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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