Chemistry 121(01) Winter 2009 Introduction to Organic Chemistry and Biochemistry Instructor Dr. Upali Siriwardane (Ph.D. Ohio State) E-mail: [email protected] Office: 311 Carson Taylor Hall ; Phone: 318-257-4941; Office Hours: MTW 9:00 am - 11:00 am; Chapter 13: Unsaturated Hydrocarbons TR 9::00 - !0:00 am & 1:00-2:00 pm. December 19, Test 1 (Chapters 12-14) Sections 4.1-4.5 January 2 Test 1 (Chapters 15-16) February 6 (Chapters 17-19) February 27, (Chapters 20-22) March 2, 2009, Make Up Exam: Bring Scantron Sheet 882-E Chemistry 121, Winter 2008, LA Tech 1-1 Chemistry 121, Winter 2008, LA Tech 1-2 Chapter 13: Unsaturated Hydrocarbons Unsaturated hydrocabons 13.2 Characteristics of Alkenes and Cycloalkenes Hydrocarbons with carbon-carbon double bonds 13.3 Names for Alkenes and Cycloalkenes 13.4 Line-Angle Formulas for Alkenes and triple bonds 13.5 Isomerism in Alkenes 13.6 Naturally Occurring Alkenes double bonds: alkenes 13.7 Physical Properties of Alkenes 13.8 Chemical Reactions of Alkenes triple bonds: alkynes 13.9 Polymerization of Alkenes: Addition Polymers 13.10 Alkynes three alternating double bond in 6 carbon ring: 13.11 Aromatic Hydrocarbons aromatics 13.12 Names for Aromatic Hydrocarbons 13.13 Aromatic Hydrocarbons: Physical Properties and Sources 13.14 Chemical Reactions of Aromatic Hydrocarbons 13.15 Fused-Ring Aromatic Compounds Chemical Connections: Ethene: A Plant Hormone and High-Volume Industrial Chemical; Cis-Trans Isomerism and Vision; Carotenoids: A Source of Color; Fused-Ring Aromatic Hydrocarbons and Cancer Chemistry 121, Winter 2008, LA Tech 1-3 Chemistry 121, Winter 2008, LA Tech 1-4 Units of Unsaturation Unsaturated Hydrocarbons: Compounds that have have fewer hydrogens than H saturated hydrocarbons (C H ). Two hydrogen are Alkenes Alkynes Arenes n 2n+2 H C H considered as unit of unstauration C C C C H C H Cycloalkane ring CnH2n (one unit of unsat.) H ethene ethyne benzene Unsaturated hydrocarbons: C H C H C H 2 4 2 2 6 6 bond C H (one unit of unsat.) C H C H C H n 2n n 2n n 2n−2 n n bond C H (two units of unsat.) unsaturated unsaturated Aromatic n 2n−2 alkene alkyne Arene Chapters 13 Chapters 13 Chapter 13 Chemistry 121, Winter 2008, LA Tech 1-5 Chemistry 121, Winter 2008, LA Tech 1-6 Structure of Alkenes Planar Structure of Alkenes • Alkene: contains a carbon-carbon double bond and has According to the orbital overlap model, a double bond consists of the general formula CnH2n 2 • The two carbon atoms of a double bond and the four • a σ bond formed by overlap of sp hybrid orbitals atoms bonded to them lie in a plane, with bond angles • a π bond formed by overlap of parallel 2p orbital of approximately 120° Rotating by 90°breaks the pi bond 121.7° 124.7° H H H CH 3 CC CC H H H H Ethylene Propene Chemistry 121, Winter 2008, LA Tech 1-7 Chemistry 121, Winter 2008, LA Tech 1-8 Structure of Alkynes The functional group of an alkyne is a carbon-carbon triple Alkenes bond A triple bond consists of Second members of the hydrocarbon family. • one σ bond formed by the overlap of sp hybrid orbitals • contain only hydrogen and carbon • two π bonds formed by the overlap of sets of parallel 2p • have single bonds and at least one C=C double orbitals bond All members have the general formula of CnH2n TwiceTwice as as many many hydrogen hydrogen asas carbon carbon Chemistry 121, Winter 2008, LA Tech 1-9 Chemistry 121, Winter 2008, LA Tech 1-10 Alkenes: Naming and Structures 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. One simple class of compound is the alkene multiple bond(s) (parent chain). If cyclic, ring is the parent. which has only C, H and single bonds. • use the infix -en- to show the presence of a carbon-carbon double bond • ethene propene 2- butene • use the infix -yn- to show the presence of a carbon-carbon • C H C H C H 2 4 3 6 4 8 triple bond • CH2CH2 CH3CH2CH2 CH3CH2CHCH3 • 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 Chemistry 121, Winter 2008, LA Tech 1-11 Chemistry 121, Winter 2008, LA Tech 1-12 IUPAC Nomenclature of Alkynes Example of IUPAC Nomenclature of Alkenes • use the infix -yn-yn to show the presence of a carbon- Cycloalkanes carbon triple bond • number the parent chain to give the 1st carbon of the 5 3 1 triple bond the lower number 6 4 2 6 4 2 4 2 5 3 1 5 3 1 • follow IUPAC rules for numbering and naming 1-Hexene 4-Methyl-1-hexene substituents 2-Ethyl-3-methyl- 1-pentene 1 1 2 34 2 4 2 5 4 1 CH 3 3 3 3 6 7 5 CH3 4 6 