Chemistry 331 - Fall 1996
Elements of Organic Chemistry I

Professor Carl C. Wamser

Chapter 3 - Alkenes. Organic Reactions

Alkenes

• 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

Alkene Nomenclature

• 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

Cycloalkene Nomenclature

• double bond assumed at C1-C2
• number in the direction that gives substituents lower numbers

3-methylcyclohexene

5,5-dimethyl-1,3-cyclopentadiene

Alkene Structure

• 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

Cis-Trans Isomers

• another example of stereoisomers
• 2-butene has two isomers:

trans-2-butene

cis-2-butene

• usually trans is more stable than cis

Cis-Trans Isomers

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

Stereochemistry Designation

cis or trans ?

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

E,Z Designation

• 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

Sequence Rules

• 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

Practice Nomenclature

(E)-3,4-dimethyl-2-octene

(Z)

Reaction Types

• 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

Reaction Mechanisms

• 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 Movement

• 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)

HCl plus Ethene

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

Addition Mechanism

• 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-

Carbocation Intermediate

• 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)

Formation of Chloroethane

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

Equilibrium

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

Kinetics

• 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?

Heats of Reaction

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

Potential Energy Diagrams

• draw exothermic reactions downhill
• draw endothermic reactions uphill

Activation Energy, Ea

• there is usually an energy barrier between reactants and products
• activation energy represents the highest amount of energy necessary while travelling along the minimum-energy (easiest) pathway from reactants to products

The Transition State

• 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)