Chapter 4: Alkenes and Alkynes

1. Chapter 4: Alkenes and Alkynes

2. Unsaturated Hydrocarbons • Unsaturated hydrocarbon:contains one or more carbon-carbon double or triple bonds • alkene:contains a carbon-carbon double bond and has the general formula CnH2n • alkyne:contains a carbon-carbon triple bond and has the general formula CnH2n-2

3. Unsaturated Hydrocarbons • Arenes:benzene and its derivatives (Ch 9)

4. Benzene and Phenyl Groups • We do not study benzene and its derivatives until Chapter 9 • but we show structural formulas of compounds containing the phenyl group before that time • the phenyl group is not reactive under any of the conditions we describe in Ch 3-8

5. Structure of Alkenes • The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°

6. 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

7. Cis-Trans Isomerism • Because of restricted rotation about a C-C double bond, groups on adjacent carbons are either cis or trans to each other • Remember same order of attachment, different arrangement in space

8. Cis-Trans Isomerism • trans alkenes are more stable thancis alkenes because of nonbonded interaction strain between alkyl substituents of the same side of the double 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. Nomenclature of Alkenes • IUPAC Nomenclature • use the -en- at the end to show the presence of a carbon-carbon double bond • number the parent chain to give the 1st carbon of the double bond the lower number • follow IUPAC rules for numbering and naming substituents • for a cycloalkene, the double bond must be numbered 1,2

11. Nomenclature of Alkenes • IUPAC nomenclature

12. Numbering System (cont) e.g.

13. IUPAC nomenclature

14. Nomenclature of Alkenes • Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names • CH2= methylene • CH2=CH─ vinyl • CH2=CHCH2 ─ allyl

15. Question Name this compound:

16. Answer 2-methyl-2-butene

17. Nomenclature of Alkynes • IUPAC nomenclature • use the -yn- at the end 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

18. IUPAC nomenclature

19. Numbering System (cont) e.g. 3-Methyl-1-butyne

20. Question Name this compound: http://www.chemsynthesis.com/molimg/1/big/16/16207.gif

21. Answer 2,2-dimethyl-3-heptyne

22. Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond are alkadienes, alkatrienes, and so on. e.g. 2-methyl-1,4-pentadiene

23. Polyenes

24. Alkylidene GroupsDouble Bonds Fused to Rings

25. Name These

27. Configuration: Cis-Trans • The cis,trans system:configuration is determined by the orientation of atoms of the main chain • Draw cis-2- butene and trans-2-butene • Cis and trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. • Neither 1-butene and 2-methylpropene have no stereoisomers.

28. Stability of Alkenes Stability of cis- and trans isomers • Interconversion does not occur spontaneously • Interconversion can be brought about by treating the alkene with a strong acid • Cis isomers are less stable than trans isomers because of the steric strain between the two larger substituents on the same side of the double-bond

29. Relative Stabilities of Alkenes

30. Sequence Rules: The E,Z Designation • E,Z system • Sequence rules used to assign priorities to the substituent groups on the double-bond carbons (alkenes) • Z – for German zusammen “together” • High priority groups on each carbon are on same side of the double-bond • E – for German entgegen “opposite” • High priority groups on each carbon are on opposite sides of the double-bond

31. E/Z System

32. Configuration: E,Z • Priority rules 1. Priority is based on atomic number; the higher the atomic number, the higher the priority 2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference

33. Configuration - E,Z 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

34. “E” is More Stable than “Z”

35. Physical Properties • Alkenes and alkynes are nonpolar compounds • 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

36. Terpenes • Terpene:a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

37. 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