Chapter 05 Structure and Preparation of Alkenes: Elimination Reactions part 01

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1 hapter 05 Structure and Preparation of Alkenes: Elimination Reactions part 01 EM 341: Spring 2012 Prof. Greg ook 2012 Gregory R ook

2 Alkenes ompounds that contain double bonds. Ethylene (Ethene) Gregory R ook

3 Degrees of Unsaturation General formula for alkene is n2n Number of 's if saturated - Number of 's that are present 2n+2 - # - #X + #N DU = 2 DU = Gregory R ook

4 Alkene Nomenclature Step 1 - find the longest chain containing the double bonds (or multiple double bonds) Step 2 - number the chain from the end nearest the double bond. Double bonds have priority over alkyl and halide substituents but not alcohols Step 3 - name the molecule using -ene as the suffix to replace the -ane Gregory R ook

5 Alkene Examples 1-hexene (hex-1-ene) 2-methyl-2-butene (2-methylbut-2-ene) l 3-methyl-1,5-hexadiene 4-chlorocyclohexene Gregory R ook

6 Alkene Examples O Gregory R ook

7 ommon names a vinyl group l vinyl chloride vinylic hydrogens an isoprenyl group an allyl group Br O isoprenyl bromide allyl alcohol allylic hydrogens 2 2 an methylene group methylene cyclopentane Gregory R ook

8 Structure and Bonding A double bond is sp 2 hybridized (pi bond) Gregory R ook

9 Structure and Bonding Bond Dissociation Energy + Δ = +730 kj/mol (172 kcal/mol) + Δ = +375 kj/mol 90 kcal/mol) Gregory R ook

10 Alkene Stereoisomers Double bonds have restricted rotation rotate pi-bond broken Gregory R ook

11 Alkene Stereoisomers butene 2-methyl-propene 3 3 trans-2-butene 3 3 cis-2-butene Gregory R ook

12 Alkene Isomerization β-carotene from carrots metabolized vitamin A metabolized O O When trans-retinal absorbs a photon of light, it isomerizes one of the double bonds to the cis isomer. This changes the shape of the molecule, and that changes the shape of the rhodopsin protein that it is buried in. This sends a signal to the brain that the eye has been struck by a photon. This is critical for vision. hν trans-retinal cis-retinal O Gregory R ook

13 E-Z Naming System l Br l F F Br Z (zusammen, together) E (entgegen, opposite) Gregory R ook

14 E-Z Naming System l Br l F F Br Z (zusammen, together) E (entgegen, opposite) ahn-ingold-prelog rules for assigning priorities Look at the atom directly connected to the carbon of the double bond. Rank the atoms according to their atomic number. The higher atomic number gets priority over the lower Gregory R ook

15 E-Z Naming System If the distinction cannot be made at the first atom, look at the next level of atoms attached to those groups. Only look out as far as you need to make a distinction and NO FURTER. l An E alkene Gregory R ook

16 E-Z Naming System If the distinction cannot be made at the first atom, look at the next level of atoms attached to those groups. Only look out as far as you need to make a distinction and NO FURTER. l 2 O A Z alkene Gregory R ook

17 E-Z Naming System If the distinction cannot be made at the first atom, look at the next level of atoms attached to those groups. Only look out as far as you need to make a distinction and NO FURTER. l 2 2 O ( 3 ) Gregory R ook

18 E-Z Naming System Multiple bonds are equivalent to the same number of single bonds IN BOT DIRETIONS. l 2 ( 3 ) 2 A Z alkene Gregory R ook

19 Substituent Effects μ D μ D μ D μ D Gregory R ook

20 Substituent Effects in general, the more substituted the alkene, the more stable it will be antibonding p orbital p orbital bonding Gregory R ook

21 Substituent Effects steric strain is also important - trans alkenes more stable than cis alkenes + cat. heat 24% 76% trans isomer is less crowded Gregory R ook

22 ycloalkenes cyclopropene cyclopentene cyclohexene cyclooctene Gregory R ook Friday, February 24, 12

23 ycloalkenes cyclopropene cyclopentene cyclohexene cyclooctene or trans less stable or cis more stable 39 kj/mol Gregory R ook Friday, February 24, 12

24 Preparation of Alkenes industrial dehydrogenation - practical only for small alkenes Gregory R ook

25 Dehydration of Alcohols O + cat. + O Gregory R ook

26 Dehydration of Alcohols O + cat. + O O O 2 SO SO O + O O 2 SO 4 < O Gregory R ook

27 Regioselectivity of Dehydration Zaitsev Rule - Elimination occurs to form the most substituted double bond preferentially (the hydrogen on the carbon with the fewest hydrogens is lost preferentially) O O 2 SO PO 4 heat + 10% 90% + 84% 16% Gregory R ook

28 Stereoselectivity of Dehydration If possible, elimination occurs to form the trans alkene preferentially O 2 SO 4 heat + 75% 25% Gregory R ook