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Unit 5

Unit 5. Alkenes and Alkynes. Alkenes. Nomenclature Physical Properties Synthesis Reactions. Alkenes. Contain at least one carbon-carbon double bond (C=C). Also called olefins. Are unsaturated. (Alkanes are saturated.). Properties of C=C Bonds. sigma ( σ ) bonds. pi ( π ) bond.

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Unit 5

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  1. Unit 5 Alkenes and Alkynes

  2. Alkenes Nomenclature Physical Properties Synthesis Reactions

  3. Alkenes • Contain at least one carbon-carbon double bond (C=C). • Also called olefins. • Are unsaturated. (Alkanes are saturated.)

  4. Properties of C=C Bonds sigma (σ) bonds pi (π) bond • C=C consists of a σ bond and a π bond. • C=C is shorter than a C-C bond.

  5. Properties of C=C Bonds • The pi bond forces the six atoms involved to be planar. • The e- density of the pi bond is above and below the plane.

  6. Alkenes • σ bond dissociation energy = 347 kJ/mol • π bond dissociation energy = 264 kJ/mol • The pi bond is relatively easy to break, which makes C=C a functional group. • The pi bond blocks nucleophilic attack.

  7. Elements of Unsaturation • Alkenes contain C=C. • Alkynes contain C≡C. • Nonaromatic rings contain two fewer H’s than the corresponding straight chain compound. • Compounds containing one or more of the above contain elements of unsaturation.

  8. Elements of Unsaturation • An element of unsaturation corresponds to two fewer H atoms than in a saturated formula, CnH(2n+2). CH3CH2CH2CH2CH3 pentane, C5H12 CH3CH=CHCH2CH3 2-pentene, C5H10 1 E.U. cyclopentane, C5H10 1 E.U.

  9. Elements of Unsaturation • If the compound contains • a halogen, count the halogen as an H. • oxygen, ignore it when counting. • nitrogen, count as ½ C. • E.U. = ½ (2*(number of C + ½ number of N) + 2 - (number of H + X)) • Knowing the elements of unsaturation in a compound helps you find a structure from a molecular formula, something you may find useful in NMR.

  10. Elements of Unsaturation • Find the E.U. of C6H3NBrCl. • E.U. = ½ (2*(number of C + ½ number of N) + 2 - (number of H + X)) • E.U. = ½ (2*(6 + ½) + 2 - 5) • E.U. = ½ (2*6.5 + 2 - 5) • E.U. = ½ (15-5) • E.U. = ½ (10) • E.U. = 5

  11. Elements of Unsaturation • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum. • C3H4Cl2 • E.U. = ½ (2*3 + 2 - (4+2)) = ½ (8-6) = 1 E.U.

  12. Elements of Unsaturation • C3H4Cl2 1 E. U. SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

  13. Elements of Unsaturation • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum. • C4H4O2 • E.U. = ½ (2*4 + 2 - 4) = ½ (10-4) = 3 E.U.

  14. Elements of Unsaturation • C4H4O2 3 E. U. SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

  15. Elements of Unsaturation • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum. • C4H4O2 • E.U. = ½ (2*4 + 2 - 4) = ½ (10-4) = 3 E.U.

  16. Elements of Unsaturation • C4H4O2 3 E. U. SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

  17. Nomenclature of Alkenes • Apply the same rules you learned for the alkanes. • Use the root name of the longest chain containing the double bond, but change -ane to -ene. • Number the carbon atoms from the end closer to the double bond.

