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CHAPTER 20 Unsaturated Hydrocarbons 20.1 BONDING IN UNSATURATED HYDROCARBONS Alkene – general formula {CnH2n} Unsaturated hydrocarbon; suffix is -ene (C=C) must contain at least one carbon=carbon (double bond) Diene: a compound that contains 2 C=C bonds. Useful as starting components for polymers 20.2 NOMENCLATURE OF ALKENES 20.3 GEOMETRIC ISOMERISM IN ALKENES 20.4 CYCLOALKENES Number the carbon atoms of the double bond as “1” and “2” in the direction that gives the substituent encountered first the smaller number. Continue numbering around the ring. Identify, name and number any substituents as is done for (cyclo)alkanes. 20.5 PREPARATION AND PHYSICAL PROPERTIES OF ALKENES slightly lower boiling/melting pt than alkanes due to lower molar mass insoluble in water (nonpolar) zero-4 carbons a gas at room temp, 5-17 C liquid, above 17 solid Produced through elimination reactions such as cracking of petroleum & dehydration of alcohols. Cracking or pyrolysis is the process of heating saturated hydrocarbons to very high temperatures in the presence of a catalyst. Dehydration results in the elimination of water from a reactant molecule. 20.6 CHEMICAL PROPERTIES OF ALKENES Addition at the C=C double bond is the most common reaction Another common reaction of alkenes is oxidation at the double bond Ethene is called ethylene gas (the gas that is emitted from ripening fruit and is a plant growth hormone) 20.7 ALKYNES: NOMENCLATURE AND PREPARATION Alkyne – general formula {CnH2n-2} Unsaturated hydrocarbon suffix is -yne (C≡C) 20.8 PHYSICAL AND CHEMICAL PROPERTIES OF ALKYNES must contain at least one carbon≡carbon (triple bond) gas at room temperature slightly soluble in water but still considered nonpolar Ethyne is commonly called acetylene; used in oxyacetylene cutting/welding torches and as an intermediate in the manufacture of many other substances. HC≡CH very reactive due to triple bond; undergoes addition reactions very readily. Produce alkenes or alkanes upon addition reactions. Some of which are the starting points for further reactions – such as polyvinyl chloride (PVC). Acetylene (ethyne) is prepared inexpensively from calcium carbide and water (the old minor’s lamp reaction) or through cracking methane 20.9 AROMATIC HYDROCARBONS: STRUCTURE general formula {C6H6} Unsaturated hydrocarbon; ending may be “-benzene” benzene ring; single/double bond – 6 C liquids or solids at room temperature nonpolar; therefore, insoluble in water do not tend to undergo addition or elimination reactions, but do undergo substitution reactions – where one or more of the H atoms is substituted. many are carcinogenic (cancer producing) found in coal tar & tar from cigarette smoke Some of these monosubstituted benzenes have special names that become parent names. 20.10 NAMING AROMATIC COMPOUNDS The C6H5 group is known as the phenyl group and the name phenyl is sometimes used to name compounds. What is the “phenyl” part? Polysubstituted benzenes – more than one substituent on a benzene ring. Naming: Carbon atoms are numbered starting at one of the substituted groups. Numbering may be either clockwise or counterclockwise but must be done in the direction that gives the lowest possible numbers to the substituent groups. When the compound is named as a derivative of one of the special parent compounds (found on previous page), the substituent of the parent compound is considered to be on carbon 1 of the ring. Draw the following: 2, 6-diiodophenol and toluene Name: 20.11 POLYCYCLIC AROMATIC COMPOUNDS Polycyclic aromatic compounds – other aromatic ring systems besides benzene Structures consist of 2 or more rings in which 2 carbon atoms are common to 2 rings. 20.12 SOURCES AND PHYSICAL PROPERTIES OF AROMATIC HYDROCARBONS Halogenation – when benzene reacts with chlorine or bromine in the presence of a catalyst. Replaces H. Nitration – when benzene reacts with a mixture of concentrated nitric acid and conc. sulfuric acid at about 50°C, nitrobenzene is produced. Nitro (-NO2) is substituted for H. Draw the structure of 1,3,5-trinitrobenzene and the explosive TNT, 2,4,6-trinitrotoluene. Alkylation – an alkyl halide is reacted with benzene in the presence of AlCl3 catalyst and the alkyl group substitutes for an H atom. REVIEW FUNCTIONAL GROUPS HOMEWORK: Required: 2-56 (even)