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Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions

Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions. 5.1 Alkene Nomenclature. Alkenes. Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula C n H 2n said to be "unsaturated". H 2 C. CH 2. H 2 C.

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Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions

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  1. Chapter 5Structure and Preparation of Alkenes:Elimination Reactions

  2. 5.1Alkene Nomenclature

  3. Alkenes • Alkenes are hydrocarbons that contain a carbon-carbon double bond • also called "olefins" • characterized by molecular formula CnH2n • said to be "unsaturated"

  4. H2C CH2 H2C CHCH3 Alkene Nomenclature Propene Ethene

  5. H2C CHCH2CH3 Alkene Nomenclature 1-Butene or But-1-ene • 1) Find the longest continuous chain that includes the double bond. • 2) Replace the -ane ending of the unbranched alkane having the same number of carbons with -ene. • 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon.

  6. H2C CHCHCH2Br CH3 Alkene Nomenclature • 4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. • The compound shown above is4-bromo-3-methyl-1-butene. • (or 4-bromo-3-methylbut-1-ene)

  7. H2C CHCHCH2OH CH3 Alkene Nomenclature • 4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. • The compound shown above is2-methyl-3-buten-1-ol. • (or 2-methyl-but-3-en-1-ol)

  8. H2C H2C H2C CCH3 Alkenyl Groups • methylene • vinyl • allyl • isopropenyl CH H2C CHCH2

  9. Cycloalkene Nomenclature Cyclohexene • 1) Replace the -ane ending of the cycloalkane having the same number of carbons with -ene.

  10. 3 2 CH3 4 1 6 5 CH2CH3 Cycloalkene Nomenclature 6-Ethyl-1-methylcyclohexene • 1) Replace the -ane ending of the cycloalkane having the same number of carbons with -ene. • 2) Number through the double bond in thedirection that gives the lower number to the first-appearing substituent.

  11. 5.2Structure and Bonding in Alkenes

  12. Structure of Ethylene • bond angles: H-C-H = 117° • H-C-C = 121° • bond distances: C—H = 110 pm • C=C = 134 pm planar

  13. Bonding in Ethylene • Framework of  bonds • Each carbon is sp2 hybridized     

  14. Bonding in Ethylene • Each carbon has a half-filled p orbital.

  15. Bonding in Ethylene • Side-by-side overlap of half-filled p orbitals gives a  bond.

  16. 5.3Isomerism in Alkenes

  17. Isomers Isomers are different compounds thathave the same molecular formula.

  18. Constitutional isomers Stereoisomers same connectivity;different arrangementof atoms in space different connectivity Isomers

  19. Constitutional isomers Stereoisomers Isomers consider the isomeric alkenes of molecular formula C4H8

  20. H3C H CH2CH3 H C C C C H3C H H H H3C CH3 H H3C C C C C H H H CH3 1-Butene 2-Methylpropene trans-2-Butene cis-2-Butene

  21. H3C H CH2CH3 H C C C C H3C H H H H3C CH3 C C H H 1-Butene 2-Methylpropene Constitutional isomers cis-2-Butene

  22. H3C H CH2CH3 H C C C C H3C H H H H H3C C C H CH3 1-Butene 2-Methylpropene Constitutional isomers trans-2-Butene

  23. H3C CH3 H H3C C C C C H H H CH3 Stereoisomers trans-2-Butene cis-2-Butene

  24. Stereochemical Notation • trans (identical or analogous substituents on opposite sides) cis (identical or analogous substituents on same side)

  25. Rotation Barrier in Alkenes Interconversion of stereoisomericalkenes does not normally occur.Requires that component of doublebond be broken. cis trans

  26. Rotation Barrier in Alkenes

  27. 5.4Naming Steroisomeric Alkenesby the E-Z Notational System

  28. C C Stereochemical Notation CH2(CH2)6CO2H CH3(CH2)6CH2 • Cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond). • Cis and trans are ambiguous when analogies are not obvious. Oleic acid H H

  29. Cl Br C C H F Example • What is needed:1) Systematic body of rules for ranking substituents • 2) New set of stereochemical symbols other than cis and trans

  30. C C The E-Z Notational System • E : higher ranked substituents on opposite sides • Z : higher ranked substituents on same side higher lower

  31. C C The E-Z Notational System • E : higher ranked substituents on opposite sides • Z : higher ranked substituents on same side lower higher

  32. higher higher C C C C lower lower Zusammen The E-Z Notational System • E : higher ranked substituents on opposite sides • Z : higher ranked substituents on same side higher lower lower higher Entgegen

  33. higher higher C C C C lower lower Zusammen The E-Z Notational System Question: How are substituents ranked? Answer: They are ranked in order of decreasing atomic number. higher lower lower higher Entgegen

  34. The Cahn-Ingold-Prelog (CIP) System • The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog • Their rules for ranking groups were devised in connection with a different kind of stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene stereochemistry.

  35. higher higher Br Cl C C F H lower lower Table 5.1 CIP Rules • (1) Higher atomic number outranks lower atomic number Br > F Cl > H

  36. higher higher Br Cl C C F H lower lower Table 5.1 CIP Rules • (1) Higher atomic number outranks lower atomic number Br > F Cl > H (Z )-1-Bromo-2-chloro-1-fluoroethene

  37. —C(H,H,H) —C(C,H,H) Table 5.1 CIP Rules • (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. —CH2CH3 outranks —CH3

  38. Table 5.1 CIP Rules • (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. —CH(CH3)2 outranks —CH2CH2OH —C(C,H,H) —C(C,C,H)

  39. Table 5.1 CIP Rules • (4) Evaluate substituents one by one. Don't add atomic numbers within groups. —CH2OH outranks —C(CH3)3 —C(O,H,H) —C(C,C,C)

  40. Table 5.1 CIP Rules • (5) An atom that is multiply-bonded to another atom is considered to be replicated as a substituent on that atom. —CH=O outranks —CH2OH —C(O,O,H) —C(O,H,H)

  41. Table 5.1 CIP Rules • A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text.

  42. 5.5Physical Properties of Alkenes

  43. H H C C H H H3C H C C H H  = 0.3 D Dipole Moments • What is direction of dipole moment? • Does a methyl group donate electrons to the double bond, or does it withdraw them?  = 0 D

  44.  = 1.4 D H H C C H H H Cl C C H H H3C H C C H H  = 0.3 D Dipole Moments • Chlorine is electronegative and attracts electrons.  = 0 D

  45.  = 1.4 D H H C C H Cl H3C H C C Cl H H3C H  = 1.7 D C C H H  = 0.3 D Dipole Moments • Dipole moment of 1-chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene.

  46.  = 1.4 D H H C C H Cl H3C H C C Cl H H3C H  = 1.7 D C C H H  = 0.3 D Dipole Moments Therefore, a methyl group donates electrons to the double bond.

  47. R—C+ is more stable than H—C+ • R—C is more stable than • H—C R—C is more stable than H—C Alkyl Groups Stabilize sp2 HybridizedCarbon by Releasing Electrons

  48. 5.6Relative Stabilities of Alkenes

  49. H R C C H H H R C C C C C C R' H disubstituted Double Bonds are Classified According tothe Number of Carbons Attached to Them monosubstituted R' H R R R' H H H disubstituted disubstituted

  50. R" R" R R C C C C R' H R' R"' trisubstituted tetrasubstituted Double Bonds are Classified According tothe Number of Carbons Attached to Them

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