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Chapter 9 Organic Chemistry II Alkanes Formulas CH4 C2H6 C3H8 C4H10 C5H12 Name methane ethane propane butane pentane Formulas C6H14 C7H16 C8H18 C9H20 C10H22 Name hexane heptane octane nonane decane 1. The parent name is that of the longest continuous chain of C atoms. 2. An alkane minus one H atom is named as an alkyl group. 3. Indicate the locations where replacements are made. Number in the direction that gives the smaller numbers for the locations of the branches. 4. Use prefixes when there is more than one alkyl branches of the same kind & for other types of substituents. 5. Cyclic Alkanes — indicates by cyclo before the hydrocarbon name. Alkenes • Alkenes, containing at least one carbon-carbon double bond, are unsaturated hydrocarbons (olefin) and have the general formula CnH2n where n = 1, 2, 3,… • Cis- and Trans- Isomerism: because of the -bonding, there is not free rotation about the double bond. Therefore, the following isomers are possible: two Cl 's are on the same side of the double bond Cl Cl C H Cl C H C H cis-dichloroethylene two Cl 's are on opposite side of the double bond C H Cl trans-dichloroethylene • Nomenclature: 1. Drop ane and add ene CH2=CH2 ethene (ethylene) CH2=CHCH2CH3 1-butene CH3CH=CH2 propene CH3CH=CHCH3 2-butene Alkenes 2. In naming branched chain alkenes, 1) the longest continuous chain must contain the double bond; 2) the double bond is given the lowest number. CH3 5 CH3 1 CH 2 CH2 3 CH 4 CH2 CH3 CH3 4,5-dimethyl-2-hexene CH2 CH CH 1 2 3 CH2 C CH 2 3 CH2 4 1,3-butadiene CH3 1,2-butadiene 6 1 4 3. Polyenes (have several double bonds): use prefixes to denote the number of double bonds and number the longest continuous chain to locate them. 4. Cyclic Olefins: use prefix cyclo and number to locate double bonds or radicals on the ring. cyclohexene 1,3-cyclohexadiene 3-methylcyclohexene 3,6-dimethyl-1,4cyclohexadiene Alkynes • Alkynes, containing at least one carbon-carbon triple bond, have the general formula CnH2n-2 where n = 1, 2, 3,… • Nomenclature: alkynes end in yne. Rules of nomenclature are the same as for alkenes. (Note: alkyl means any of a series of univalent groups of the general formula CnH2n+1 derived from aliphatic hydrocarbons) CH≡CH ethyne (acetylene) CH≡CCH2CH3 1-butyne CH3C≡CH2 propyne CH3C≡CCH3 2-butyne • Cyclic Alkynes and Polyalkynes: 1. The linear nature of the -C≡C- group small ring alkynes are not stable. 2. Since the −C ≡ C− group is very reactive, poly-ynes are not common. CH3 CH3 1 C C 2 3 CH2 4 C CH3 6 5 CH3 5,5-dimethyl-2-hexyne CH3 1 C C C C 2 3 4 5 CH2 6 2,4-heptadiyne CH3 7 Aromatic Hydrocarbons • An aromatic hydrocarbon is a hydrocarbon of which the molecular structure incorporates one or more planar sets of six carbon atoms that are connected by delocalized electrons. • The term aromatic was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. CH2CH3 ethylbenzene Cl NH2 chlorobenzene aminobenzene 6 2 5 3 4 benzene nitrobenzene toluene furan Br Br 1 NO2 Br 1,2-dibromobenzene Br 1,3-dibromobenzene pyridine imidazole Aryl Groups phenyl (C6H5-) benzyl (C6H5CH2-) R-pyridium Polycyclic Aromatic Hydrocarbons 1-R1-3-R2-imidazolium Reactions of Hydrocarbons • Alkanes 1. Quite unreactive; used as nonpolar solvents. 2. Reactions involve the substitution of some other element for H. 3. Burned as fuel. light Substitution CH4(g) + Cl2(g) CH3Cl(g) + HCl (g) Combustion CH4(g) + 2O2(g) CO2(g) + 2H2O(l) DH = -890.4 kJ • Alkenes and Alkynes 1. Addition reactions: add small molecules across the multiple bonds. The bond breaks and two s bonds are formed. 2. If small molecules of HX are added to an unsymmetric alkene or alkyne, the addition is such that the H goes to the C having the greater # of H's. Addition CH CH(g) + HBr(g) Addition CH2 CHBr(g) + HBr(g) CH2 CHBr(g) CH3CHBr2(g) 3. Cracking Pt C2H6(g) catalyst CH2 CH2(g) + H2(g) 4. Hydrogenation CH CH(g) + H2(g) CH2 CH2(g) • Aromatic compounds 1. Aromatic compounds undergo substitution rather than addition reactions. 2. Benzene and derivatives convert to cyclohexane and derivatives when treated with H2 at 450 K and 10 atm with a finely divided nickel catalyst. 