carbon double
  25. 25.  Optical isomerism ◦ Criteria:  A chiral carbon, i.e. an asymmetrical carbon atom with four different substituent groups  No plane of symmetry ◦ Optical isomers exist as a pair of non– superimposable mirror images of each other. ◦ Optical isomers are also known as
  26. 26.  Enantiomers ◦ Identical chemical properties and physical properties ◦ Rotate plane of polarised light in opposite
  27. 27.  Racemic mixture ◦ Equal amounts of both enantiomers. ◦ Optically inactive, as there is no net rotation of plane-polarised light. Although both enantiomers rotate plane-polarised light in opposite directions, the rotations cancel
  28. 28.  Drawing optical isomers ◦ Optical isomers must be drawn as three- dimensional structures according to the convention used in the example below:35 (Source: Higher 2 Chemistry Syllabus 2013/9647)