Chemistry 331 - Winter 1996
Elements of Organic Chemistry I

Professor Carl C. Wamser

Chapter 3 - Alkenes. Organic Reactions

• C=C double bond
• CnH2n general formula
• unsaturated
• not "saturated" with maximum hydrogens
• unsaturated fats have some double bonds (easier to digest)
• cycloalkanes also are CnH2n

• parent alkene is the longest continuous carbon chain that includes the double bond
• number from the end that gives the double bond the lower number
• use -ene suffix and only the first number of the double bond
• name substituents as usual

• double bond assumed at C1-C2
• number in the direction that gives substituents lower numbers
• 3-methylcyclohexene
• 5,5-dimethyl-1,3-cyclopentadiene

• C=C double bond is one sigma bond and one pi bond
• sp2 hybridization (trigonal planar)
• pi bond doesn't rotate
• unlike ethane, ethene has no other conformers

• another example of stereoisomers
• 2-butene has two isomers:
• trans-2-butene
• cis-2-butene
• usually trans is more stable than cis

• an alkene can have cis-trans isomers only when each C in the double bond is attached to 2 different groups

• note that cis-trans works only for disubstituted alkenes
• E,Z designation is more general

• assign priorities to the two groups on each C of the double bond (high, low for each C)
• if the the two high priority groups are on the same side, it is Z
• if the the two high priority groups are on opposite sides, it is E

• assign priority based on the atoms directly attached to the carbons of the double bond
• higher priority goes to:
• higher atomic number
• break ties by considering the next bonded atoms

• (E)-3,4-dimethyl-2-octene
• (Z)

• additions
• A + B --> C
• H-Cl + CH2=CH2 --> CH3-CH2-Cl
• eliminations
• X --> Y + Z
• CH3-CH2-Cl --> CH2=CH2 + H-Cl
• substitutions
• A-B + C-D --> A-C + B-D
• CH4 + Cl2 --> CH3-Cl + H-Cl
• rearrangements
• X --> Y
• cyclopropane --> propene

• a step-by-step account of how a reaction occurs (and why)
• bond-breaking steps:
• homolytic: one electron to each fragment
• » generates radicals
• heterolytic: both electrons to one fragment
• » generates ions
• bond-making steps:
• homogenic or heterogenic

• electron-pushing arrows indicate the flow of electrons in a mechanism
• electrophile:
• electron-deficient species seeking a pair of electrons
• (a Lewis acid )
• nucleophile:
• electron-rich species that can provide a pair of electrons
• a Lewis base

• CH2=CH2 + H-Cl --> CH3-CH2-Cl
• an electrophilic addition
• reaction type: addition
• reagent type: an electrophile
• HCl, actually H+, a strong Lewis acid

• pi bond is relatively reactive, especially towards electrophiles
• it provides a good source of electrons
• addition of H+ to CH2=CH2 forms a new C-H sigma bond
• the electrons for the new bond came from the pi bond
• the other C is left with only 6 e-

• an intermediate is formed in the reaction mechanism
• CH2=CH2 + H+ --> CH3-CH2+
• carbocation: a carbon atom with only 3 bonds (6 e-) and a positive charge
• structure: sp2 hybridized (trigonal)

• the reaction is completed as chloride anion (a nucleophile) adds to the carbocation (an electrophile)
• CH3-CH2+ + Cl- ---> CH3-CH2-Cl

• for a general reaction:
• a A + b B ==> c C + d D
• equilibrium constant, K
• K = [C]c [D] d / [A] a [B] b
• favorable reactions have large K
• unfavorable reactions have small K

• rates of reaction
• may or may not correlate with the favorability of the equilibrium
• rate depends on the mechanism
• is there a good way to get from reactants to products?

• delta H enthalpy change
• delta H = H(products) - H(reactants)
• exothermic
• delta H < 0 (negative)
• reaction gives off heat
• endothermic
• H > 0 (positive)
• reaction absorbs heat

• draw exothermic reactions downhill
• draw endothermic reactions uphill

• there is usually an energy barrier between reactants and products
• activation energy represents the highest amount of energy necessary to carry out the reaction (starting from reactants)

• structure of the molecule(s) at the highest point along the reaction pathway
• the stability of the transition state (relative to reactants) determines Ea (rate of reaction)

• name alkenes and cycloalkenes
• identify cis-trans stereoisomers
• apply the sequence rules
• assign E, Z designations
• classify organic reaction types: addition, elimination, substitution, rearrangement
• identify bond polarities in functional groups
• identify nucleophiles and electrophiles
• identify the steps in the reaction mechanism for CH2=CH2 + HCl
• distinguish and apply kinetics and equilibrium concepts
• distinguish delta H and Ea for a reaction
• draw and identify the components of potential energy diagrams
• predict favorability of a reaction from kinetic or equilibrium data