1	Chemistry 121(01) Winter 2009
2	Chapter 13: Unsaturated Hydrocarbons Sections 4.1-4.5
3	Chapter 13: Unsaturated Hydrocarbons
4	Unsaturated hydrocabons Hydrocarbons with carbon-carbon double bonds and triple bonds double bonds: alkenes triple bonds: alkynes three alternating double bond in 6 carbon ring: aromatics
5	Unsaturated Hydrocarbons:
6	Units of Unsaturation
7	Structure of Alkenes
8	Planar Structure of Alkenes According to the orbital overlap model, a double bond consists of a s bond formed by overlap of sp² hybrid orbitals a p bond formed by overlap of parallel 2p orbital Rotating by 90°breaks the pi bond
9	Structure of Alkynes The functional group of an alkyne is a carbon-carbon triple bond A triple bond consists of one s bond formed by the overlap of sp hybrid orbitals two p bonds formed by the overlap of sets of parallel 2p orbitals
10	Alkenes Second members of the hydrocarbon family. contain only hydrogen and carbon have single bonds and at least one C=C double bond All members have the general formula of C_nH_2n
11	Alkenes: Naming and Structures One simple class of compound is the alkene which has only C, H and single bonds. ethene propene 2- butene C₂H₄C₃H₆C₄H₈ CH₂CH₂CH₃CH₂CH₂CH₃CH₂CHCH₃
12	IUPAC Nomenclature of Alkenes and Alkynes name the longest continuous carbon chain containing the multiple bond(s) (parent chain). If cyclic, ring is the parent. use the infix -en- to show the presence of a carbon-carbon double bond use the infix -yn- to show the presence of a carbon-carbon triple bond number the parent chain to give the 1st carbon of the double/triple bond the lower number If both double and triple are present and cannot have the same #, then double bonds take priority. follow IUPAC general rules for numbering and naming substituents for a cycloalkene, the double bond must be numbered 1,2
13	IUPAC Nomenclature of Alkynes use the infix -yn- to show the presence of a carbon-carbon triple bond number the parent chain to give the 1st carbon of the triple bond the lower number follow IUPAC rules for numbering and naming substituents
14	Example of IUPAC Nomenclature of Alkenes Cycloalkanes
15	Nomenclature of Alkenes: Common Names Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names
16	Alkenes First four members of the alkanes Name # of C Condensed formula Ethene 2 CH₂=CH₂ Propene 3 CH₃CH=CH₂ 2-Butene 4 CH₃CH=CHCH₃ Called a homologous series “Members differ by number of CH₂ groups”
17	Naming alkenes and alkynes Find the longest carbon chain. Use as base name with an ene or yne ending. Number the chain to give lowest number for the carbons of the double or triple bond. Locate any branches on chain. Use base names with a yl ending. For multiple branch of the same type, modify name with di, tri, ... Show the location of each branch with numbers. List multiple branches alphabetically - the di, tri, ... don’t count..
18	Cis and trans Geometrical isomers of alkenes two groups are said to be located cis to each other if they lie on the same side of a plane with respect to the double bond. If they are on opposite sides, their relative position is described as trans.
19	Geometrical Stereoisomerism Because of restricted rotation about a C-C double bond, groups on adjacent carbons are either cis or trans to each other
20	Physical Properties Alkenes and alkynes are nonpolar compounds the only attractive forces between their molecules are dispersion forces Their physical properties are similar to those of alkanes of similar carbon skeletons those that are liquid at room temperature are less dense than water (1.0 g/m L) they dissolve in each other and in nonpolar organic solvents they are insoluble in water
21	Cis-Trans Isomerism trans alkenes are more stable than cis alkenes because of nonbonded interaction strain between alkyl substituents of the same side of the double bond
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23	Cis-Trans Isomerism in Cycloalkenes the configuration of the double bond in cyclopropene through cycloheptene must be cis; these rings are not large enough to accommodate a trans double bond cyclooctene is the smallest cycloalkene that can accommodate a trans double bond
24	Cis-Trans Isomerism Dienes, trienes, and polyenes for an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2ⁿ cis-trans isomers are possible consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here
25	Naturally Occurring AlkenesCis-Trans Isomerism vitamin A has five double bonds four of the five can show cis-trans isomerism vitamin A is the all-trans isomer
26	Naturally Occurring Alkenes: The Terpenes Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene
27	Terpenes: Polymers of Isoprene myrcene, C₁₀H₁₆, a component of bayberry wax and oils of bay and verbena menthol, from peppermint
28	Terpenes Vitamin A (retinol) the four isoprene units in vitamin A are shown in red they are linked head to tail, and cross linked at one point (the blue bond) to give the six-membered ring
29	Reactions of alkenes Combustion C₂H₄ + 4 O₂ 2 CO₂ + 2 H₂O + heat Alkynes also under go combustion reactions similarly
30	Addition Reactions The exposed electrons of double bonds make alkenes more reactive than alkanes and show addition reactions.