CH 12 5 3 3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne 3-Methylcyclo- 1,6-Dimethylcyclo- pentene hexene Chemistry 121, Winter 2008, LA Tech 1-13 Chemistry 121, Winter 2008, LA Tech 1-14 Nomenclature of Alkenes: Common Names Alkenes Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names First four members of the alkanes Name # of C Condensed formula CH 3 CH =CH CH CH= CH CH C=CH 2 2 3 2 3 2 Ethene 2 CH2=CH2 IUPAC: Ethene Propene 2-Methylpropene Co m mo n: Ethy l e ne Propylene Isobutylene Propene 3 CH3CH=CH2 2-Butene 4 CH3CH=CHCH3 Called a homologous series • “Members differ by number of CH2 groups” Chemistry 121, Winter 2008, LA Tech 1-15 Chemistry 121, Winter 2008, LA Tech 1-16 Naming alkenes and alkynes Cis and trans Geometrical isomers of alkenes Find the longest carbon chain. Use as base name with an ene or yne ending. two groups are said to be located cis to each other if they Number the chain to give lowest number for the two groups are said to be located cis to each other if they carbons of the double or triple bond. lie on the same side of a plane with respect to the double bond. Locate any branches on chain. Use base names with a yl ending. If they are on opposite sides, their relative position is described as trans. 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.. Chemistry 121, Winter 2008, LA Tech 1-17 Chemistry 121, Winter 2008, LA Tech 1-18 Geometrical Stereoisomerism Physical Properties Because of restricted rotation about a C-C double bond, Alkenes and alkynes are nonpolar compounds groups on adjacent carbons are either cis or trans to • the only attractive forces between their molecules are each other dispersion forces Their physical properties are similar to those of alkanes of similar carbon skeletons H H H CH3 • those that are liquid at room temperature are less dense C C C C than water (1.0 g/m L) H3 C CH3 H3 C H • they dissolve in each other and in nonpolar organic solvents cis-2-Butene trans-2-Butene mp -139°C, bp 4°C mp -106°C, bp 1°C • they are insoluble in water Chemistry 121, Winter 2008, LA Tech 1-19 Chemistry 121, Winter 2008, LA Tech 1-20 Cis-Trans Isomerism • trans alkenes are more stable than cis alkenes because Geometric isomers of nonbonded interaction strain between alkyl substituents of the same side of the double bond There are two possible arrangements. Example 2-butene H3C CH3 H3C H C=C C=C H H H CH3 cis trans Largest groups are Largest groups are on the same side. on opposite sides. Chemistry 121, Winter 2008, LA Tech 1-21 Chemistry 121, Winter 2008, LA Tech 1-22 Cis-Trans Isomerism in Cycloalkenes Cis-Trans Isomerism • the configuration of the double bond in cyclopropene through Dienes, trienes, and polyenes cycloheptene must be cis; these rings are not large enough to accommodate a trans double bond • for an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n cis-trans H CH3 isomers are possible • consider 2,4-heptadiene; it has four cis-trans isomers, H C H3 • cyclooctene is the smallest cycloalkene that can accommodate a two of which are drawn here trans double bond Double bond 2 2 C2 -C3 C4 -C5 4 4 trans trans trans ci s cis trans trans,trans-2,4- trans,cis-2,4- cis cis heptadiene heptadiene trans-Cy clo octene cis-Cyclooctene Chemistry 121, Winter 2008, LA Tech 1-23 Chemistry 121, Winter 2008, LA Tech 1-24 Naturally Occurring Naturally Occurring Alkenes: The Terpenes AlkenesCis-Trans Isomerism Terpene: a compound whose carbon skeleton can be • vitamin A has five double bonds divided into two or more units identical with the carbon skeleton of isoprene • four of the five can show cis-trans isomerism • vitamin A is the all-trans isomerenzyme- catalyzed oxidation OH Vitamin A (retinol) O head2 4 tail H 1 3 2-Methyl-1,3-butadiene Vitamin A aldehyde (retinal) (Isoprene) Chemistry 121, Winter 2008, LA Tech 1-25 Chemistry 121, Winter 2008, LA Tech 1-26 Terpenes: Polymers of Isoprene Terpenes Vitamin A (retinol) • myrcene, C10H16, a • the four isoprene units in vitamin A are shown in red component of bayberry • they are linked head to tail, and cross linked at one wax and oils of bay and verbena point (the blue bond) to give the six-membered ring • menthol, from peppermint OH OH Chemistry 121, Winter 2008, LA Tech 1-27 Chemistry 121, Winter 2008, LA Tech 1-28 Reactions of alkenes Addition Reactions The exposed electrons of double bonds make Combustion alkenes more reactive than alkanes and show addition reactions.