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CHEM 2331H
T.Andrew Taton

Chapter 7 Book Notes Structure and Synthesis of Alkenes 7.1) Introduction • Alkenes (aka olefins b/c oily): hydrocarbons w/ C=C double bond o among most important industrial chemicals, many found in plants and animals too • Pi bonds have less energy than sigma bonds  more reactive than sigma bonds • C=C double bond’s relatively reactive  functional group, alkenes characterized by rxn of double bonds 7.2) The Orbital Description of theAlkene Double Bond • Double bond has 2 pairs of electrons, which by pauli exclusion principle, each pair needs its own orbital • 7.2A) The Sigma Bond Framework o ex: ethylene: each C bonded to 3 other atoms, no nonbonding electrons   3 hybrid orbitals needed sp hybridization  120 degree bond angles 2 o Bonding: sp orbitals of C overlap with s from H  C-H bond here is shorter than C-H bond in ethane b/c sp has more s character than sp orbital  s is closer to nucleus than p  shorter bonds o For C=C bond, shorter than ethane b/c of same reason above and b/c of 2 pi nd bond • 7.2B) The Pi Bond o 2 bond = pi bond, half = above sigma, half = under, can’t twist  isomers 7.3) Elements of Unsaturation • 7.3A) Elements of Unsaturation in Hydrocarbons o Alkenes = unsaturated b/c can add H in presence of catalyst o Alkanes = saturated b/c can’t react w/ anymore H o Elements of unstaturation: features that decrease #H in hydrocarbons, like ring/pi bond; each element takes away 2 more H’s • 7.3B) Elements of Unsaturation with Heteroatoms o Heteroatoms: atoms other than C or H…can be element of unsaturation o Halogens: 1 halogen replaces 1 H o Oxygen: can be added w/o changing # H’s  ignore O atoms o Nitrogen: N can replace C but is trivalent  only 1 additional H (not 2)  N = 1/2C 7.4) Nomenclature ofAlkenes • Simple ones are named like alkanes, but change prefix from –ane to –ene • If more than 3 C’s, use # for double bond location, start #ing from end closest to double bond o cycloalkenes assumed to have double bond in #1 position • IUPAC: instead of putting # before name, put before suffix alk-#-ene • diene: 2 double bonds; triene: 3 double bonds; tetraene: 4 double bonds o use #s to specify locations of each double bonds • Alkenes as substituents: called alkenyl groups; have systematic/common names • Common names: most named by IUPAC system, but some have common names too 7.5) Nomenclature of cis-trans Isomers • 7.5A) Cis-Trans Nomeclature o Cis-trans isomerism (geometric isomerism)  cis = 2 similar groups on same side of double bond  trans = similar groups on opposite sides of bond  But not all alkenes have cis-trans isomers b/c either C of double bond can have identical groups o Trans cycloalkenes = unstable unless big ring (8+ C’s) to accommodate double bond  all cycloalkenes assumed to be cis unless specified (cis not usually added to name) • 7.5B) E-Z Nomenclature o E-Z system: patterned after Cahn-Ingold-Prelog convention for asymmetric C’s  used b/c can’t always use cis/trans if more than 2 different groups o Separate double bonds into 2 ends o Assign priorities to groups using same rules for R and S (just 1 and 2 now) per end o If priority #s are cis  Z, if trans,  E o If alkene has more than 1 double bond, do this for each double bond o E-Z is always an option, but required when double bond isn’t clearly cis/trans • 7.6) Commercial Importance ofAlkenes • B/c C=C double bond readily converted to other functional groups, alkenes = important intermediates in polymers, drugs, pesticides, and other valuable chemicals • Ethylene produced at largest volume; most = polymerized  polyethylene o rest used to synthesize ethanol, acetic acid, ethylene glycol, vinyl chloride o also = plant hormone, accelerates fruit ripening (ex: tomatoes picked raw, treated w/ ethylene  red just before on display) • Propylene: lot becomes polypropylene o rest  propylene glycol, acetone, isopropyl alcohol, etc. • Polymer: large molecule made of many monomer molecules (individual units) o Alkene monomer can polymerize by chain rxn (additional alkene molecules added to end of growing polymer chain)  addition polymers (add individual alkene units) o Polyolefins: made from monofunctional (single functional group) alkenes (ex: ethylene, propene) 7.7) Stability ofAlkenes • Heat of hydrogenation: heat given off during catalytic hydrogenation o used to compare alkene energies and stabilities o Like heat of combustion but smaller/more accurate #s • 7.7A) Heats of Hydrogenation o When alkene treated w/ hydrogen in presence of platinum catalyst, H adds to double bond  alkene → alkane o Rxn is mildly exothermic • 7.7B) Substitution Effects o Proves why certain products are more stable as claimed in last chapter (Zaitsev’s rule) o Heat of hydrogenation measures energy content in pi bond o Can compare alkenes if they hydrogenate to give alkanes of similar energy o Alkyl groups on double bond = stabilized  Alkyl groups = electron-donating, contribute electron density to pi bond  bulky substituent groups = best situated as far apart as possible   alkyl groups are best separated by most highly substituted double bond o double-bond isomers: differ only in position of double bond • 7.7C) Energy Differences in cis-trans isomers o trans isomers are more stable than cis isomers b/c alkyl substituents separated farther • 7.7D) Stability of Cycloalkenes o most cycloalkenes react like acyclic alkenes (ring only makes difference if ring strain from small ring or trans)  5+membered ring can easily accommodate double bonds and still act like straight chain alkenes  But 3 and 4 membered rings have ring strain o Cyclobutene  has more energy than cyclobutane  ring strain  more energy   more reactive than typical double bond o Cyclopropene  lot of strain, scientists used to think can’t even be formed b/c would snap open  now can be synthesized, and stored in cold  but still highly unusual o Trans Cycloalkenes:  trans = more stable for acyclic  but for cyclic, 2 alkyl groups in trans are so far apart, need lot of C’s to complete ring…needs 10+ • 7.7E) Bredt’s Rule o Bredt’s Rule: bridged bicyclic compound can’t have double bond @ bridgehead position unless one of the rings contains at leas
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