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32	Halogenation Halogenation - Addition of halogen to the double bond. Textbook page xx.
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34	Hydrogenation
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36	Hydration Addition of water to the double bond. Textbook page86.
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38	Markovnikov Rule Non symmetric alkene In hydrohalogenation and hydration reations hydrogen adds to the double-bonded carbon with the most hydrogens
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42	Reactions of alkynes Alkynes undergo hydration, halogenation, and hydrohalogenation just like alkenes. A special application is the carbide lamp (oxidation of alkyne). 2 C (coke) + CaO (lime) + heat ---> CaC₂ (calcium carbide) + CO CaC₂ + H₂O ---> H-CºC-H (acetylene) + Ca(OH)² Acetylene serves as combustion fuel for the carbide lamp.
43	Aromatic hydrocarbons Aromatic hydrocarbons - organic compounds that had aromas and had different chemical properties from alkane Benzene is the parent compound for the aromatic hydrocarbons. Textbook, page90. Consider benzene. C₆H₆
44	Resonance Structures of Benzene Resonance structures or contributing structures = when two or more structure can be drawn for a compound. In thiscase, the real structure is something between the proposed structures. Textbook, page 90-91.
45	Naming Aromatic Hydroarbons. Monosubstituted benzenes: Ar-CH₂CH₃ ethylbenzene Ar-CH₂-CH₂-CH₂-CH₃ butylbenzene Ar-CH₃ (methylbenzene) toluene Ar-X (halobenzene) bromobenzene, Ar-NO₂ nitrobenzene Ar-SO₃H benzenesulfonic acid Ar-NH₂ a nitrile substituent
46	Nomenclature Disubstituted benzenes locate substituents by numbering or use the locators ortho (1,2-), meta (1,3-), and para (1,4-) Where one group imparts a special name, name the compound as a derivative of that molecule
47	Nomenclature Polysubstituted benzenes with three or more substituents, number the atoms of the ring if one group imparts a special name, it becomes the parent name if no group imparts a special name, number to give the smallest set of numbers, and then list alphabetically
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49	Disubstituted benzenes: Textbook, page 352. 2,6-dibromotoluene p-diethylbenzene 3,5-dinitrotoluene p-cholonitrobenzene o-nitrobenzenesulfonic acid 4-benzyl-1-octene m-cyanotoluene
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53	Benzylic Oxidation Benzene is unaffected by strong oxidizing agents such as H₂CrO₄ and KMnO₄ halogen and nitro substituents are unaffected by these reagents an alkyl group with at least one hydrogen on the benzylic carbon is oxidized to a carboxyl group
54	Benzylic Oxidation if there is more than one alkyl group, each is oxidized to a -COOH group terephthalic acid is one of the two monomers required for the synthesis of poly(ethylene terephthalate), a polymer that can be fabricated into Dacron polyester fibers and into Mylar films
55	Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon
56	Reactions of Benzene
57	Nitration The electrophile is NO₂⁺, generated in this way
58	Friedel-Crafts Alkylation Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group
59	Friedel-Crafts Acylations Treating an aromatic ring with an acid chloride in the presence of AlCl₃ acid (acyl) chloride: a derivative of a carboxylic acid in which the -OH is replaced by a chlorine