Alkenes: Structure and Reactivity
nAfter this
chapter you should be able to:
nDescribe bonding in alkenes
nDescribe and name isomers using cis-trans and Z,E systems
nCalculate stability of alkenes
nDescribe electrophilic addition and predict products using
Markovnikov’s rule
nKnow relative stability of carbocation and use stability to
predict products
Introduction
nAlkenes =
olefin = unsaturated
nsp2 hybridization
= trigonal planar carbons
nOverlapping
p-orbitals form p bonds
np bonds can NOT rotate without breaking
nCis-trans
isomerism possible
nInteractions occur across multiple alternating double bonds systems through resonance
Degree
of Unsaturation
n
CnH2n+2
for alkane
n How many fewer pairs of hydrogens
n
Other
considerations
n Add halogens to hydrogens
n Ignore the Oxygens
n 
Subtract Nitrogens from hydrogens
n Number of Double Bonds + Rings
Nomenclature
n Number longest 
continuous carbon chain that contains the double bond beginning with end
closest to double bond.
n 
Indicate the
position of the double bond with a number-
n Drop ane ending
and use –ene suffix
n As a substituent
group the double bond takes precedence over substituent groups in numbering.
n For poly-enes use position numbers for each double bond and use suffix…
n 
-diene
n -triene
n -tetraene …
n Start numbering from end with lowest numbered double bond.
Numbering begins
with double bond
n Lowest number to substituent groups
Nomenclature
n For cycloalkenes
n
Nomenclature:
Cis –Trans
n No free rotation
about double bond can occur without breaking p bond
Nomenclature E,Z
system
n More general than cis-trans
n Consider carbons
of double 
bond separately
n Rank atoms
attached to each carbon by atomic weight with larger having higher priority
n 
For chains move
further down attached chains until atomic weight difference can distinguish
priority
n Multiple bonds
count as if two atoms are
attached.
Nomenclature
E,Z system
Alkene
Stability
n 
Cis-trans
isomers can interconvert upon treatment with strong acid
n Given that the
equilibrium mixture is 76% trans calculate the energy difference between the
two states. DG= -RT lnK
n DG= -8.314J/kmol x 298K x ln(24/76)
n DG= -2.86kJ/mol
n Relative
stability of cis-trans isomers can also be found by the difference in heats of
combustion of the two isomers.
n -2682.2 -(-
2685.5) = 3.3kJ/mol
Alkene
Stability
n The stability of
intermediates of addition reactions are stabilized by hyperconjugation
Electrophilic
Addition: C=C + HX
n 
Markovnivokov’s
Rule:
n “Them as has,
gets.” (From Huck Finn)
n The carbon with
the most hydrogens gets the hydrogen.
Stability of the
Carbocation (C+) Intermediate.
n The most highly
substituted carbon can stabilize the carbocation to the largest degree.
n Partly because
of inductive effect
n Direct attraction
of electrons from adjacent carbons
n Partly because
of hyperconjugation.
n The more hydrogens
attached to adjacent carbons, the greater number of hyperconjugation
possibilities with the vacant p orbital
The
Hammond Postulate
n 
The structure of
a transition state (activated complex) resembles the structure of the nearest
stable species.
n Transition state
for endergonic steps resemble products
n Transition state for exergonic step resembles reactant
Energy
Profile for Protonation in Addition
Reaction Mechanism
Carbocation
Rearrangement
Hydride
Shift for More
Stable Carbocation
Carbocation
Rearrangement
Alkyl Shift for More
Stable Carbocation