3. Bz is an excellent ligand in organometallic chemistry of low-valent metals, e.g. the sandwich Cr(C6H6)2 and half-sandwich [RuCl2(C6H6)]2 complexes H CH2CH3 H H + CH3CH2Cl H H H H H AlCl3 catalyst + HCl H H H Functional Groups • Functional group compounds: Replace a H on a hydrocarbon with a group of atoms other than C and H. Such groups are called functional groups. They impart the specific chemical reactivity to the compound. Organic Halides • Organic Halide: halogen replaces a hydrogen on an alkane. • Name halogen as a radical: a) Drop the elemental ending on the halogen and add o, i.e. -F = fluoro, -Cl = chloro, -Br = bromo, -I = iodo; b) halogen is given the lowest possible number. CH3Cl chloromethane (methyl chloride) CH2Cl2 dichloromethane (methylene chloride) CHCl3 trichloromethane (chloroform) CCl4 tetrachloromethane (carbon tetrachloride) CH3 1 I CH3 CH CH 2 3 CH3 4 2-iodo-3-methylbutane Br 1,2dibromobenzene Br Organic Halides • Use: 1. Starting materials for other organic compounds because the halogen group is fairly easy to remove. 2. Solvents. however the use of halogenated solvents is being phased out because of environmental concerns. Chloroform is a common solvent in the laboratory because it is relatively unreactive, miscible with most organic liquids, and conveniently volatile. It is used as a solvent in the pharmaceutical industry and for producing dyes and pesticides. 3. Coolants (CCl2F2 = Freon). The widespread use of chlorofluorocarbons is now thought to be one of the major causes for decrease in the ozone layer. Carbon tetrachloride is a reagent in synthetic chemistry and was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent. It is a colourless liquid with a "sweet" smell that can be detected at low levels. Alcohols • Alcohol is formed by replacing a H on an alkane by an OH group. General formula is R-OH where R = hydrocarbon fragment. 1,2-ethanediol 1, 2, 3 - propanetriol glycerol or glycerine • Nomenclature: a) the parent name is taken from the l.c.c. having the OH; b) drop the e on the alkane name and add ol; c) When necessary, number the l.c.c. to locate the OH. OH 4-methyl-2-pentanol CH3 CH CH3 CH2 CH CH3 Classification of Alcohols • Primary Alcohols: OH is on a C that is bonded to at least two H's. That is, the OH is on an end carbon. Examples: CH3OH, CH3CH2OH • Secondary Alcohols: OH is on a C that is bonded to one H. That is, the C is bonded to two other carbons. Examples: CH3CH(OH)CH3 isopropanol • Tertiary Alcohols: OH is on a C that is not bonded to a H. That is, the C is bonded to three other carbons.. Examples: CH3C(CH3)(OH)CH3 2-methyl-2-propanol (t-butyl alcohol) Production of Ethanol Biological C6H12O6(aq) enzyme 2CH3CH2OH(aq) + 2CO2(g) Commercial CH2=CH2(g) + H2O(g) H2SO4 CH3CH2OH(g) Metabolic Oxidation of Ethanol CH3CH2OH alcohol dehydrogenase CH3CHO + H2 Ethers • An Ether is an organic compound that contains an ether group – an O- atom connected to two alkyl or aryl groups – of general formula R–O–R’. H2SO4 CH3OH + HOCH3 catalyst CH3OCH3 + H2O • Nomenclature: the name for simple ethers with no or few other functional groups are a composite of the two substituents followed by ‘ether’. For example, CH3OC2H5 methyl ethyl ether, C6H5OC6H5 diphenylether. • CH3O- = methoxide ion; CH3O- = methoxyl group • Used as solvents and anaesthetics • Highly flammable and toxic • Peroxide formation: ethers with a CH group next to the ether O form peroxides. The reaction requires oxygen (or air) and is accelerated by light, metal catalysts and aldehydes. The resulting peroxides can be explosive. Important Ethers Ethelene oxide The smallest cyclic ether Dimethyl ether An aerosol spay propellant Diethyl ether A common low boiling solvent (34.6oC), and an early anaesthetic Dimethoxyethane A high boiling solvent (85oC) Dioxane A cyclic ether and high boiling solvent (101oC) Tetrahydrofuran A cyclic ether, one of the most polar simple ethers that is used as a solvent Anisole An aryl ether and a major constituent of (methoxybenzene) the essential oil of anise seed Crown ethers Cyclic polyether that used as phase transfer catalysts. Polyethylene glycol A linear polyether, e.g. used in cosmetics and pharmaceuticals