hapter 05 Structure and Preparation of Alkenes: Elimination Reactions part 01 EM 341: Spring 2012 Prof. Greg ook 2012 Gregory R ook
Alkenes ompounds that contain double bonds. Ethylene (Ethene) 2 2012 Gregory R ook
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 = 2 3 2012 Gregory R ook
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 4 2012 Gregory R ook
Alkene Examples 1-hexene (hex-1-ene) 2-methyl-2-butene (2-methylbut-2-ene) l 3-methyl-1,5-hexadiene 4-chlorocyclohexene 5 2012 Gregory R ook
Alkene Examples O 6 2012 Gregory R ook
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 7 2012 Gregory R ook
Structure and Bonding A double bond is sp 2 hybridized (pi bond) 8 2012 Gregory R ook
Structure and Bonding Bond Dissociation Energy + Δ = +730 kj/mol (172 kcal/mol) + Δ = +375 kj/mol 90 kcal/mol) 9 2012 Gregory R ook
Alkene Stereoisomers Double bonds have restricted rotation rotate pi-bond broken 10 2012 Gregory R ook
Alkene Stereoisomers 2 3 3 3 1-butene 2-methyl-propene 3 3 trans-2-butene 3 3 cis-2-butene 11 2012 Gregory R ook
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 12 2012 Gregory R ook
E-Z Naming System l Br l F F Br Z (zusammen, together) E (entgegen, opposite) 13 2012 Gregory R ook
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. 13 2012 Gregory R ook
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 3 2 3 An E alkene 14 2012 Gregory R ook
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 2 2 2 3 A Z alkene 14 2012 Gregory R ook
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 ) 2 14 2012 Gregory R ook
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 2 2 15 2012 Gregory R ook
Substituent Effects μ 0.3-0.5 D μ 0.3-0.5 D μ 0.3-0.5 D μ 0.3-0.5 D 16 2012 Gregory R ook
Substituent Effects in general, the more substituted the alkene, the more stable it will be antibonding p orbital p orbital bonding 17 2012 Gregory R ook
Substituent Effects steric strain is also important - trans alkenes more stable than cis alkenes + cat. heat 24% 76% trans isomer is less crowded 18 2012 Gregory R ook
ycloalkenes cyclopropene cyclopentene cyclohexene cyclooctene 19 2012 Gregory R ook Friday, February 24, 12
ycloalkenes cyclopropene cyclopentene cyclohexene cyclooctene or trans less stable or cis more stable 39 kj/mol 19 2012 Gregory R ook Friday, February 24, 12
Preparation of Alkenes industrial dehydrogenation - practical only for small alkenes 750 + 750 3 3 + 20 2012 Gregory R ook
Dehydration of Alcohols O + cat. + O 21 2012 Gregory R ook
Dehydration of Alcohols O + cat. + O O O 2 SO 4 160 2 SO 4 140 + O + O O 2 SO 4 < 100 + O 21 2012 Gregory R ook
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 4 80 3 PO 4 heat + 10% 90% + 84% 16% 22 2012 Gregory R ook
Stereoselectivity of Dehydration If possible, elimination occurs to form the trans alkene preferentially O 2 SO 4 heat + 75% 25% 23 2012 Gregory R ook