  18. Nomenclature of Alkenes • When the chain contains more than three C atoms, use a number to give the location of the double bond. CH3CH=CH2 propene common: propylene CH3CH=CHCH2CH3 2-pentene pent-2-ene trans-6-chloro-5-methylhex-2-ene

  19. Nomenclature of Alkenes • Cycloalkenes are assumed to have the double bond in the number 1 position. 3-chlorocyclopentene

  20. Nomenclature of Alkenes • The double bond is always between carbon 1 and carbon 2. • Number such that you get to the substituent sooner. 3-bromo-4-methylcyclopentene

  21. Nomenclature of Alkenes • A compound with two C=C’s is a diene; a compound with three C=C’s is a triene, etc. CH3CH=CHCH=CHCH=CH2 1,3,5-heptatriene hepta-1,3,5-triene 2-chlorocyclopentadiene

  22. Nomenclature of Alkenes • Alkenes as substituents are called alkenyl groups. =CH2 methylene group methenyl -CH=CH2 vinyl group ethenyl -CH2-CH=CH2 allyl group 2-propenyl

  23. Phenyl and nitro groups • The benzene ring as a substituent is called a phenyl group. • Nitro groups are common substituents on benzene ring. -NO2 nitro group phenyl group

  24. Nomenclature of Alkenes • Alkenes as substituents are often named using common names. 4-methylenecyclopentene H2C=CHCl vinyl chloride H2C=CH-CH2Cl allyl chloride

  25. Nomenclature of Alkenes • cis-trans (geometric) isomers • Must have an identical group on each C of the double bond, but not two identical groups on one C of the double bond. • cis: identical groups are on the same side of the double bond. • trans: identical groups are on opposite sides of the double bond.

  26. Nomenclature of Alkenes cis-2,3-dichlorobut-2-ene cis-2,3-dichloro-2-butene trans-2,3-dichlorobut-2-ene trans-2,3-dichloro-2-butene 1,1-dichloro-2-methylpropene

  27. Nomenclature of Alkenes • trans cycloalkenes are unstable unless the ring has at least 8 C atoms. • All cycloalkenes are assumed to be cis unless they are specifically named trans. trans-cyclodecene cis-cyclodecene

  28. Nomenclature of Alkenes • E-Z nomenclature • Can be used instead of cis-trans, but must be used when cis-trans doesn’t apply. • E-Z is patterned after the Cahn-Ingold-Prelog convention for asymmetric C atoms. • Assign priorities to the two groups on each C of the double bond. • If the higher priority groups are on the same side of C=C, the isomer is “Z”. • If the higher priority groups are on opposite sides of C=C, the isomer is “E”.

  29. Nomenclature of Alkenes cis-2,3-dichloro-2-butene (Z)-2,3-dichloro-2-butene (E)-2-bromo-1-chloropropene 2-ethyl-1-pentene (Z)-2-bromo-1-chloropropene

  30. Nomenclature of Alkenes • E-Z nomenclature • Designate the isomer present for each double bond. (2Z,4E,6Z)-3-chloroocta-2,4,6-triene (2Z,4E,6Z)-3-chloro-2,4,6-octatriene

  31. Uses of Alkenes • Intermediates in the synthesis of • polymers (e.g., polyethylene, polypropylene, polystyrene) • drugs • pesticides • Ethylene is used to make • ethanol • acetic acid • vinyl chloride • ethylene glycol

  32. Stability of Alkenes • Information comes from heats of hydrogenation. • The more exothermic the heat of hydrogenation, the less stable the bond.

  33. Stability of Alkenes • The most stable double bonds are those with the most alkyl groups attached (Zaitsev’s rule). • alkyl groups are e--donating • serves to separate bulky alkyl groups • trans isomers are generally more stable than cis.

  34. Stability of Alkenes • Cyclopropeneand cyclobutene have extra ring strain due to the C=C. • Larger rings can accommodate the C=C without loss of stability.

  35. Stability of Alkenes • Bredt’s rule: A bridged bicyclic compound cannot have a C=C at a bridgehead position unless one of the rings contains at least eight C atoms.

  36. Physical Properties of Alkenes • Boiling points and densities are similar to those of the corresponding alkanes. • Although classified nonpolar, alkenes are slightly more polar than alkanes. • Pi electrons are more polarizable. • Vinylic bonds are slightly polar.

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