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Organic Chemistry: Introduction IB Topic 10 Fundamentals of Organic Chemistry Section 10.1 What is organic chemistry? Organic Chemistry • The study of carbon, the compounds it makes and the reactions it undergoes. • Over 16 million carbon-containing compounds are known. Carbon • Carbon can form multiple bonds with other carbon and with atoms of other elements. • Carbon can make four bonds since it has 4 valence electrons and most often bonds to H, O, N and S. • Because the C-C single bond and the C-H bond are strong, carbon compounds are stable. • Carbon can form chains and rings. Hydrocarbons • Hydrocarbons are organic compounds that only contain carbon and hydrogen • Some types of hydrocarbons include Alkanes CnH2n+2 Alkenes CnH2n Alkynes CnH2n-2 Hydrocarbons • Classified as: • Saturated - all C-C single bonds • Unsaturated - contain double or triple C-C bonds • Aliphatic – straight chains with single C-C bonds • Cyclic – ring structures with single C-C bonds • Aromatic – ring structures of alternating single and double C-C bonds (arenes) Homologous Series • A homologous series is a series of related compounds that have the same functional group. Homologous compounds… • For example, differ from each other by a – CH2 – unit (methylene group) • Can all be represented by a general formula (alkanes – CnH2n+2) • Have similar chemical properties • Have physical properties that vary in a regular manner as the number of carbon atoms present increases Homologous series – Alkanes #C Prefix Alkane (ane) CnH2n+2 1 meth CH4 methane 2 eth C2H6 ethane 3 prop 4 but 5 pent 6 hex Trends in Boiling Points What is the trend? Alkane Formula Boiling Pt./oC methane CH4 -162.0 ethane C2H6 -88.6 propane C3H8 -42.2 butane C4H10 -0.5 Trends in Boiling Points • Intermolecular forces present: • Simple alkanes, alkenes, alkynes → van der Waals’ forces (nonpolar) → lower b.p. • Aldehydes, ketones, esters & presence of halogens (polar) → dipole: dipole forces → slightly higher b.p. • Alcohol, carboxylic acid & amine → hydrogen bonding (w/ O, N, F) → even higher b.p. Formulas Empirical Formula: Smallest whole number ratio of atoms in a formula Molecular Formula: Formula showing the actual numbers of atoms Molecular Formula Empirical Formula CH4 CH4 C2H6 CH3 C6H12O6 C4H8 C8H16 Formulas Structural Formula • Bond angles are drawn as though 90o. The true shape around C with 4 single bonds is tetrahedral and the angle is 109.5o. • Show every atom and every bond. Can use condensed structural formulas. • Hexane: CH3CH2CH2CH2CH2CH3 (condensed s.f.) M.F. = C6H14 E.F. = C3H7 Writing structural formulae • Write the condensed structural formula for the following two compounds. • For branches, write them in parantheses with the C they branch from. Isomers • Isomers: different compounds that have the same molecular formula • Structural isomers: an isomer in which the atoms are joined in a different order so that they have different structural formulae • Three kinds: chain isomers, positional isomers, functional group isomers • Warning! Some molecules may seem like isomers, but there is free movement around C-C single bonds; they can rotate. Isomers • Chain isomers: branches are placed in different spots on the C-C backbone Isomers • Positional isomers: important functional groups are moved around on the C-C backbone, but the backbone does not change Isomers • Functional group isomers: certain atoms are rearranged on the C-C backbone to form different functional groups Drawing structural formulae • Draw out the structural formulas and write the condensed formula for all isomers that can be formed by: • CH4 • C2H6 • C3H8 • C4H10 Naming compounds 1. Determine the longest carbon chain 2. Use the prefix to denote the number carbons in the chain 3. Use the suffix “-ane” to indicate that the substance is an alkane 4. If the chain is branched, the name of the side chain will be written before the main chain and will end with “–yl” Naming compounds Methylpropane Methylbutane Dimethylbutane Naming compounds 1 Meth- 6 Hex- 2 Eth- 7 Hept- 3 Prop- 8 Oct- 4 But- 9 Non- 5 Pent- 10 Dec- Naming compounds For chains longer than 4 carbons with side chains: 5. Number the carbons in the chain consecutively, starting at the end nearest side chains. 6. Designate the location of each substituent group by an appropriate number and name. And with 2 or more side chains: 5. Use prefixes di-, tri-, tetra-, to indicate when there are multiple side chains of the same type. 6. Use commas to separate numbers and hyphens to separate numbers or letters. 7. Name the side chains in alphabetical order. Naming compounds • • • • Alkenes have one (or more) carbon to carbon double bonds Suffix changes to “-ene” When there are 4 or more carbon atoms in a chain, the location of the double bond is indicated by a number. Begin counting the carbons closest to the end with the C=C bond Numbering the location of the double bond(s) takes precedence over the location of side chains 1-butene 2-butene 24 Functional Groups (p. 243) Naming Carbon Rings • To name a molecule with a ring, follow the steps as previously described for branches and functional groups. • The numbering of the base chain/ring begins at the functional group. Double/triple bonds start the ring, if present. • For the base chain/ring, insert a “cyclo-” prefix before the Latin number prefix. • Example: Classifying molecules With reference to the carbon that is directly bonded to an alcohol or amine group or a halogen: • Primary = carbon atom is only bonded to one other carbon • Secondary = carbon atom is bonded to two other carbons • Tertiary = carbon atom is bonded to three other carbons Aromatic Hydrocarbons • Presence of a benzene ring. • August Kekule proposed a structure with a ring of C with alternating single and double bonds. • Unsymmetrical with different lengths of bonds. • Experimentally, it was discovered that the bonds are, in fact, the same length (140 pm) • Single bond – 154 pm; double bond – 134 pm • Bond order of 1.5 • Actually symmetrical! Aromatic Hydrocarbons Delocalized electrons from the ½ bond – resonance! Aromatic Hydrocarbons Benzene is uncharacteristically stable. When adding H to this molecule, you would expect the enthalpy to change 3 times that of adding H to cyclohexene, but it isn’t – it’s much less. This difference in energy (expected vs. actual) is known as resonance energy or delocalization energy. Functional Group Chemistry Section 10.2 Alkanes • Simplest hydrocarbons • Low bond polarity • Strong covalent bonds • C-C (346 kJ mol-1) • C-H (414 kJ mol-1) • Relatively inert • Important reactions: • Combustion • Halogenation Combustion of Alkanes • Used as fuels (propane, butane, octane, etc) for the large energy released • Volatility decreases as the length of C-chain increases – short chains used as fuel • Undergoes complete combustion in presence of excess O2 to produce CO2 and H2O. Combustion of Alkanes • Undergoes incomplete combustion in presence of limiting O2 to produce CO and H2O. • CO irreversibly binds hemoglobin the blood thus reducing its oxygen-carrying capacity. • Suffocation results Quick Question Deduce the balanced equations for the complete combustion of: 1. Propane 2. Pentane 3. Hexane Types of Reactions • Substitution: replacement of individual atoms with other single atoms or with a small group of atoms. • Addition: two molecules are added together to produce a single molecule. • Elimination: the removal of two substituents from the molecule. Halogenation of Alkanes • Halogenating alkanes increases reactivity. • Free-radical substitution and elimination • Free-radical refers to a species that is formed when a molecule undergoes homolytic fission: two electrons of a covalent bond are split evenly between two atoms resulting in two atoms with a single electron. • Heterolytic fission: both electrons in the bond are transferred to one atom resulting in cation and anion Halogenation of Alkanes Example: methane reacts with chlorine in the presence of UV light: Halogenation of Alkanes 3 stages to free-radical substitution: 1. Initiation 2. Propagation 3. Termination Halogenation of Alkanes Initiation: homolytic fission of the chlorine molecule in presence of UV light produces 2 freeradicals Halogenation of Alkanes Propagation: first stage is reaction of methane and chlorine free-radical to produce methyl radical. Halogenation of Alkanes Propagation: second stage is the reaction of methyl radical with chlorine to produce chloromethane and chlorine radical Halogenation of Alkanes Termination: reduces the concentration of radicals. Radicals begin to react with other radicals. Alkenes • Unsaturated hydrocarbons – contain at least one C-C double bond. • Double bond makes them more reactive than corresponding alkane • Undergoes addition reactions. • Test for unsaturation: • bromine water • Addition of alkene to bromine water adds Br to molecule, thus rendering it colorless. Hydrogenation • Addition of hydrogen • Important in food industry – removing the doublebond increases melting point thus making a substance that is solid rather liquid at room temperature • Partial hydrogenation of fats and oils can be harmful to health – saturated vs unsaturated fats. Halogenation of Alkenes • Electrophilic halogenation of symmetrical alkenes involves addition of elemental halogens resulting in dihalogenated alkane: Halogenation of Alkenes • Example: but-2-ene and bromine Halogenation of Alkenes Addition of hydrogen halide, HX, to a symmetrical alkane results in mono-halogenated alkane: Polymerization of Alkenes • Plastics industry utilizes addition polymerization • Reaction of many smaller monomers with a C=C linking together to form a polymer. • Monomer ethane supplied by petrochemical industry undergoes addition polymerization to form polyethene. • Any monomer with a C-C double bond can undergo polymerization wherever the double bond is located. Polymerization of Alkenes Alcohols Can undergo complete combustion reactions to form CO2 and H2O. Oxidation of Alcohols Oxidation of Alcohols • Oxidation of primary alcohols occurs in two steps. First step produces an aldehyde. Second step produces a carboxylic acid. Oxidation of Alcohols • Aldehydes can be recovered using distillation. • Distillation involves the gentle evaporation of liquids utilizing the difference in boiling points. The gas is collected and cooled into a pure distillate. Oxidation of Alcohols • Oxidation of secondary alcohols produces a ketone. Oxidation of Alcohols • To get a carboxylic acid, the aldehyde has to remain in the solution with the oxidizing agent for a longer amount of time. Instead of distillation, a reflux column is used. • Refluxing is a technique that involves the cyclic evaporation and condensation of a volatile reaction mixture, preserving the solvent as it is does not evaporate. Condensation reaction of alcohol and carboxylic acid • Esters are derived from carboxylic acids • Applications include flavoring agents, medications, solvents, and explosives. • Esterification – a reversible reaction that occurs when a carboxylic acid and an alcohol are heated in the presence of a catalyst (e.g. H2SO4) Nucleophilic substitution • A halogenoalkane can undergo other reactions. • The polar carbon-halogen bond, C-X, creates an electron deficient carbon making it open to ‘attack’ by electron-rich species known as nucleophiles – species that contain a lone pair of electrons and sometimes a full negative charge. Electrophilic substitution • Benzene does not readily undergo addition reactions but will undergo electrophilic substitution reactions. • Electrophiles – electron-poor substance capable of accepting an electron pair. • The double-bond attracts the electrophile but the stability of benzene leads to substitution NOT addition (like with alkenes). Nucleophilic substitution Electrophilic substitution